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Photoproduction of Vector Mesons at Large Transfer: At forward angles, the cross-sections of photoproduction of vector mesons
($\rho$, $\omega$, and $\phi$) are well accounted for by the exchange of the
Pomeron at high energies, while contributions of $t$ channel exchange of
Reggeons are significant at low energies. At large angles, the impact parameter
becomes small enough to prevent their constituents to build up the exchanged
Reggeons or Pomeron. Two gluon exchange appears to dominate above $-t\simeq 1$
GeV$^2$, especially in the $\phi$ channel. | hep-ph |
Analysis of the decay constants of the heavy pseudoscalar mesons with
QCD sum rules: In this article, we recalculate the contributions of all vacuum condensates
up to dimension-6, in particular the one-loop corrections to the quark
condensates $\alpha_s<\bar{q}q>$ and partial one-loop corrections to the
four-quark condensates $\alpha_s^2<\bar{q}q>^2$, in the operator product
expansion. Then we study the masses and decay constants of the heavy
pseudoscalar mesons $D$, $D_s$, $B$ and $B_s$ using the QCD sum rules with two
choices: {\bf I} we choose the $\bar{MS}$ masses by setting $m=m(\mu)$ and take
perturbative corrections up to the order $\mathcal{O}(\alpha_s)$; {\bf II} we
choose the pole masses $m$, take perturbative corrections up to the order
$\mathcal{O}(\alpha_s^2)$ and set the energy-scale to be the heavy quark pole
mass $\mu=m_Q$. In the case of {\bf I}, the predictions
$f_D=(208\pm11)\,\rm{MeV}$ and $f_B=(189\pm15)\,\rm{MeV}$ are consistent with
the experimental data within uncertainties, while the prediction
$f_{D_s}=(241\pm12)\,\rm{MeV}$ is below the lower bound of the experimental
data $f_{D_s}=(260.0\pm5.4)\,\rm{MeV}$. In the case of {\bf II}, the
predictions $f_D=(211\pm14)\,\rm{MeV}$, $f_B=(190\pm17)\,\rm{MeV}$,
$f_{D_s}=(258\pm13)\,\rm{MeV}$ and $f_{D_s}/f_D=1.22\pm0.08$ are all in
excellent agreements with the experimental data within uncertainties. | hep-ph |
Threshold behavior of Feynman diagrams: the master two-loop propagator: An asymptotic expansion of the two-loop two-point ``master'' diagram with two
masses $m$ and $M$, on the mass shell $Q^2=M^2$, is presented. The treatment of
the non-analytical terms arising in the expansion around the branching point is
discussed. Some details of the calculation of a new class of two-loop integrals
are given. | hep-ph |
Virtual Corrections to the NLO Splitting Functions for Monte Carlo: the
non-singlet case: Construction of a QCD cascade at the NLO level requires recalculation of the
splitting functions in a different manner [1]. We describe the calculation of
some of the virtual contributions to the non-singlet splitting function. In
order to be compatible with the earlier calculated real contributions [2], the
principal value prescription for regularizing the infrared singularities must
be used in a new way. We illustrate this new scheme on simple examples. For the
calculations we wrote a Mathematica package called Axiloop. We describe its
current status. | hep-ph |
Induced top Yukawa coupling and suppressed Higgs mass parameters: In the scenarios with heavy top squarks, mass parameters of the Higgs field
must be fine-tuned due to a large logarithmic correction to the soft scalar
mass. We consider a new possibility that the top Yukawa coupling is small above
TeV scale. The large top mass is induced from strong Yukawa interaction of the
Higgs with another gauge sector, in which supersymmetry breaking parameters are
given to be small. Then it is found that the logarithmic correction to the
Higgs soft scalar mass is suppressed in spite of the strong coupling and the
fine-tuning is ameliorated. We propose an explicit model coupled to a
superconformal gauge theory which realizes the above situation. | hep-ph |
DUNE potential as a New Physics probe: Neutrino experiments, in the next years, aim to determine with precision all
the six parameters of the three-neutrino standard paradigm. The complete
success of the experimental program is, nevertheless, attached to the
non-existence (or at least smallness) of Non-Standard Interactions (NSI). In
this work, anticipating the data taken from long-baseline neutrino experiments,
we map all the weakly coupled theories that could induce sizable NSI, with the
potential to be determined in these experiments, in particular DUNE. Once
present constraints from other experiments are taken into account, in
particular charged-lepton flavor violation, we find that only models containing
leptoquarks (scalar or vector) and/or neutral isosinglet vector bosons are
viable. We provide the explicit matching formulas connecting weakly coupled
models and NSI, both in propagation and production. Departing from the weakly
coupled completion with masses at TeV scale, we also provide a global fit on
all NSI for DUNE, finding that NSI smaller than $10^{-2}$ cannot be probed even
in the best-case scenario. | hep-ph |
Partonic structure of the virtual photon: Interactions of virtual photons are analyzed in terms of their parton
distribution functions. It is shown that the concept of parton distribution
functions is phenomenologically very useful even for highly virtual photonds
involved in hard collisions. The role of the longitudinal photons for proper
interpretation of the data on jet cross--sections in the region of moderate
photon virtualities accessible at HERA is explored. | hep-ph |
Inelastic dark matter nucleus scattering: Direct detection experiments aim at the detection of dark matter in the form
of weakly interacting massive particles (WIMPs) by searching for signals from
elastic dark matter nucleus scattering. Additionally, inelastic scattering in
which the nucleus is excited is expected from nuclear physics and provides an
additional detectable signal. In the context of a low-energy effective field
theory we investigate the experimental reach to these inelastic transitions for
xenon-based detectors employing a dual-phase time projection chamber. We find
that once a dark matter signal is established, inelastic transitions enhance
the discovery reach and we show that they allow a better determination of the
underlying particle physics. | hep-ph |
Hard spectator-scattering in B -> pi pi decays at NNLO: We compute the 1-loop (NNLO) corrections to hard spectator-scattering in
tree-dominated hadronic B decays. Depending on the values of hadronic input
parameters the corrections are shown to have a significant impact on the B ->
pi pi branching fractions. | hep-ph |
Dilaton Stabilisation in $D$-term Inflation: Dilaton stabilisation is usually considered to pose a serious obstacle to
successful $D$-term inflation in superstring theories. We argue that the
physics of gaugino condensation is likely to be modified during the
inflationary phase in such a way as to enhance the gaugino condensation scale.
This enables dilaton stabilisation during inflation with the $D$-term still
dominating the vacuum energy at the stable minimum. | hep-ph |
Bell's Inequality and $τ$-Physics at LEP: In this talk given at the TAU92 Workshop, Columbus, OH, Sept. 92, we
summarize results presented in more detail in a recent paper by S. Abel, M.
Dittmar and the author where we gave a general proof that Bell's inequality can
not be tested at a collider experiment. In particular, a measurement of
correlated tau-spins at LEP does not constitute a test of local realistic
theories via Bell's inequality. The central point of the argument is that such
tests, where the spins of two particles are inferred from a scattering
distribution, can always be described by a local hidden variable theory. In
response to questions at the workshop we go beyond the paper and show that an
old experiment involving the measurement of the correlated spins of the two
photons emitted in positronium decay via Compton-scattering is also not a
viable test of Bell's inequality. | hep-ph |
Neutrino induced meson productions in forward limit: We study neutrino-induced meson productions off the nucleon in the forward
limit by applying the PCAC hypothesis to our dynamical coupled-channels (DCC)
model. The DCC model reasonably describes pi N, gamma N -> pi N, eta N, K
Lambda, K Sigma data in the resonance region. We give a prediction for nu N ->
pi N, pi pi N, eta N, K Lambda, K Sigma$ reactions cross sections. We compare
our results with those from the Rein-Sehgal model, and find a significant
difference. | hep-ph |
The linear BESS model and the double Higgs-strahlung production: In this paper we evaluate, in the context of the linear BESS model the
cross-section for the double Higgs-strahlung process. We find that, within the
bounds given by the actual experimental data, significant deviations with
respect to the SM may arise. In the linear BESS model not only the
self-couplings of the Higgs are modified, but also the Z-Higgs couplings. We
think that this is a generic feature of any extension of the SM and, in our
opinion, it should be kept in mind in analyzing the future data on the process
studied here. | hep-ph |
Density spikes near black holes in self-interacting dark matter halos
and indirect detection constraints: Self-interacting dark matter (SIDM) naturally gives rise to a cored
isothermal density profile, which is favored in observations of many dwarf
galaxies. The dark matter distribution in the presence of a central black hole
in an isothermal halo develops a density spike with a power law of $r^{-7/4}$,
which is shallower than $r^{-7/3}$ as expected for collisionless dark matter
(CDM). Thus, indirect detection constraints on dark matter annihilations from
the density spike could be relaxed in SIDM. Taking the most dense satellite
galaxy of the Milky Way Draco as an example, we derive upper limits on the
annihilation cross section and the black hole mass for both SIDM and CDM halos.
For the former case, Draco could host an intermediate mass black hole even if
dark matter is composed of thermal relics. We further explore the constraints
from the Milky Way and M87, which host supermassive black holes, and show the
upper limits on the annihilation cross section are significantly weakened in
SIDM. Our results also indicate that the Event Horizon Telescope could provide
a unique test of SIDM spikes. | hep-ph |
Weak Radiative Hyperon Decays in Chiral Perturbation Theory: The parity-conserving $a$ and parity-violating $b$ amplitudes for weak
radiative hyperon decay are studied using chiral perturbation theory. The
imaginary parts of $a$ and $b$ are computed using unitarity. The real part of
$b$ is dominated by a one-loop infrared divergent graph which is computed. The
real part of $a$ has a large theoretical uncertainty and cannot be calculated
reliably. Counterterms for the $a$ and $b$ amplitudes are classified using
$CPS$ symmetry. The experimental values for decay widths and asymmetries are
consistent with theory, with the exception of the asymmetry parameter for the
$\Sigma^+ \rightarrow p \gamma$ decay. | hep-ph |
Report of the Topical Group on Cosmic Probes of Dark Matter for Snowmass
2021: Cosmological and astrophysical observations currently provide the only
robust, positive evidence for dark matter. Cosmic probes of dark matter, which
seek to determine the fundamental properties of dark matter through
observations of the cosmos, have emerged as a promising means to reveal the
nature of dark matter. This report summarizes the current status and future
potential of cosmic probes to inform our understanding of the fundamental
nature of dark matter in the coming decade. | hep-ph |
First exotic hadron with open heavy flavor: $cs\bar u\bar d$ tetraquark: The LHCb Collaboration has reported resonant activity in the channel $D^+
K^-$, identifying two components: $X_0(2900)$ with $J^P = 0^+$ at $2866 {\pm}
7$ MeV, $\Gamma_0=57{\pm} 13$ MeV and $X_1(2900)$ with $J^P = 1^-$ at $2904
{\pm} 7$ MeV, $\Gamma_1=110{\pm} 12$ MeV. We interpret the $X_0(2900)$
component as a $cs \bar u\bar d$ isosinglet compact tetraquark, calculating its
mass to be $2863 {\pm} 12$ MeV. This is the first exotic hadron with open heavy
flavor. The analogous $bs\bar u\bar d$ tetraquark is predicted at $6213 {\pm}
12$ MeV. We discuss possible interpretations of the heavier and wider
$X_1(2900)$ state and examine potential implications for other systems with two
heavy quarks. | hep-ph |
Investigating $R_D$ and $R_{D^*}$ anomalies in a Left-Right model with
an Inverse Seesaw: We investigate $R_D$ and $R_{D^*}$ anomalies in a low scale left-right
symmetric model based on $SU(3)_C\times SU(2)_L\times SU(2)_R\times
U(1)_{B-L}\times Z_2$ with a simplified Higgs sector consisting of only one
bidoublet and one $SU(2)_R$ doublet. The Wilson coefficients relevant to the
transition $b\to c\tau \nu$ are derived by integrating out the charged Higgs
$H^\pm$ boson, which gives the dominant contributions. We emphasize that the
charged Higgs effects, with the complex right-handed quark mixing matrix, can
account for both $R_D$ and $R_{D^*}$ anomalies simultaneously, while adhering
to the constraints from ${\rm BR}(B^-_c \to \tau^- \bar{\nu}_\tau) $. | hep-ph |
Phase Transition to RS: Cool, not Supercool: Motivated by the warped conifold compactification, we model the infrared (IR)
dynamics of confining gauge theories in a Randall-Sundrum (RS)-like setup by
modifying the stabilizing Goldberger-Wise (GW) potential so that it becomes
large (in magnitude) in the IR and back-reacts on the geometry. We study the
high-temperature phase by considering a black brane background in which we
calculate the entropy and free energy of the strongly back-reacted solution. As
with Buchel's result for the conifold (arXiv:2103.15188), we find a minimum
temperature beyond which the black brane phase is thermodynamically unstable.
In the context of a phase transition to the confining background, our results
suggest that the amount of supercooling that the metastable black brane phase
undergoes can be limited. It also suggests the first-order phase transition
(and the associated gravitational waves from bubble collision) is not
universal. Our results therefore have important phenomenological implications
for early universe model building in these scenarios. | hep-ph |
Phenomenology of the BFKL pomeron and Unitarity Corrections at low x: The low $x$ limit of deep inelastic electron proton scattering is considered
using methods of perturbative QCD. In the first part we investigate the
phenomenological consequences of the resummation of leading logarithms in $1/x$
given by the BFKL pomeron. We apply the BFKL pomeron to the inclusive structure
function $F_2$, to the diffractive production of vector mesons at large
momentum transfer, to inclusive photon diffractive dissociation in DIS and to
quark-antiquark production with large transverse momenta in DIS diffractive
dissociation. For the last process we perform extensive numerical calculations
based on the double logarithmic approximation. The BFKL pomeron is known to
violate unitarity. In the second part the first next-to-leading corrections
which have to be taken into account to restore unitarity of the scattering
amplitude are investigated. A compact configuration space representation of the
two to four gluon transition vertex is derived. Conformal symmetry of the
vertex is proven and its relation to a conformal covariant three point function
is established. The important role of the spectral function $\chi_4$ of the
four gluon state is pointed out. We relate this function to the twist expansion
of the four gluon amplitude. Motivated by this relation we develop a method to
perform the twist expansion of the amplitude. Based upon first results of our
analysis we draw conclusions concerning the singularity structure of the
function $\chi_4$. | hep-ph |
Probing neutrino magnetic moment and unparticle interactions with
Borexino: We discuss the limits on the neutrino magnetic moment and hypothetical
interactions with a hidden unparticle sector, coming from the first neutrino
data release of the Borexino experiment. The observed spectrum in Borexino
depends weakly on the solar model used in the analysis, since most of the
signal comes from the mono-energetic 7Be neutrinos. This fact allows us to
calibrate the nu-e scattering cross section through the spectral shape. In this
way, we have derived a limit on the magnetic moment for the neutrinos coming
from the Sun (in which a nu_mu and nu_tau component is present): mu_nu<8.4E-11
mu_B (90%CL) which is comparable with those obtained from low energy reactor
experiments. Moreover, we improve the previous upper limit on magnetic moment
of the nu_tau by three orders of magnitude and the limit on the coupling
constant of the neutrino with a hidden unparticle sector. | hep-ph |
Chiral Phase Transition in QCD and Vector Manifestation: Spontaneous chiral symmetry breaking is one of the most important features in
low-energy QCD. The chiral symmetry is expected to be restored at very high
temperature and/or density. Accompanied by the chiral phase transition,
properties of hadrons will be changed especially near the critical point. The
study of the phenomena associated with the chiral phase transition will give us
some clues on the connection between the chiral symmetry and the low-energy
hadron dynamics.
We develop the theory based on the hidden local symmetry (HLS) at finite
temperature, which is an effective field theory of QCD and includes pions and
vector mesons as the dynamical degrees of freedom, and study the chiral phase
transition in hot matter. We show that the chiral symmetry is restored as the
vector manifestation (VM), in which the massless degenerate pion (and its
flavor partners) and the longitudinal $\rho$ meson (and its flavor partners) as
the chiral partner. We also present several predictions based on the VM. We
estimate the critical temperature $T_c$ and show the following phenomena near
$T_c$: the vector charge susceptibility becomes equal to the axial-vector
charge susceptibility; the vector dominance of the electromagnetic form factor
of the pion is largely violated; the pion velocity is close to the speed of
light. Furthermore, we show that the remnant of the VM can be clearly seen in
the system of heavy mesons. We expect that the VM and its predictions are
testable by current and future experiments and the lattice analysis. | hep-ph |
Probing a new decay of vector-like top partner mediated by heavy
Majorana neutrino via single production: Models beyond the Standard Model have been proposed to simultaneously solve
the problems of naturalness and neutrino mass, in which heavy Majorana
neutrinos and vector-like top partners are usually predicted. A new decay
channel of the top partner mediated by the heavy Majorana neutrino can thus
appear: $T\to b\,W^{+}\to b\,\ell^{+}\ell^{+}q\bar{q'}$. We study in this paper
the observability of this decay process through single production of the top
partner at the 14 TeV LHC: $pp\to T/\bar{T}$+jets$\to
b/\bar{b}+\mu^{\pm}\mu^{\pm}$+jets. $2\sigma$ exclusion bounds on the top
partner mass and mixing parameters are given by Monte-Carlo simulation, which
surpass those from the search through VLT pair production in the mass range of
$m_{T}>1.3$ TeV. | hep-ph |
Effects of lepton number violating interactions on $t \bar t$ production
at NLC: We discuss the effects of lepton number violating interactions namely,
R-parity violation and leptoquarks on top quark pair production at the upcoming
$e^+ e^-$ linear colliders. Effects of SU(2) singlet, doublet and triplet
leptoquark interactions are investigated. R-parity violating minimal
supersymmetric standard model also allows certain kinds of lepton number
violating interactions which are same as singlet leptoquarks with left-handed
interactions. We have calculated the cross-section of $e^+ e^- \to t \bar t$ in
presence of the above interactions. With conservative values of lepton number
violating coupling strengths we got enhancement of top-pair production
cross-section in all of the above cases. | hep-ph |
Scaling law for the $Υ(4S) \to B \bar B$ and $ψ(3770) \to D
\bar D$ decay constants from effective sum rules: Sum rules for exclusive production of heavy meson pairs in $e^+e^-$
annihilation are used to evaluate the $\Upsilon(4S) \to B \bar B$ and
$\psi(3770) \to D \bar D$ decay widths. Infinitely heavy quark limit is
discussed, so that scaling law for the quarkonium-meson coupling constant is
derived. A value of the $B\bar B$ pair contribution into the leptonic constant
$f_{\Upsilon(4S)}$ is estimated. | hep-ph |
Branching ratios of $B_s\to (K^+K^-,K^0\bar{K}^0)$ and $B_d\to π^+
π^- and determination of \boldmath{$γ(φ_3)$: We explored various cases for the branching ratios (BRs) of $B_s\to K^+K^-$,
$B_s\to K^0\bar{K}^0$ and $B_d\to \pi^+ \pi^-$ decays. We study the possibility
of determining $\gamma $ by using the following the measurements: (a) BRs of
$B_s\to K^+K^-$, and $B_s\to K^0\bar{K}^0$; (b) the ratio of direct CP
asymmetries in $B_d\to \pi^+ \pi^-$ and $B_s\to K^+K^-$; (c) the mix-induced CP
asymmetry in $B_d\to \pi^+ \pi^-$; and (d) the angle of $\beta$. | hep-ph |
Top quark asymmetry and dijet resonances: CDF recently reported an anomaly in the $m_{jj}$ distribution of dijet events
produced in association with a $W$ boson. If this anomaly is associated with a
new flavor conserving vector resonance, $V$, one might have expected to observe
effects in the analogous $m_{jj}$ distribution produced in association with a
$\gamma$. No such excess is observed. A single $u-t-V$ flavor changing
coupling, however, can contribute to the $m_{jj}$ anomaly while being
consistent with other resonance searches. Furthermore, it gives a potential
explanation of the observed forward-backward asymmetry in top quark production. | hep-ph |
NLO QCD Predictions for associated t-tbar-h production in Hadronic
Collisions: We present the next-to-leading-order (NLO) QCD corrections to the inclusive
total cross section for the production of a Higgs boson in association with a
top anti-top quark pair within the Standard Model at the Tevatron and the LHC. | hep-ph |
Calculation of the scattering amplitudes: General equations for the calculation of amplitudes are presented. As an
illustration of application of proposed formulae we calculate electron-electron
scattering amplitudes. | hep-ph |
Manifestaion of SUSY in B decays: SUSY effects on various flavor changing neutral current processes are
discussed in the minimal supergravity model and the SU(5) grand unified theory
with right-handed neutrino supermultiplets. In particular, in the latter case
the neutrino Yukawa coupling constants can be a source of the flavor mixing in
the right-handed-down-type-squark sector. It is shown that due to this mixing
the time-dependent CP asymmetry of radiative B decay can be as large as 30% and
the ratio of $B_s$-$\bar{B}_s$ mixing and $B_d$-$\bar{B}_d$ mixing deviates
from the prediction in the standard model and the minimal supergravity model
without the neutrino interaction. | hep-ph |
Pseudoscalar--meson decuplet--baryon coupling constants in light cone
QCD: Taking into account the $SU(3)_f$ breaking effects, the strong coupling
constants of the $\pi$, $K$ and $\eta$ mesons with decuplet baryons are
calculated within light cone QCD sum rules method. It is shown that all
coupling constants, even in the case of $SU(3)_f$ breaking, are described in
terms of only one universal function. It is shown that for $\Xi^{\ast 0} \to
\Xi^{\ast 0} \eta $ transition violation of $SU(3)_f$ symmetry is very large
and for other channels when $SU(3)_f$ symmetry is violated, its maximum value
constitutes $10%\div15%$. | hep-ph |
The QCD phase structure at finite temperature and density: We discuss the phase structure of QCD for $N_f=2$ and $N_f=2+1$ dynamical
quark flavours at finite temperature and baryon chemical potential. It emerges
dynamically from the underlying fundamental interactions between quarks and
gluons in our work. To this end, starting from the perturbative high-energy
regime, we systematically integrate-out quantum fluctuations towards low
energies by using the functional renormalisation group. By dynamically
hadronising the dominant interaction channels responsible for the formation of
light mesons and quark condensates, we are able to extract the phase diagram
for $\mu_B/T \lesssim 6$. We find a critical endpoint at
$(T_\text{CEP},{\mu_B}_{\text{CEP}})=(107, 635)\,\text{MeV}$. The curvature of
the phase boundary at small chemical potential is $\kappa=0.0142(2)$, computed
from the renormalised light chiral condensate $\Delta_{l,R}$. Furthermore, we
find indications for an inhomogeneous regime in the vicinity and above the
chiral transition for $\mu_B\gtrsim 417$ MeV. Where applicable, our results are
in very good agreement with the most recent lattice results. We also compare to
results from other functional methods and phenomenological freeze-out data.
This indicates that a consistent picture of the phase structure at finite
baryon chemical potential is beginning to emerge. The systematic uncertainty of
our results grows large in the density regime around the critical endpoint and
we discuss necessary improvements of our current approximation towards a
quantitatively precise determination of QCD phase diagram. | hep-ph |
Gauge Dependence of Four-Fermion QED Green Function and Atom-Like Bound
State Calculations: We derive a relation between four-fermion QED Green functions of different
covariant gauges which defines the gauge dependence completely. We use the
derived gauge dependence to check the gauge invariance of atom-like bound state
calculations. We find that the existing QED procedure does not provide gauge
invariant binding energies. A way to a corrected gauge invariant procedure is
pointed out. | hep-ph |
Ernest Henley and the shape of baryons: Calculations of pion-baryon couplings, baryon quadrupole and octupole
moments, baryon spin and orbital angular momentum done in collaboration with
Ernest Henley are reviewed. A common theme of this work is the shape of
baryons. Also, a personal account of my work with Ernest Henley during the
period 1999-2013 is given. | hep-ph |
Braaten-Pisarski Action, Disoriented Chiral Condensate, and Chiral
Symmetry Non-Restoration: The QCD effective action at high T shows a manifest global chiral symmetry.
And calculations show that the order parameter \psibarpsi vanishes above T_c.
It has been popular to refer to this T_c as chiral symmetry restoration
temperature because it fits into our prejudice that chiral symmetry is like an
`ordered' state, and at high T it must become disordered. In fact, NJL ground
state is not an ordered spin state. The recent scenario of a generic class of
disoriented chiral condensate offers an example where \psibarpsi in each little
domain is nonzero, but the average over all space of \psibarpsi vanishes. Such
a dcc ground state continues to break chiral invariance. But how do you
reconcile this with the apparent chiral symmetry at high T? The
Braaten-Pisarski action is a good laboratory to investigate the subtleties of
high temperature chiral symmetry. By carrying out a canonical quantization of
this highly nonlocal action, I demonstrate how the thermal vacuum at high T
conserves the new \beta-chirality but breaks the old T=0 chirality. Lattice
calculations show that the pion develops a screening mass at high T. Our
continuum field theory calculations show that the QCD pion remains massless for
all T. I conclude the talk by showing how the hot pion manages to accomodate
the two results by propagating in the early universe with a halo. | hep-ph |
Can an invisible Higgs boson be seen via diffraction at the LHC?: We study the possibility of observing an `invisible' Higgs boson in central
exclusive diffractive production at the LHC. We evaluate the cross section
using, as a simple example, the Standard Model with a heavy fourth generation,
where the invisible decay mode $H \to \nu_4 \bar{\nu}_4$ dominates, with the
heavy neutrino mass $M(\nu_4) \simeq 50$ GeV. We discuss the possible
requirements on trigger conditions and the background processes. | hep-ph |
MLAnalysis: An open-source program for high energy physics analyses: We present a python-based program for phenomenological investigations in
particle physics using machine learning algorithms, called \verb"MLAnalysis".
The program is able to convert LHE and LHCO files generated by
\verb"MadGraph5_aMC@NLO" into data sets for machine learning algorithms, which
can analyze the information of the events. At present, it contains three
machine learning (ML) algorithms: isolation forest (IF) algorithm, nested
isolation forest (NIF) algorithm, kmeans anomaly detection (KMAD), and some
basic functionality to analyze the kinematic features of a data set. Users can
use this program to improve the efficiency of searching for new physics
signals. | hep-ph |
On next to soft corrections to Drell-Yan and Higgs Boson productions: We present a framework that resums threshold enhanced large logarithms to all
orders in perturbation theory for the production of a pair of leptons in
Drell-Yan process and of Higgs boson in gluon fusion as well as in bottom quark
annihilation. We restrict ourselves to contributions from diagonal partonic
channels. These logarithms include the distributions $((1-z)^{-1}
\ln^i(1-z))_+$ resulting from soft plus virtual (SV) and the logarithms
$\ln^i(1-z)$ from next-to-SV (NSV) contributions. We use collinear
factorisation and renormalisation group invariance to achieve this. The former
allows one to define a Soft-Collinear (SC) function which encapsulates soft and
collinear dynamics of the perturbative results to all orders in strong coupling
constant. The logarithmic structure of these results are governed by universal
infrared anomalous dimensions and process dependent functions of Sudakov
differential equation that the SC satisfies. The solution to the differential
equation is obtained by proposing an all-order ansatz in dimensional
regularisation, owing to several state-of-the-art perturbative results
available to third order. The $z$ space solutions thus obtained provide an
integral representation to sum up large logarithms originating from both soft
and collinear configurations, conveniently in Mellin $N$ space. We show that in
$N$ space, tower of logarithms $a_s^n/N^\alpha \ln^{2n-\alpha} (N),
a_s^n/N^\alpha \ln^{2n-1-\alpha}(N) \cdots $ etc for $\alpha =0,1$ are summed
to all orders in $a_s$. | hep-ph |
Threshold factorization of the Drell-Yan process at next-to-leading
power: We present a factorization theorem valid near the kinematic threshold
$z=Q^2/\hat{s}\to 1$ of the partonic Drell-Yan process $q\bar q\to\gamma^*+X$
for general subleading powers in the $(1-z)$ expansion. We then consider the
specific case of next-to-leading power. We discuss the emergence of collinear
functions, which are a key ingredient to factorization starting at
next-to-leading power. We calculate the relevant collinear functions at
$\mathcal{O}(\alpha_s)$ by employing an operator matching equation and we
compare our results to the expansion-by-regions computation up to the
next-to-next-to-leading order, finding agreement. Factorization holds only
before the dimensional regulator is removed, due to a divergent convolution
when the collinear and soft functions are first expanded around $d=4$ before
the convolution is performed. This demonstrates an issue for threshold
resummation beyond the leading-logarithmic accuracy at next-to-leading power. | hep-ph |
A Step Toward Model Comparison: Connecting Electroweak-Scale Observables
to BSM through EFT and Bayesian Statistics: Recognizing the potential of effective field theories to posit multiple BSM
scenarios in similar footing, with a possibility to compare them, we inspect
the effects of 11 single scalar-multiplet extensions of the SM on the combined
set of electroweak precision observables and Higgs signal strength data, by
systematically integrating out the heavy multiplets and computing the resulting
SMEFT operators and Wilson coefficients (WCs) up to one-loop level. Noting that
multiple BSM models give rise to a degenerate set of WCs, we then perform
Bayesian statistical inference both directly on the BSM parameters and on the
associated set of independent WCs. Using the posteriors of the BSM parameters,
we infer the respective (correlated) WC-distributions and compare both the
model-independent and dependent analyses by overlaying the 2-D marginal
WC-posteriors from both processes, thus laying the ground for a data-driven
attempt to compare diverse BSM theories of different origins, and hopefully, a
possible way to approach the intractable inverse problem. We also demonstrate,
with an example model, the crucial role of theoretical constraints to rule out
large chunks of BSM parameter spaces. The entirety of numerical results is
available in GitHub. | hep-ph |
Phenomenological study of the anisotropic quark matter in the 2-flavor
Nambu-Jona-Lasinio model: With the two flavor Nambu-Jona-Lasinio (NJL) model we carry out a
phenomenological study on the chiral phase structure, mesonic properties and
transport properties in a momentum-space anisotropic quark matter. To calculate
transport coefficients we have utilized the kinetic theory in the relaxation
time approximation, where the momentum anisotropy is embedded in the estimation
of both distribution function and the relaxation time. It is shown that an
increase of the anisotropy parameter $\xi$ may results in a catalysis of chiral
symmetry breaking. The critical endpoint (CEP) is shifted to smaller
temperatures and larger quark chemical potentials as $\xi $ increases, the
impact of momentum anisotropy on temperature of CEP is almost the same as that
on the quark chemical potential of CEP. The meson masses and the associated
decay widths also exhibit a significant $\xi$ dependence. It is observed that
the temperature behavior of scaled shear viscosity $\eta/T^3$ and scaled
electrical conductivity $\sigma_{el}/T$ exhibit a similar dip structure, with
the minima of both $\eta/T^3$ and $\sigma_{el}/T$ shifting toward higher
temperatures with increasing $\xi$. Furthermore, we demonstrate that the
Seebeck coefficient $S$ decreases when temperature goes up and its sign is
positive, indicating the dominant carriers for converting the temperature
gradient to the electric field are up-quarks. The Seebeck coefficient $S$ is
significantly enhanced with a large $\xi$ for the temperature below the
critical temperature. | hep-ph |
Double-parton scattering effects in double charm production within gluon
fragmentation scenario: We discuss charm $D^0 D^0$ meson-meson pair production in the forward
rapidity region related to the LHCb experimental studies at $\sqrt{s}$ = 7 TeV.
We consider double-parton scattering mechanisms of double $c \bar c$ production
and subsequent standard $cc \to D^{0}D^{0}$ scale-independent hadronization as
well as new double $g$ and mixed $g c\bar c $ production mechanisms with $gg
\to D^{0}D^{0}$ and $gc \to D^{0}D^{0}$ scale-dependent hadronization. The new
scenario with gluon fragmentation components results also in a new
single-parton scattering mechanism of $gg$ production which is also taken here
into account. Results of the numerical calculations are compared with the LHCb
data for several correlation observables. The new mechanisms lead to a larger
cross sections and to slightly different shapes of the calculated correlation
observables. | hep-ph |
DGLAP evolution for DIS diffraction production of high masses: In this paper we develop the DGLAP evolution for the system of produced
gluons in the process of diffractive production in DIS, directly from the
evolution equation in Color Glass Condensate approach. We are able to describe
the available experimental data with small value of the QCD coupling
($\bar{\alpha_S} \approx 0.1$). We conclude that in diffractive production, we
have a dilute system of emitted gluons and in the order to describe them, we
need to develop the next-to-leading order approach in perturbative QCD. | hep-ph |
Instanton induced transverse single spin asymmetry for $π^0$
production in $pp$-scattering: We calculate the production cross-section and the transverse single-spin
asymmetry for pion in $p^{\uparrow}+p\to \pi^0 + X$. Our computation is based
on existence of the instanton induced effective quark-gluon and
quark-gluon-pion interactions with a strong spin dependency. In this framework
we calculate the cross section without using fragmentation functions. We
compare predictions of the model with data from RHIC. Our numerical results,
based on the instanton liquid model for QCD vacuum, are in agreement with
unpolarized cross section data. The asymmetry grows with the transverse
momentum of pion $k_t$ in accordance with experimental observations. It reach
value $\sim 10\%$ but at higher $k_t$ than experiment shows. | hep-ph |
MicroBooNE and the $ν_e$ Interpretation of the MiniBooNE Low-Energy
Excess: A new generation of neutrino experiments is testing the $4.8\sigma$ anomalous
excess of electron-like events observed in MiniBooNE. This is of huge
importance for particle physics, astrophysics, and cosmology, not only because
of the potential discovery of physics beyond the Standard Model, but also
because the lessons we will learn about neutrino-nucleus interactions will be
crucial for the worldwide neutrino program. MicroBooNE has recently released
results that appear to disfavor several explanations of the MiniBooNE anomaly.
Here, we show quantitatively that MicroBooNE results, while a promising start,
unquestionably do not probe the full parameter space of sterile neutrino models
hinted at by MiniBooNE and other data, nor do they probe the $\nu_e$
interpretation of the MiniBooNE excess in a model-independent way. Our analysis
code is fully available in
https://github.com/Harvard-Neutrino/MicroBooNE-analysis-2021. | hep-ph |
Elliptic flow from color-dipole orientation in pp and pA collisions: For ultrarelativistic proton-proton and proton-nucleus collisions, we perform
an exploratory study of the contribution to the elliptic flow $v_2$ coming from
the orientation of the momentum of the produced particles with respect to the
reaction plane. Via the CGC factorization valid at high energies, this
contribution is related to the orientation of a color dipole with respect to
its impact parameter, which in turn probes the transverse inhomogeneity in the
target. Using the McLerran-Venugopalan model (with impact-parameter dependence)
as an effective description for the soft gluon distribution in the (proton or
nuclear) target, we present a semi-analytic calculation of the dipole
scattering amplitude, including its angular dependence. We find that the
angular dependence is controlled by soft gluon exchanges and hence is genuinely
non-perturbative. The effects of multiple scattering turn out to be essential
(in particular, they change the sign of $v_2$). We find that sizable values for
$v_2$, comparable to those observed in the LHC data and having a similar
dependence upon the transverse momenta of the produced particles, can be easily
generated via peripheral collisions. In particular, $v_2$ develops a peak at a
transverse momentum which scales with the saturation momentum in the target. | hep-ph |
Polarization in B->VV Decays: Factorizable amplitudes in B decays to light vector meson pairs give a
longitudinal polarization satisfying 1- f_L =O(1/m_b^2). This remains formally
true when non-factorizable graphs are included in QCD factorization, and is
numerically realized in B->Rho Rho. In \Delta S=1 decays a QCD penguin
annihilation graph can effectively contribute at leading power to the
transverse and longitudinal amplitudes. The observed longitudinal polarization,
f_L (B->phi K^*) \approx 50%, can therefore be accounted for in the SM. The
ratio of perpendicular to parallel transverse rates provides a sensitive test
for new right-handed currents. The transverse b->sg dipole operator amplitudes
are highly suppressed. CP violation measurements can therefore discriminate
between new contributions to the dipole and four quark operators. SU(3)_F
violation in QCD penguin amplitudes can easily be O}(1), in general, due to
annihilation. Implications for B->Rho K^* polarization and New Physics searches
are pointed out. | hep-ph |
Four-Quark Mesons: The features of a model interpreting the light scalar mesons as
diquark-antidiquark bound states and the consequences of its natural extension
to include heavy quarks are briefly reviewed. | hep-ph |
Hadrons on the worldline, holography, and Wilson flow: Holographic principles have impacted the way we look at strong coupling
phenomena in quantum chromodynamics, strongly interacting extensions of the
standard model, and {condensed-matter} physics. In real world settings,
however, we still lack understanding of why and when such an approach is
justified. Therefore, here, without invoking any such principle a priori, we
demonstrate how such a picture arises in the worldline formulation of quantum
field theory. Among other connections to holographic models, a warped AdS5
geometry, a quantum mechanical picture, and hidden local symmetry emerge, as
well as a Wilson flow (gradient flow), which extends the four-dimensional
sources to five-dimensional fields and a link to the Gutzwiller trace formula.
The worldline formulation also reproduces the non-relativistic case, which is
important for condensed-matter physics. | hep-ph |
Reconciling sterile neutrinos with big bang nucleosynthesis: We re-examine the big bang nucleosynthesis (BBN) bounds on the mixing of
neutrinos with sterile species. These bounds depend on the assumption that the
relic neutrino asymmetry $L_{\nu}$ is very small. We show that for $L_{\nu}$
large enough (greater than about $10^{-5}$) the standard BBN bounds do not
apply. We apply this result to the sterile neutrino solution to the atmospheric
neutrino anomaly and show that for $L_{\nu} > 7 \times 10^{-5}$ it is
consistent with BBN. The BBN bounds on sterile neutrinos mixing with electron
neutrinos can also be weakened considerably. | hep-ph |
Discovery potential of the next-to-minimal supergravity-motivated model: Applying a likelihood analysis to the next-to-minimal supergravity-motivated
model, we identify parameter space regions preferred by present experimental
limits from collider, astrophysical, and low energy measurements. We then show
that favored regions are amenable to detection by a combination of the CERN
Large Hadron Collider and an upgraded Cryogenic Dark Matter Search, provided
that the more than three sigma discrepancy in the difference of the
experimental and the standard theoretical values of the anomalous magnetic
moment of the muon prevails in the future. | hep-ph |
Amplitude Factorization in the Electroweak Standard Model: We lay out the basis of factorization at the amplitude level for processes
involving the entire Standard Model. The factorization appears in a generalized
eikonal approximation in which we expand around a quasi-soft limit for massive
gauge bosons, fermions, and scalars. We use the chirality-flow formalism
together with a flow basis for isospin to express loop exchanges or emissions
as operators in chirality and isospin flow. This forms the basis for amplitude
evolution with parton exchange and branching in the full Standard Model,
including the electroweak sector. | hep-ph |
An experiment to detect gravity at sub-mm scale with high-Q mechanical
oscillators: Silicon double paddle oscillators are well suited for the detection of weak
forces because of their high Q factor (about 10^5 at room temperature). We
describe an experiment aimed at the detection of gravitational forces between
masses at sub-mm distance using such an oscillator. Gravitational excitation is
produced by a rotating aluminium disk with platinum segments. The force
sensitivity of this apparatus is about 10 fN at room temperature for 1000 s
averaging time at room temperature. The current limitations to detection of the
gravitational force are mentioned. | hep-ph |
Vector-like quarks with non-renormalizable interactions: We study the impact of the leading non-renormalizable terms in the effective
field theory that describes general extensions of the Standard Model with
vector-like quarks. Dropping the usual assumption of renormalizability has
several phenomenological consequences for the production and decay of the heavy
quarks and also for Higgs physics. The most dramatic effects, including those
associated with a long lifetime, occur for vector-like quarks with non-standard
quantum numbers. | hep-ph |
Sfermion masses in the supersymmetric economical 3-3-1 model: Sfermion masses and eigenstates in the supersymmetric economical 3-3-1 model
are studied. By lepton number conservation, the exotic squarks and
superpartners of ordinary quarks are decoupled. Due to the fact that in the
3-3-1 models, one generation of quarks behaves differently from other two, by
R-parity conservation, the mass mixing matrix of the squarks in this model are
smaller than that in the Minimal Supersymmetric Standard Model (MSSM). Assuming
substantial mixing in pairs of highest flavours, we are able to get mass
spectrum and eigenstates of all the sfermions. In the effective approximation,
the slepton mass splittings in the first two generations, are consistent with
those in the MSSM, namely: $ m^2_{\tilde{l}_L} - m^2_{\tilde{\nu}_{l L}} =
m_W^2 \cos 2\ga$ $(l=e, \mu)$. In addition, within the above effective limit,
there exists degeneracy among sneutrinos in each multiplet:
$m^2_{\tilde{\nu}_{l L}} = m^2_{\tilde{\nu}_{l R}}$. In contradiction to the
MSSM, the squark mass splittings are different for each generation and not to
be $ m_W^2 \cos 2\ga$. | hep-ph |
Weak interaction corrections to hadronic top quark pair production:
contributions from quark-gluon and $b \bar b$ induced reactions: As an addendum to our previous evaluation of the weak-interaction corrections
to hadronic top-quark pair production we determine the leading weak-interaction
contributions due to the subprocesses $b {\bar b} \to t {\bar t}$ and $g q
({\bar q}) \to t {\bar t} q ({\bar q})$. For several distributions in $t {\bar
t}$ production at the LHC we find that these contributions are non-negligible
as compared to the weak corrections from the other partonic subprocesses. | hep-ph |
Low Energy Theorems of Hidden Local Symmetries: We prove to all orders of the loop expansion the low energy theorems of
hidden local symmetries in four-dimensional nonlinear sigma models based on the
coset space $G/H$, with $G$ and $H$ being arbitrary compact groups. Although
the models are non-renormalizable, the proof is done in an analogous manner to
the renormalization proof of gauge theories and two-dimensional nonlinear sigma
models by restricting ourselves to the operators with two derivatives (counting
a hidden gauge boson field as one derivative), i.e., with dimension 2, which
are the only operators relevant to the low energy limit. Through loop-wise
mathematical induction based on the Ward-Takahashi identity for the BRS
symmetry, we solve renormalization equation for the effective action up to
dimension-2 terms plus terms with the relevant BRS sources. We then show that
all the quantum corrections to the dimension-2 operators, including the finite
parts as well as the divergent ones, can be entirely absorbed into a
re-definition (renormalization) of the parameters and the fields in the
dimension-2 part of the tree-level Lagrangian. | hep-ph |
Neutrino quantum states and spin light in matter: On the basis of the exact solutions of the modified Dirac equation for a
massive neutrino moving in matter we develop the quantum theory of the spin
light of neutrino ($SL\nu$). The expression for the emitted photon energy is
derived as a function of the density of matter for different matter
compositions. The dependence of the photon energy on the helicities of the
initial and final neutrino states is shown explicitly. The rate and radiation
power of the $SL\nu$ in matter are obtained with the emitted photon linear and
circular polarizations being accounted for. The developed quantum approach to
the $SL\nu$ in matter (which is similar to the Furry representation of
electrodynamics) can be used in the studies of other processes with neutrinos
in the presence of matter | hep-ph |
Implication of the B -> rho rho data on the B -> pi pi puzzle: We point out that the B -> rho rho data have seriously constrained the
possibility of resolving the B -> pi pi puzzle from the large observed B^0 ->
pi^0 pi^0 branching ratio in the available theoretical approaches. The
next-to-leading-order (NLO) contributions from the vertex corrections, the
quark loops, and the magnetic penguin evaluated in the perturbative QCD (PQCD)
approach have saturated the experimental upper bound of the B^0 -> rho^0 rho^0
branching ratio, and do not help. The NLO PQCD predictions for the B^0 ->
rho^\mp rho^\pm and B^\pm -> rho^\pm rho^0 branching ratios are consistent with
the data. The inclusion of the NLO jet function from the soft-collinear
effective theory into the QCD-improved factorization approach, though enhancing
the B^0 -> pi^0 pi^0 branching ratio sufficiently, overshoots the bound of the
B^0 -> rho^0 rho^0 branching ratio, and deteriorates the predictions for the
B^\pm -> pi^0 K^\pm and B^0 -> pi^\mp K^\pm direct CP asymmetries. | hep-ph |
Self-consistent statistical error analysis of $ππ$ scattering: We analyze the conditions under which a statistical error analysis can be
carried out in the case of $\pi\pi$ scattering, namely the normality of
residuals in the conventional $\chi^2$-fit method. Here we check that the
current and benchmarking analyses only present very small violations of the
normality requirements. In particular, we show how it is possible to amend
slightly the selection of the experimental data, and improve the normality of
residuals.
As an example, we discuss the $0^{++}$ channel and the implications for the
$f_0(500)$ and $f_0(980)$ resonances, obtaining that the new selection of data
provides very similar and compatible results. In addition, the effect on the
$f_0(500)$ and $f_0(980)$ resonance pole parameters is almost negligible, which
reinforces the central results and the uncertainty analysis performed in these
benchmarking determinations. | hep-ph |
Quantum corrections to leptogenesis from the gradient expansion: Using the closed-time-path formalism we quantify gradient corrections to the
kinetic equations for leptogenesis, that are neglected in the standard
Boltzmann approach. In particular we show that an additional CP-violating
source term arises, which is non-zero even when all species are in local
thermal equilibrium. In the early universe it is proportional to the expansion
rate and would vanish for static equilibrium configurations, in accordance with
the Sakharov conditions. We find that for thermal leptogenesis in a standard
cosmological background the additional source term is small. However, it can
become the dominant source in the limit of ultra-strong washout. | hep-ph |
Matter-gravity interaction in a multiply warped braneworld,: The role of a bulk graviton in predicting the signature of extra dimensions
through collider-based experiments is explored in the context of a multiply
warped spacetime. In particular it is shown that in a doubly warped braneworld
model, the presence of the sixth dimension, results in enhanced concentration
of graviton Kaluza Klein (KK) modes compared to that obtained in the usual
5-dimensional Randall-Sundrum model. Also, the couplings of these massive
graviton KK modes with the matter fields on the visible brane turn out to be
appreciably larger than that in the corresponding 5- dimensional model. The
significance of these results are discussed in the context of KK graviton
search at the Large Hadron Collider (LHC). | hep-ph |
Transversity Distribution Functions in the Valon Model: By using the valon model, we calculate the Transverse Momentum Distribution
functions, TMDs, inside the Nucleon. TMDs indicate the probability to find
partons with spin aligned (anti- aligned) to the transversely polarized
nucleon. The results are in good agreements with all available experimental
data and also global fits. | hep-ph |
Do W_L and H form a p-wave bound state?: We examine the possibility of bound state formation in the W_L H --> W_L H
channel. The dynamical calculation using the N/D method indicates that when the
interactions among the Goldstone and Higgs bosons become sufficiently strong, a
p-wave state [I^G(J^P)=1^-(1^+)] may emerge. | hep-ph |
Gribov's Equation for the Green Function of Light Quarks: Gribov's scenario of supercritical charges in QCD is investigated. We perform
a numerical study of the corresponding equation for the Green function of light
quarks. This is done in an approximation which neglects all pion contributions.
Different types of solutions in the Euclidean region are discussed and the mass
function of the quark is calculated. The solutions of the equation are shown to
have a qualitatively different behaviour if the strong coupling constant
alpha_s exceeds a critical value alpha_c = 0.43 in the infrared region. Chiral
symmetry breaking is found to occur at supercritical coupling. The analytic
structure of the solutions is investigated. Earlier results obtained by Gribov
are confirmed and extended. | hep-ph |
Supersymmetric D-term Inflation, Reheating and Affleck-Dine Baryogenesis: The phenomenology of supersymmetric models of inflation, where the
inflationary vacuum energy is dominated by D-terms of a U(1), is investigated.
Particular attention is paid to the questions of how to arrange for sufficient
e-folds of inflation to occur, what kind of thermal history is expected after
the end of inflation, and how to implement successful baryogenesis. Such models
are argued to require a more restrictive symmetry structure than previously
thought. In particular, it is non-trivial that the decays of the fields driving
D-inflation can reheat the universe in such a way as to avoid the strong
gravitino production constraints. We also show how the initial conditions for
Affleck-Dine baryogenesis can arise in these models and that the simplest flat
directions along which baryon number is generated can often be ruled out by the
constraints coming from decoherence of the condensate in a hot environment. At
the end, we find that successful reheating and baryogenesis can take place in a
large subset of D-inflationary models. | hep-ph |
Covariant Description of Flavor Conversion in the LHC Era: A simple covariant formalism to describe flavor and CP violation in the
left-handed quark sector in a model independent way is provided. The
introduction of a covariant basis, which makes the standard model approximate
symmetry structure manifest, leads to a physical and transparent picture of
flavor conversion processes. Our method is particularly useful to derive robust
bounds on models with arbitrary mechanisms of alignment. Known constraints on
flavor violation in the K and D systems are reproduced in a straightforward
manner. Assumptions-free limits, based on top flavor violation at the LHC, are
then obtained. In the absence of signal, with 100 fb^{-1} of data, the LHC will
exclude weakly coupled (strongly coupled) new physics up to a scale of 0.6 TeV
(7.6 TeV), while at present no general constraint can be set related to Delta
t=1 processes. LHC data will constrain Delta F=2 contributions via same-sign
tops signal, with a model independent exclusion region of 0.08 TeV (1.0 TeV).
However, in this case, stronger bounds are found from the study of CP violation
in D-bar D mixing with a scale of 0.57 TeV (7.2 TeV). In addition, we apply our
analysis to models of supersymmetry and warped extra dimension. The minimal
flavor violation framework is also discussed, where the formalism allows to
distinguish between the linear and generic non-linear limits within this class
of models. | hep-ph |
Advantages of exclusive γγproduction to probe high mass
systems: We recall that the exclusive production of high mass objects via \gamma\gamma
fusion at the LHC is not strongly suppressed in comparison with inclusive
\gamma\gamma fusion. Therefore it may be promising to study new objects
produced by the \gamma\gamma subprocess in experiments with exclusive
kinematics. We list the main advantages of exclusive experiments. We discuss
the special advantage of observing $\gamma\gamma \to X \to \gamma Z$ exclusive
events. | hep-ph |
Top quark anomalous tensor couplings in the two-Higgs-doublet models: We compute the one loop right and left anomalous tensor couplings ($g_R$ and
$g_L$, respectively) for the top quark, in the aligned two-Higgs-doublet model.
They are the magnetic-like couplings in the most general parameterization of
the $tbW$ vertex. We find that the aligned two-Higgs doublet model, that
includes as particular cases some of the most studied extensions of the Higgs
sector, introduces new electroweak contributions and provides theoretical
predictions that are very sensitive to both new scalar masses and the neutral
scalar mixing angle. For a large area in the parameters space we obtain
significant deviations in both the real and the imaginary parts of the
couplings $g_R$ and $g_L$, compared to the predictions given by the electroweak
sector of the Standard Model. The most important ones are those involving the
imaginary part of the left coupling $g_{L}$ and the real part of the right
coupling $g_R$. The real part of $g_L$ and the imaginary part of $g_R$ also
show an important sensitivity to new physics scenarios. The model can also
account for new CP violation effects via the introduction of complex alignment
parameters that have important consequences on the values for the imaginary
parts of the couplings. The top anomalous tensor couplings will be measured at
the LHC and at future colliders providing a complementary insight on new
physics, independent from the bounds in top decays coming from B physics and $b
\rightarrow s \gamma$. | hep-ph |
Neutralino Dark Matter with Inert Higgsinos and Singlinos: We discuss neutralino dark matter arising from supersymmetric models with
extra inert Higgsinos and singlinos, where inert means that their scalar
partners do not get vacuum expectation values. As an example, we consider the
extended neutralino sector of the E$_6$SSM, which predicts three families of
Higgs doublet pairs, plus three singlets, plus a $Z'$, together with their
fermionic superpartners. We show that the two families of inert doublet
Higgsinos and singlinos predicted by this model provide an almost decoupled
neutralino sector with a naturally light LSP which can account for the cold
dark matter relic abundance independently of the rest of the model, providing
that the ratio of the two usual Higgs doublets satisfies tan beta < 2. | hep-ph |
Lambda Oscillations and the Conservation Laws: Lowe, Bassalleck, Burkhardt, Rusek, Stephenson, and Goldman assert, under the
assumption of decays at fixed space-time points, that Kaon induced Lambda
oscillations disappear. We find, under the same assumption, that energy
conservation and momentum conservation also disappear. Ordinary particles can
exhibit quantum oscillations. | hep-ph |
Vacuum energy and the cosmological constant: The accelerating expansion of the Universe points to a small positive value
for the cosmological constant or vacuum energy density. We discuss recent ideas
that the cosmological constant plus LHC results might hint at critical
phenomena near the Planck scale. | hep-ph |
A description of the ratio between electric and magnetic proton form
factors by using space-like, time-like data and dispersion relations: We use the available information on the ratio between the electric and
magnetic proton form factors coming from recently published space-like data and
from the few available time-like data. We apply a dispersive procedure on these
data to evaluate the behaviour of this ratio, as a complex function, for all
values of q^2. | hep-ph |
Nonuniversality of indirect CP asymmetries in $D \to ππ, KK$ decays: We point out that, if the direct CP asymmetries in the $D \to \pi^+ \pi^-$
and $D \to K^+ K^-$ decays are unequal, the indirect CP asymmetries as measured
in these modes are necessarily unequal. This nonuniversality of indirect CP
asymmetries can be significant with the right amount of new physics
contributions, a scenario that may be fine-tuned, but is still viable. A
model-independent fit to the current data allows different indirect CP
asymmetries in the above two decays. This could even be contributing to the
apparent tension between the difference CP asymmetries $\Delta A_{\rm CP}$
measured through the pion-tagged and muon-tagged data samples at the LHCb. This
also implies that the measurements of $A_\Gamma$ and $y_{\rm CP}$ in the $\pi^+
\pi^-$ and $K^+ K^-$ decay modes can be different, and averaging over these two
modes should be avoided. In any case, the complete analysis of CP violation
measurements in the $D$ meson sector needs to take into account the possibility
of different indirect CP asymmetries in the $\pi^+\pi^-$ and $K^+ K^-$
channels. | hep-ph |
Inclusive electromagnetic decays of the heavy quarkonium at next to
leading log accuracy: We show that perturbation theory may give reasonable numbers for the decays
of the bottomonium and charmonium ground states to $e^+e^-$ and to
$\gamma\gamma$. To reach this conclusion it is important to perform the
resummation of logs. In particular, we obtain the value $\Gamma(\eta_b (1S) \to
\gamma\gamma)=0.35 \pm 0.1 ({\rm th.}) \pm 0.05 (\lQ)$ KeV. | hep-ph |
Inverse Problem of Cosmic-Ray Electron/Positron from Dark Matter: We discuss the possibility of solving the inverse problem of the cosmic-ray
electron/positron from decaying/annihilating dark matter, and show simple
analytic formulae to reconstruct the source spectrum of the electron/positron
from the observed flux. We also illustrate our approach by applying the
obtained formula to the just released Fermi data as well as the new HESS data. | hep-ph |
Higgs Physics at the LHC: Some Theory Aspects: In these lecture notes we review some prospect for the upcoming LHC
experiments in view of the exploration of the Standard Model (SM) or its
minimal Supersymmetric extension (MSSM). We focus on some theoretical aspects
concerning the Higgs sector of the two models. We give results for the
precision observables M_W and m_t and their impact on the indirect
determination of the Higgs sector. We furthermore review some prospects for the
direct measurements in the SM and MSSM Higgs sector. | hep-ph |
Hadronic Higgs boson decay at order $α_s^4$ and $α_s^5$: We compute corrections to the decay of the Standard Model Higgs boson to
hadrons, to the fourth order in the strong coupling constant $\alpha_s$. We use
an effective theory in which the Higgs boson couples directly to bottom quarks
and to gluons, via top quark--mediated effective couplings. Numerically, our
results are of a comparable size to the previously-known "massless"
contributions and complete the order $\alpha_s^4$ corrections to the hadronic
decay of the Higgs boson. In these proceedings we also provide an independent
cross check of the gluonic Higgs boson decay at order $\alpha_s^5$. | hep-ph |
Hadronic Light-by-light Scattering Effect on Muon g-2: The hadronic light-by-light scattering contribution to muon $g-2$ is examined
using low energy effective theories of QCD, the Nambu-Jona-Lasinio model and
hidden local chiral symmetry, as guides. Our result is $- 36 \times 10^{-11}$
with an uncertainty of $\pm 16 \times 10^{-11}$, which includes our best
estimate of model dependence. This is within the expected measurement
uncertainty of $40\times 10^{-11}$ in the forthcoming experiment at Brookhaven
National Laboratory. Our result removes one of the main theoretical obstacles
in verifying the existence of the weak contribution to the muon $g-2$. | hep-ph |
CP violation in bilinear R-parity violation and its consequences for the
early universe: Supersymmetric models with bilinear R-parity violation (BRpV) provide a
framework for neutrino masses and mixing angles to explain neutrino oscillation
data. We consider CP violation within the new physical phases in BRpV and
discuss their effect on the generation of neutrino masses and the decays of the
lightest supersymmetric particle (LSP), being a light neutralino with mass
$\sim 100$ GeV, at next-to-leading order. The decays affect the lepton and via
sphaleron transitions the baryon asymmetry in the early universe. For a rather
light LSP, asymmetries generated before the electroweak phase transition via
e.g. the Affleck-Dine mechanism are reduced up to two orders of magnitude, but
are still present. On the other hand, the decays of a light LSP themselves can
account for the generation of a lepton and baryon asymmetry, the latter in
accordance to the observation in our universe, since the smallness of the BRpV
parameters allows for an out-of-equilibrium decay and sufficiently large CP
violation is possible consistent with experimental bounds from the
non-observation of electric dipole-moments. | hep-ph |
Vector gauge boson radiation from compact binary systems in a gauged
$L_μ-L_τ$ scenario: The orbital period of a compact binary system decays mainly due to quadrupole
gravitational radiation, which agrees with the observation to within one
percent. Other types of radiation such as ultralight scalar or pseudoscalar
radiation, massive vector boson radiation also contribute to the decay of
orbital period as long as the mass of the emitted particle is less than the
orbital frequency of the compact binary system. We obtain an expression of the
energy loss due to the radiation of massive vector field from the neutron
star-neutron star and neutron star-white dwarf binaries. Due to large chemical
potential of the degenerate electrons, neutron stars have large muon charge. We
derive the energy loss due to $U(1)_{L_\mu-L_\tau}$ gauge boson radiation from
the binaries. For the radiation of vector boson, the mass is restricted by
$M_{Z^\prime}<\Omega \simeq10^{-19}eV$ are the orbital frequencies of the
compact star binaries. Using the formula of orbital period decay, we obtain
constraints on the coupling constant of the gauge boson in the gauged
$L_\mu-L_\tau$ theory for the four compact binary systems. For vector gauge
boson muon coupling we find that for $M_{Z^\prime}<10^{-19}eV$, constraints on
the coupling constant is $g<\mathcal{O}(10^{-20})$. We also obtain the
exclusion plots of the massive vector proca field and the gauge field which can
couple to muons. | hep-ph |
Relic Abundance of Neutralinos in Heterotic String Theory: Weak Coupling
vs. Strong Coupling: The relic abundance of stable neutralinos is investigated in $E_8 \times
E_8'$ heterotic string theory when supersymmetry is spontaneously broken by
hidden-sector gaugino condensates. In the weak coupling regime, very large
scalar masses (compared to gaugino masses) are shown to lead to a too large
relic abundance of the neutralinos, incompatible with cosmological observations
in most of parameter space. The problem does not arise in the strong coupling
regime (heterotic M-theory) because there scalar and gaugino masses are
generically of the same order of magnitude. | hep-ph |
The Hunting of the MR Model: We consider experimental signatures of the standard model's minimal
supersymmetric extension with a continuous $U(1)_R$ symmetry (MR model). We
focus on the ability of existing and planned electron-positron colliders to
probe this model and to distinguish it from both the standard model and the
standard model's minimal supersymmetric extension with a discrete $R$-parity. | hep-ph |
Heavy neutral 2HDM Higgs Boson Pair Production at CLIC Energies: In this work, the neutral Higgs boson pair production is analyzed at Compact
Linear Collider (CLIC) to be operating at center of mass energies
$\sqrt{s}$=1400 GeV (stage 2) and $\sqrt{s}$=3000 GeV (stage 3). The Higgs
bosons to be searched for are neutral CP-even (H) and CP-odd (A) within the
framework of two Higgs doublet model (2HDM) in the mass range 300 < mH/A < 1000
GeV. All types of the CP-conserving model are studied and the signal
observability is evaluated taking into account the main SM background processes
like ZZ, tt and the SM-like Higgs boson associated production (hZ). Results are
presented for a set of model parameters and Higgs boson masses in terms of
signal distributions over the background as well as the integrated luminosity
needed for 5{\sigma} discovery. It is shown that the heavy mass region is well
observable at CLIC in types 3 (flipped) and 4 (lepton-specific) in the regions
not excluded by LHC so far, while in type 1 the signal observation is
challenging due to the large jet multiplicity in the tt final state. | hep-ph |
CP violation and limits on New Physics including recent $B_s$
measurements: We analyse present constraints on the SM parameter space and derive, in a
model independent way, various bounds on New Physics contributions to
$B_d^0$--$\bar B_d^0$ and $B_s^0$--$\bar B_s^0$ mixings. Our analyses include
information on a large set of asymmetries, leading to the measurement of the
CKM phases $\gamma$ and $\bar\beta$, as well as recent data from D0 and CDF
related to the $B_s^0$--$\bar B_s^0$ system such as the measurement of $\Delta
M_{B_s}$, $A_{SL}$ and $\Delta\Gamma_{s}^{CP}$. We examine in detail several
observables such as the asymmetries $A_{sl}^d$, $A_{SL}$, the width differences
$\Delta\Gamma_{d}$ and $\Delta\Gamma_{s}^{CP}$ and discuss the r\^ole they play
in establishing the limits on New Physics. The present data clearly favour the
SM, with the New Physics favoured region placed around the SM solution. A New
Physics solution significantly different from the SM is still allowed, albeit
quite disfavoured (2.6% probability). We analyse the presently available
indirect knowledge on the phase $\bar\chi$ entering in $B_s^0$--$\bar B_s^0$
mixing and study the impact of a future measurement of $\bar\chi$ to be
achieved at LHC, through the measurement of the time-dependent CP asymmetry in
$B_s\to J/\Psi \Phi$ decays. | hep-ph |
The Fundamental Constants in Physics: We discuss the fundamental constants of physics in the Standard Model and
possible changes of these constants on the cosmological time scale. The Grand
Unification of the strong, electromagnetic and weak interactions implies
relations between the time variation of the finestructure constant and of the
QCD scale. An experiment in quantum optics at the MPQ in Munich, which was
designed to look for a time variation of the QCD scale, is discussed. | hep-ph |
Asymmetric Dark Matter via Spontaneous Co-Genesis: We investigate, in the context of asymmetric dark matter (DM), a new
mechanism of spontaneous co-genesis of linked DM and baryon asymmetries,
explaining the observed relation between the baryon and DM densities,
Omega_DM/Omega_B ~ 5. The co-genesis mechanism requires a light scalar field,
phi, with mass below 5 eV which couples derivatively to DM, much like a `dark
axion'. The field phi can itself provide a final state into which the residual
symmetric DM component can annihilate away. | hep-ph |
Connection between diphoton and triboson channels in new physics
searches: Diphoton channel provides a clean signature in searches for new physics. In
this paper, we discuss a connection between the diphoton channel
($\gamma\gamma$) and triboson channels ($Z\gamma\gamma$, $ZZ\gamma$,
$WW\gamma$) imposed by the $SU(2)_{L}\times U(1)_{Y}$ symmetry of the Standard
Model (SM) in certain classes of models. To illustrate this idea we choose a
simple model that has all these channels. In this model, the same physics can
give rise to $\gamma+$MET instead of $\gamma\gamma$ and 2 bosons plus missing
energy instead of 3-boson channels. We analyze existing constraints and
previous searches and show that channels $WW\gamma$ and especially
$Z\gamma+$MET have a potential to discover new physics at the LHC. | hep-ph |
Universality of Nonperturbative Effects in Event Shapes: Nonperturbative effects in event shape distributions can be characterized by
shape functions derived in the eikonal approximation or, equivalently, from
soft-collinear effective theory. The use of energy flow operators and the boost
invariance of the Wilson lines of soft gluons in the shape functions leads to a
proof of universality for power corrections to the mean values of event shapes,
without invoking the single gluon approximation. | hep-ph |
Cornering pseudoscalar-mediated dark matter with the LHC and cosmology: Models in which dark matter particles communicate with the visible sector
through a pseudoscalar mediator are well-motivated both from a theoretical and
from a phenomenological standpoint. With direct detection bounds being
typically subleading in such scenarios, the main constraints stem either from
collider searches for dark matter, or from indirect detection experiments.
However, LHC searches for the mediator particles themselves can not only
compete with -- or even supersede -- the reach of direct collider dark matter
probes, but they can also test scenarios in which traditional monojet searches
become irrelevant, especially when the mediator cannot decay on-shell into dark
matter particles or its decay is suppressed. In this work we perform a detailed
analysis of a pseudoscalar-mediated dark matter simplified model, taking into
account a large set of collider constraints and concentrating on the parameter
space regions favoured by cosmological and astrophysical data. We find that
mediator masses above 100-200~GeV are essentially excluded by LHC searches in
the case of large couplings to the top quark, while forthcoming collider and
astrophysical measurements will further constrain the available parameter
space. | hep-ph |
Exclusive meson pair production in gamma* gamma scattering at small
momentum transfer: We study the exclusive production of pi pi and rho pi in hard gamma* gamma
scattering in the forward kinematical region where the virtuality of one photon
provides us with a hard scale in the process. The newly introduced concept of
Transition Distribution Amplitudes (TDA) is used to perform a QCD calculation
of these reactions thanks to two simple models for TDAs. Cross sections for rho
pi and pi pi production are evaluated and compared to the possible background
from the Bremsstrahlung process. This picture may be tested at intense
electron-positron colliders such as CLEO and B factories. The cross section e
gamma -> e' pi0 pi0 is finally shown to provide a possible determination of the
pi0 axial form factor, FA, at small t, which seems not to be measurable
elsewhere. | hep-ph |
CP violation in the semileptonic top decay in two-Higgs doublet model: CP violation in semileptonic top-quark decay is investigated by exactly using
one charged- and three neutral-Higgs bosons obtained by solving the Higgs mass
matrix in two-Higgs doublet model. The CP-violating up-down asymmetry of
leptons from $W$ boson decays is shown to be $1 \times 10^{-4} - 4 \times
10^{-3}$ for the region of ${\rm tan} \beta \ll 1$, where ${\rm tan} \beta $ is
the ratio of vacuum expectation values for the two neutral Higgs bosons. | hep-ph |
Coherent photoproduction of low-$p_{T}$ charmonium in peripheral heavy
ion collisions within the color dipole model: We calculate the centrality dependence for coherent photoproduction of very
low-$p_{T}$ $J/\psi$ at Relativistic Heavy Ion Collider (RHIC) and Large Hadron
Collider (LHC) energies within the impact parameter dependent saturated color
dipole model. By using the large equivalent photon fluxes, we present the
differential cross section of very low-$p_{T}$ $J/\psi$ produced by coherent
photonuclear in peripheral heavy-ion collisions. The numerical results
demonstrate that our calculation are agree with $J/\psi$ data in peripheral
heavy ion collisions at Relativistic Heavy Ion Collider (RHIC) energies. | hep-ph |
Magnetic Inelastic Dark Matter: Iodine is distinguished from other elements used in dark matter direct
detection experiments both by its large mass as well as its large magnetic
moment. Inelastic dark matter utilizes the large mass of iodine to allay
tensions between the DAMA annual modulation signature and the null results from
other experiments. We explore models of inelastic dark matter that also take
advantage of the second distinct property of iodine, namely its large magnetic
moment. In such models the couplings are augmented by magnetic, rather than
merely electric, interactions. These models provide simple examples where the
DAMA signal is compatible with all existing limits. We consider dipole moments
for the WIMP, through conventional magnetism as well as "dark" magnetism,
including both magnetic-magnetic and magnetic-electric scattering. We find
XENON100 and CRESST should generically see a signal, although suppressed
compared with electric inelastic dark matter models, while KIMS should see a
modulated signal comparable to or larger than that of DAMA. In a large portion
of parameter space, de-excitation occurs promptly, producing a ~ 100 keV photon
inside large xenon experiments alongside the nuclear recoil. This effect could
be searched for, but if not properly considered may cause nuclear recoil events
to fail standard cuts. | hep-ph |
From R_AA via correlations to jets - the long road to tomography: The main motivation to investigate hard probes in heavy ion collisions is to
do tomography, i.e. to infer medium properties from the in-medium modification
of hard processes. Yet while the suppression of high P_T hadrons has been
measured for some time, solid tomographic information is slow to emerge. This
can be traced back to theoretical uncertainties and ambiguities in modelling
both medium evolution and parton-medium interaction. Ways to overcome these
difficulties are to constrain models better and to focus on more differential
observables. Correlations of high P_T hadrons offer non-trivial information
beyond what can be deduced from single hadron suppression. They reflect not
only the hard reaction being modified by the medium, but also the back reaction
of the medium to the hard probe. Models for hard back-to-back correlations are
now very well constrained by a wealth of data and allow insights into the
nature of the parton-medium interaction as well as first true tomographic
results. Models of full in-medium jet evolution are being actively developed,
but have yet to make substantial contact with data. Progress is slower in the
understanding of low P_T correlations, the ridge and the cone, although a
qualitative understanding of the nature of the physics behind these
correlations starts to emerge. | hep-ph |
Bayesian Selection of sign(mu) within mSUGRA in Global Fits Including
WMAP5 Results: We study the properties of the constrained minimal supersymmetric standard
model (mSUGRA) by performing fits to updated indirect data, including the relic
density of dark matter inferred from WMAP5. In order to find the extent to
which mu < 0 is disfavoured compared to mu > 0, we compare the Bayesian
evidence values for these models, which we obtain straightforwardly and with
good precision from the recently developed multi-modal nested sampling
('MultiNest') technique. We find weak to moderate evidence for the mu > 0
branch of mSUGRA over mu < 0 and estimate the ratio of probabilities to be P(mu
> 0)/P(mu < 0) = 6-61 depending on the prior measure and range used. There is
thus positive (but not overwhelming) evidence that mu > 0 in mSUGRA. The
MultiNest technique also delivers probability distributions of parameters and
other relevant quantities such as superpartner masses. We explore the
dependence of our results on the choice of the prior measure used. We also use
the Bayesian evidence to quantify the consistency between the mSUGRA parameter
inferences coming from the constraints that have the largest effects: (g-2)_mu,
BR(b -> s gamma) and cold dark matter (DM) relic density Omega_{DM}h^2. | hep-ph |
(Light) Stop Signs: Stop squarks with a mass just above the top's and which decay to a nearly
massless LSP are difficult to probe because of the large SM di-top background.
Here we discuss search strategies which could be used to set more stringent
bounds in this difficult region. In particular, we note that both the rapidity
difference Delta y(t,tbar) and spin correlations (inferred from, for example,
Delta phi(l+,l-)) are sensitive to the presence of stops. We emphasize that
systematic uncertainties in top quark production can confound analyses looking
for stops, making theoretical and experimental progress on the understanding of
Standard Model top production at high precision a very important task. We
estimate that spin correlation alone, which is relatively robust against such
systematic uncertainties, can exclude a 200 GeV stop at 95% confidence with 20
fb^-1 at the 8 TeV LHC. | hep-ph |
Corrections to the ${\bf SU(3)\times SU(3)}$ Gell-Mann-Oakes-Renner
relation and chiral couplings $L^r_8$ and $H^r_2$: Next to leading order corrections to the $SU(3) \times SU(3)$
Gell-Mann-Oakes-Renner relation (GMOR) are obtained using weighted QCD Finite
Energy Sum Rules (FESR) involving the pseudoscalar current correlator. Two
types of integration kernels in the FESR are used to suppress the contribution
of the kaon radial excitations to the hadronic spectral function, one with
local and the other with global constraints. The result for the pseudoscalar
current correlator at zero momentum is $\psi_5(0) = (2.8 \pm 0.3) \times
10^{-3} GeV^{4}$, leading to the chiral corrections to GMOR: $\delta_K = (55
\pm 5)%$. The resulting uncertainties are mostly due to variations in the upper
limit of integration in the FESR, within the stability regions, and to a much
lesser extent due to the uncertainties in the strong coupling and the strange
quark mass. Higher order quark mass corrections, vacuum condensates, and the
hadronic resonance sector play a negligible role in this determination. These
results confirm an independent determination from chiral perturbation theory
giving also very large corrections, i.e. roughly an order of magnitude larger
than the corresponding corrections in chiral $SU(2) \times SU(2)$. Combining
these results with our previous determination of the corrections to GMOR in
chiral $SU(2) \times SU(2)$, $\delta_\pi$, we are able to determine two low
energy constants of chiral perturbation theory, i.e. $L^r_8 = (1.0 \pm 0.3)
\times 10^{-3}$, and $H^r_2 = - (4.7 \pm 0.6) \times 10^{-3}$, both at the
scale of the $\rho$-meson mass. | hep-ph |
Dark Matters in Gauged B-3L_i Model: We study a dark matter model with local B-3L_i symmetry that is known as
anomaly free and requires a single right-handed neutrino. Here we have two dark
matter candidates; that is, fermionic or bosonic one. We focus on analyzing
each of the case within the light mass region, which is required by the
perturbative theory of the Higgs quartic coupling. | hep-ph |
Resolved Photon Processes: We review the present level of knowledge of the hadronic structure of the
photon, as revealed in interactions involving quarks and gluons ``in" the
photon. The concept of photon structure functions is introduced in the
description of deep--inelastic $e \gamma$ scattering, and existing
parametrizations of the parton densities in the photon are reviewed. We then
turn to hard \gamp\ and \gaga\ collisions, where we treat the production of
jets, heavy quarks, hard (direct) photons, \jpsi\ mesons, and lepton pairs. We
also comment on issues that go beyond perturbation theory, including recent
attempts at a comprehensive description of both hard and soft \gamp\ and \gaga\
interactions. We conclude with a list of open problems. | hep-ph |
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