921 research outputs found
Double charmonium production in exclusive bottomonia decays
This paper is devoted to the leading twist exclusive bottomonia decays with
double charmonium in the final state. Using models of the twist-2 charmonia
distribution amplitudes the widths of these decays are calculated within light
cone formalism. In addition, the processes under consideration are studied
within nonrelativistic QCD. In our analysis we have found that the production
of some of the -wave charmonia mesons with is allowed already
at the leading twist approximation. This means that the selection rules which
predict the suppression of such decays are violated. The mechanism which lies
behind this violation is discussed.Comment: Minor changes, some references adde
Probing Quark Distribution Amplitudes Through Generalized Parton Distributions at Large Momentum Transfer
In the large momentum transfer limit, generalized parton distributions can be
calculated through a QCD factorization theorem which involves
perturbatively-calculable hard kernels and light-cone parton distribution
amplitudes of hadrons. We illustrate this through the
distribution for the pion and proton, presenting the hard kernels at leading
order. As a result, experimental data on the generalized parton distributions
in this regime can be used to determine the functional form of the parton
distribution amplitudes which has thus far been quite challenging to obtain.
Our result can also be used as a constraint in phenomenological GPD
parametrizations.Comment: 7 pages, 4 figures; new references and figure added, errors correcte
Mass Spectrum in SQCD and Problems with the Seiberg Duality. Another Scenario
N=1 SQCD with SU(N_c) colors and N_F flavors of light quarks is considered
within the dynamical scenario which assumes that quarks can be in two different
phases only. These are: a) either the HQ (heavy quark) phase where they are
confined, b) or they are higgsed, at the appropriate values of parameters of
the Lagrangian.
The mass spectra of this (direct) theory and its Seiberg's dual are obtained
and compared, for quarks of equal or unequal masses. It is shown that in all
cases when there is the additional small parameter at hand (it is
0<(3N_c-N_F)/N_F << 1 for the direct theory, or its analog 0<(2N_F-3N_c)/N_F <<
1 for the dual one), the mass spectra of the direct and dual theories are
parametrically different. A number of other regimes are also considered.Comment: 30 pages, purely technical improvements for readers convenienc
Light Cone Sum Rules for the pi0-gamma*-gamma Form Factor Revisited
We provide a theoretical update of the calculations of the pi0-gamma*-gamma
form factor in the LCSR framework, including up to six polynomials in the
conformal expansion of the pion distribution amplitude and taking into account
twist-six corrections related to the photon emission at large distances. The
results are compared with the calculations of the B-> pi l nu decay and pion
electromagnetic form factors in the same framework. Our conclusion is that the
recent BaBar measurements of the pi0-gamma*-gamma form factor at large momentum
transfers are consistent with QCD, although they do suggest that the pion DA
may have more structure than usually assumed.Comment: 20 pages, 14 figures, 5 table
Symmetry Relations for Trajectories of a Brownian Motor
A Brownian Motor is a nanoscale or molecular device that combines the effects
of thermal noise, spatial or temporal asymmetry, and directionless input energy
to drive directed motion. Because of the input energy, Brownian motors function
away from thermodynamic equilibrium and concepts such as linear response
theory, fluctuation dissipation relations, and detailed balance do not apply.
The {\em generalized} fluctuation-dissipation relation, however, states that
even under strongly thermodynamically non-equilibrium conditions the ratio of
the probability of a transition to the probability of the time-reverse of that
transition is the exponent of the change in the internal energy of the system
due to the transition. Here, we derive an extension of the generalized
fluctuation dissipation theorem for a Brownian motor for the ratio between the
probability for the motor to take a forward step and the probability to take a
backward step
Coulomb dissociation of a fast pion into two jets
We calculate the electromagnetic contribution to the scattering amplitude of
pion diffractive dissociation into di-jets which is described by one photon
exchange. The result shows that the factorization procedure known for the
description of exclusive reactions holds also for this quasi-exclusive process.
We find that the longitudinal momentum distribution of di-jets does not depend
on the form of the pion distribution amplitude. We discuss the magnitude of the
cross section.Comment: 7 pages, 3 .eps figures, Late
Operator Relations for SU(3) Breaking Contributions to K and K* Distribution Amplitudes
We derive constraints on the asymmetry a1 of the momentum fractions carried
by quark and antiquark in K and K* mesons in leading twist. These constraints
follow from exact operator identities and relate a1 to SU(3) breaking
quark-antiquark-gluon matrix elements which we determine from QCD sum rules.
Comparing our results to determinations of a1 from QCD sum rules based on
correlation functions of quark currents, we find that, for a1^\parallel(K*) the
central values agree well and come with moderate errors, whereas for a1(K) and
a1^\perp(K*) the results from operator relations are consistent with those from
quark current sum rules, but come with larger uncertainties. The consistency of
results confirms that the QCD sum rule method is indeed suitable for the
calculation of a1. We conclude that the presently most accurate predictions for
a1 come from the direct determination from QCD sum rules based on correlation
functions of quark currents and are given by: a1(K) = 0.06\pm 0.03,
a1^\parallel(K*) = 0.03\pm 0.02, a1^\perp(K*) = 0.04\pm 0.03.Comment: 21 page
Accuracy of the pion elastic form factor extracted from a local-duality sum rule
We analyze the accuracy of the pion elastic form factor predicted by a
local-duality (LD) version of dispersive sum rules. To probe the precision of
this theoretical approach, we adopt potential models with interactions that
involve both Coulomb and confining terms. In this case, the exact form factor
may be obtained from the solution of the Schroedinger equation and confronted
with the LD sum rule results. We use parameter values appropriate for hadron
physics and observe that, independently of the details of the confining
interaction, the deviation of the LD form factor from the exact form factor
culminates in the region Q^2~4-6 GeV^2. For larger Q^2, the accuracy of the LD
description increases rather fast with Q^2. A similar picture is expected for
QCD. For the pion form factor, existing data suggest that the LD limit may be
reached already at the relatively low values Q^2=4-10 GeV^2. Thus, large
deviations of the pion form factor from the behaviour predicted by LD QCD sum
rules for higher values of Q^2, as found by some recent analyses, appear to us
quite improbable. New accurate data on the pion form factor at Q^2=4-10 GeV^2
expected soon from JLab will have important implications for the behaviour of
the pion form factor in a broad Q^2 range up to asymptotically large values of
Q^2.Comment: 12 pages, extended version, conclusions remain unchange
Periodic and Quasi-Periodic Compensation Strategies of Extreme Outages caused by Polarization Mode Dispersion and Amplifier Noise
Effect of birefringent disorder on the Bit Error Rate (BER) in an optical
fiber telecommunication system subject to amplifier noise may lead to extreme
outages, related to anomalously large values of BER. We analyze the Probability
Distribution Function (PDF) of BER for various strategies of Polarization Mode
Dispersion (PMD) compensation. A compensation method is proposed that is
capable of more efficient extreme outages suppression, which leads to
substantial improvement of the fiber system performance.Comment: 3 pages, 1 figure, Submitted to IEEE Photonics Letter
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