926 research outputs found
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
Robust quantization of a molecular motor motion in a stochastic environment
We explore quantization of the response of a molecular motor to periodic
modulation of control parameters. We formulate the Pumping-Quantization Theorem
(PQT) that identifies the conditions for robust integer quantized behavior of a
periodically driven molecular machine. Implication of PQT on experiments with
catenane molecules are discussed.Comment: 7 pages, 4 figures. J. Chem. Phys. Communications (in press
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
Higher Twist Distribution Amplitudes of the Nucleon in QCD
We present the first systematic study of higher-twist light-cone distribution
amplitudes of the nucleon in QCD. We find that the valence three-quark state is
described at small transverse separations by eight independent distribution
amplitudes. One of them is leading twist-3, three distributions are twist-4 and
twist-5, respectively, and one is twist-6. A complete set of distribution
amplitudes is constructed, which satisfies equations of motion and constraints
that follow from conformal expansion. Nonperturbative input parameters are
estimated from QCD sum rules.Comment: 29 pages, 4 figures, eqn in (3.19) corrected, table 3 accordingly
changed, some typos fixe
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
Resummation of (-B(0) Alpha-s)**N Corrections to the Photon - Meson Transition Form-Factor Gamma* + Gamma ---> Pi0
We have resummed all the (-b_0 alpha_s)^n contributions to the photon-meson
transition form factor F_{gamma pi}. To do this, we have used the assumption of
`naive nonabelianization' (NNA). Within NNA, a series in (N_f alfa_s)^n is
interpreted as a series in (-b_0 alpha_S)^n by means of the restoration of the
full first QCD beta-function coefficient -b_0 by hand. We have taken into
account corrections to the leading order coefficient function and to the
evolution of the distribution function. Due to conformal constraints, it is
possible to find the eigenfunctions of the evolution kernel. It turns out that
the nondiagonal corrections are small, and neglecting them we obtained a
representation for the distribution function with multiplicatively renormalized
moments. For a simple shape of the distribution function, which is close to the
asymptotic shape, we find that the radiative correction decrease the LO by 30 %
and the uncertainty in the resummation lies between 10 % and 2 % for Q^2
between 2 and 10 GeV^2.Comment: 30 pages, LaTeX, 10 figures. New figures and references added. Some
changes in the tex
Photon Distribution Amplitudes in QCD
We develop a consistent technique for the calculation of real photon emission
in hard exclusive processes, which is based on the background field formalism
and allows a convenient separation of hard electromagnetic and soft hadronic
components of the photon. The latter ones are related to matrix-elements of
light-cone operators in the electromagnetic background field and can be
parametrized in terms of photon distribution amplitudes. We construct a
complete set of photon distribution amplitudes up to and including twist-4, for
both chirality-conserving and chirality-violating operators. The distribution
amplitudes involve several nonperturbative parameters and, most importantly,
the magnetic susceptibility of the quark condensate. We review and update
previous estimates of the susceptibility and also give new estimates of
parameters describing higher-twist amplitudes from QCD sum rules.Comment: 33 pages Late
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