12,380 research outputs found
Transversity Distribution Does Not Contribute to Hard Exclusive Electroproduction of Mesons
We show that in hard exclusive electroproduction, ep-->eVp, the leading-twist
hard-scattering coefficient for the production of a transversely polarized
vector meson V vanishes to all orders of perturbation theory. This implies that
this process cannot be used to measure the skewed transversity distribution of
quarks in a hadron. In contrast, a recent calculation obtained a non-zero value
at NLO. We show that this calculation is incorrect because it failed to include
the necessary collinear subtractions. Our method of proof also applies to other
processes whose hard-scattering coefficients are constrained by chirality and
helicity conservation, and thus validates helicity selection rules based on
these symmetries.Comment: 5 pages, 3 figures, ReVTe
Perturbative Field-Theoretical Renormalization Group Approach to Driven-Dissipative Bose-Einstein Criticality
The universal critical behavior of the driven-dissipative non-equilibrium
Bose-Einstein condensation transition is investigated employing the
field-theoretical renormalization group method. Such criticality may be
realized in broad ranges of driven open systems on the interface of quantum
optics and many-body physics, from exciton-polariton condensates to cold atomic
gases. The starting point is a noisy and dissipative Gross-Pitaevski equation
corresponding to a complex valued Landau-Ginzburg functional, which captures
the near critical non-equilibrium dynamics, and generalizes Model A for
classical relaxational dynamics with non-conserved order parameter. We confirm
and further develop the physical picture previously established by means of a
functional renormalization group study of this system. Complementing this
earlier numerical analysis, we analytically compute the static and dynamical
critical exponents at the condensation transition to lowest non-trivial order
in the dimensional epsilon expansion about the upper critical dimension d_c =
4, and establish the emergence of a novel universal scaling exponent associated
with the non-equilibrium drive. We also discuss the corresponding situation for
a conserved order parameter field, i.e., (sub-)diffusive Model B with complex
coefficients.Comment: 17 pages, 6 figures, to appear in Phys. Rev. X (2014
Boundary critical behavior at m-axial Lifshitz points for a boundary plane parallel to the modulation axes
The critical behavior of semi-infinite -dimensional systems with
-component order parameter and short-range interactions is
investigated at an -axial bulk Lifshitz point whose wave-vector instability
is isotropic in an -dimensional subspace of . The associated
modulation axes are presumed to be parallel to the surface, where . An appropriate semi-infinite model representing the
corresponding universality classes of surface critical behavior is introduced.
It is shown that the usual O(n) symmetric boundary term
of the Hamiltonian must be supplemented by one of the form involving a
dimensionless (renormalized) coupling constant . The implied boundary
conditions are given, and the general form of the field-theoretic
renormalization of the model below the upper critical dimension
is clarified. Fixed points describing the ordinary, special,
and extraordinary transitions are identified and shown to be located at a
nontrivial value if . The surface
critical exponents of the ordinary transition are determined to second order in
. Extrapolations of these expansions yield values of these
exponents for in good agreement with recent Monte Carlo results for the
case of a uniaxial () Lifshitz point. The scaling dimension of the surface
energy density is shown to be given exactly by , where
is the anisotropy exponent.Comment: revtex4, 31 pages with eps-files for figures, uses texdraw to
generate some graphs; to appear in PRB; v2: some references and additional
remarks added, labeling in figure 1 and some typos correcte
The annihilation of virtual photons into pseudoscalar mesons
We investigate the possibility to constrain the pion distribution amplitude
from the gamma* gamma* -> pi transition. For a surprisingly large range in the
two photon virtualities we find that the transition form factor is essentially
independent of the distribution amplitude. This in turn entails a
parameter-free prediction of QCD. The gamma* gamma* -> eta, eta' form factors
are also briefly discussed. We estimate that experimental studies might be
feasible at the existing e+ e- experiments BaBar, Belle, and CLEO.Comment: 22 pages latex, 9 figure
Two-Photon Annihilation into Baryon-Antibaryon Pairs
We study the handbag contribution to two-photon annihilation into
baryon-antibaryon pairs at large energy and momentum transfer. We derive
factorization of the process amplitude into a hard gamma gamma -> q qbar
subprocess and form factors describing the soft q qbar -> B Bbar transition,
assuming that the process is dominated by configurations where the (anti)quark
approximately carries the full momentum of the (anti)baryon. The form factors
represent moments of time-like generalized parton distributions, so-called B
Bbar distribution amplitudes. A characteristic feature of the handbag mechanism
is the absence of isospin-two components in the final state, which in
combination with flavor symmetry provides relations among the form factors for
the members of the lowest-lying baryon octet. Assuming dominance of the handbag
contribution, we can describe current experimental data with form factors of
plausible size, and predict the cross sections of presently unmeasured B Bbar
channels.Comment: 20 pages latex, 4 figures. v2: minor clarifications, references
update
Adjoint-based predictor-corrector sequential convex programming for parametric nonlinear optimization
This paper proposes an algorithmic framework for solving parametric
optimization problems which we call adjoint-based predictor-corrector
sequential convex programming. After presenting the algorithm, we prove a
contraction estimate that guarantees the tracking performance of the algorithm.
Two variants of this algorithm are investigated. The first one can be used to
solve nonlinear programming problems while the second variant is aimed to treat
online parametric nonlinear programming problems. The local convergence of
these variants is proved. An application to a large-scale benchmark problem
that originates from nonlinear model predictive control of a hydro power plant
is implemented to examine the performance of the algorithms.Comment: This manuscript consists of 25 pages and 7 figure
Boundary critical behaviour at -axial Lifshitz points: the special transition for the case of a surface plane parallel to the modulation axes
The critical behaviour of -dimensional semi-infinite systems with
-component order parameter is studied at an -axial bulk
Lifshitz point whose wave-vector instability is isotropic in an -dimensional
subspace of . Field-theoretic renormalization group methods are
utilised to examine the special surface transition in the case where the
potential modulation axes, with , are parallel to the surface.
The resulting scaling laws for the surface critical indices are given. The
surface critical exponent , the surface crossover exponent
and related ones are determined to first order in
\epsilon=4+\case{m}{2}-d. Unlike the bulk critical exponents and the surface
critical exponents of the ordinary transition, is -dependent already
at first order in . The \Or(\epsilon) term of is
found to vanish, which implies that the difference of and
the bulk exponent is of order .Comment: 21 pages, one figure included as eps file, uses IOP style file
- …