248 research outputs found
Comments on Exclusive Electroproduction of Transversely Polarized Vector Mesons
We discuss the electroproduction of light vector mesons from transversely
polarized photons. Here QCD factorization cannot be applied as shown explicitly
in a leading order calculation of corresponding Feynman diagrams. It is
emphasized that present infrared singular contributions cannot be regularized
through phenomenological meson distribution amplitudes with suppressed endpoint
configurations. We point out that infrared divergencies arise also from
integrals over skewed parton distributions of the nucleons.
In a phenomenological analysis of transverse vector meson production model
dependent regularizations have to be applied. If this procedure preserves the
analytic structure suggested by a leading order calculation of Feynman
diagrams, one obtains contributions from nucleon parton distributions and their
derivatives. In particular polarized gluons enter only through their
derivative
Quantum Information Encoding, Protection, and Correction from Trace-Norm Isometries
We introduce the notion of trace-norm isometric encoding and explore its
implications for passive and active methods to protect quantum information
against errors. Beside providing an operational foundations to the "subsystems
principle" [E. Knill, Phys. Rev. A 74, 042301 (2006)] for faithfully realizing
quantum information in physical systems, our approach allows additional
explicit connections between noiseless, protectable, and correctable quantum
codes to be identified. Robustness properties of isometric encodings against
imperfect initialization and/or deviations from the intended error models are
also analyzed.Comment: 10 pages, 1 figur
Surfaces Meeting Porous Sets in Positive Measure
Let n>2 and X be a Banach space of dimension strictly greater than n. We show
there exists a directionally porous set P in X for which the set of C^1
surfaces of dimension n meeting P in positive measure is not meager. If X is
separable this leads to a decomposition of X into a countable union of
directionally porous sets and a set which is null on residually many C^1
surfaces of dimension n. This is of interest in the study of certain classes of
null sets used to investigate differentiability of Lipschitz functions on
Banach spaces
Deeply Virtual Neutrino Scattering (DVNS)
We introduce the study of neutrino scattering off protons in the deeply
virtual kinematics, which describes under a unified formalism elastic and deep
inelastic neutrino scattering. A real final state photon and a recoiling
nucleon are detected in the few GeV ( GeV) region of momentum
transfer. This is performed via an extension of the notion of deeply virtual
Compton scattering, or DVCS, to the case of a neutral current exchange. The
relevance of this process and of other similar exclusive processes for the
study of neutrino interactions in neutrino factories for GeV neutrinos is
pointed out.Comment: 28 pages, 12 figures, revised final version, to appear in JHE
Off-Forward Parton Distributions
Recently, there have been some interesting developments involving off-forward
parton distributions of the nucleon, deeply virtual Compton scattering, and
hard diffractive vector-meson production. These developments are triggered by
the realization that the off-forward distributions contain information about
the internal spin structure of the nucleon and that diffractive
electroproduction of vector mesons depends on these unconventional
distributions. This paper gives a brief overview of the recent developments
On the number of Mather measures of Lagrangian systems
In 1996, Ricardo Ricardo Ma\~n\'e discovered that Mather measures are in fact
the minimizers of a "universal" infinite dimensional linear programming
problem. This fundamental result has many applications, one of the most
important is to the estimates of the generic number of Mather measures.
Ma\~n\'e obtained the first estimation of that sort by using finite dimensional
approximations. Recently, we were able with Gonzalo Contreras to use this
method of finite dimensional approximation in order to solve a conjecture of
John Mather concerning the generic number of Mather measures for families of
Lagrangian systems. In the present paper we obtain finer results in that
direction by applying directly some classical tools of convex analysis to the
infinite dimensional problem. We use a notion of countably rectifiable sets of
finite codimension in Banach (and Frechet) spaces which may deserve independent
interest
The Status of Lattice Calculations of the Nucleon Structure Functions
We review our progress on the lattice calculation of low moments of both the
unpolarised and polarised nucleon structure functions.Comment: 6 pages, contribution to 29th International Symposium on the Theory
of Elementary Particles, Buckow, Germany, (29 August - 2 September 1995). 6
pages, Latex, requires espcrc2.sty, epsf.st
Generalized parton distributions of the pion in chiral quark models and their QCD evolution
We evaluate Generalized Parton Distributions of the pion in two chiral quark
models: the Spectral Quark Model and the Nambu-Jona-Lasinio model with a
Pauli-Villars regularization. We proceed by the evaluation of double
distributions through the use of a manifestly covariant calculation based on
the alpha representation of propagators. As a result polynomiality is
incorporated automatically and calculations become simple. In addition,
positivity and normalization constraints, sum rules and soft pion theorems are
fulfilled. We obtain explicit formulas, holding at the low-energy quark-model
scale. The expressions exhibit no factorization in the t-dependence. The QCD
evolution of those parton distributions is carried out to experimentally or
lattice accessible scales. We argue for the need of evolution by comparing the
Parton Distribution Function and the Parton Distribution Amplitude of the pion
to the available experimental and lattice data, and confirm that the
quark-model scale is low, about 320 MeV.Comment: 25 pages, 15 figures, added discussion of the end-point behavio
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