211 research outputs found
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
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
Diffractive vector meson electroproduction at small Bjorken within GPD approach
We study light vector meson electroproduction at small within the
generalized parton distributions (GPDs) model. The modified perturbative
approach is used, where the quark transverse degrees of freedom in the vector
meson wave function and hard subprocess are considered. Our results on the
cross section and spin observables are in good agreement with experimentComment: 6 pages, 5 figures, presented at Symmetries and Spin meeting, Prague,
8- 14 July, 200
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
Twist-3 contribution to the amplitude in the Wandzura-Wilczek approximation
We have calculated the Wandzura-Wilczek contribution to the twist-3 part of
amplitude. It describes interaction of the
longitudinally polarized virtual photon with the real one, and it is suppressed
by 1/Q, where is the virtuality of the , as compared to the
twist-2 contribution. We have found that, in the Wandzura-Wilczek
approximation, factorization applies to the twist-3 amplitude.Comment: 13 pages, 4 figure
Pion and Rho Structure Functions from Lattice QCD
We calculate the lower moments of the deep-inelastic structure functions of
the pion and the rho meson on the lattice. Of particular interest to us are the
spin-dependent structure functions of the rho. The calculations are done with
Wilson fermions and for three values of the quark mass, so that we can perform
an extrapolation to the chiral limit.Comment: 30pp, LaTeX2e with 15 eps figures using epsfig. Postscript file also
available from ftp://ftp.th.physik.uni-frankfurt.de/pub/cbest/pionrho.ps or
http://www.th.physik.uni-frankfurt.de/~cbest/pionrho.p
Vector meson electroproduction at next-to-leading order
The process of a light neutral vector meson electroproduction is studied in
the framework of QCD factorization in which the amplitude factorizes in a
convolution of the nonperturbative meson distribution amplitude and the
generalized parton densities with the perturbatively calculable hard-scattering
amplitudes. We derive a complete set of hard-scattering amplitudes at
next-to-leading order for the production of vector mesons, .Comment: latex, 6 pages, with corrected Eq. (12) and added remark on
dimensional regularisatio
Generalized parton distributions and double distributions for q q-bar pions
We consider two simple covariant models for pions (one with scalar and the
other with spin-1/2 constituents). Pion generalized parton distributions are
derived by integration over the light-cone energy. The model distributions are
consistent with all known properties of generalized parton distributions,
including positivity. We also construct the corresponding double distributions
by appealing to Lorentz invariance. These ostensibly constructed double
distributions lead to different generalized parton distributions that need not
respect the positivity constraints. This inconsistency arises from the
ambiguity inherent in defining double distributions in a one-component
formalism (even in the absence of the Polyakov-Weiss term). We demonstrate that
the correct model double distributions can be calculated from non-diagonal
matrix elements of twist-two operators.Comment: 10 pages, 7 figures, RevTex4, reference added, typos correcte
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