9,776 research outputs found
QCD evolution and skewedness effects in color dipole description of DVCS
We show the role played by QCD evolution and skewedness effects in the DVCS
cross section at large within the color dipole description of the process
at photon level. The dipole cross section is given by the saturation model,
which can be improved by DGLAP evolution at high photon virtualities. We
investigate both possibilities as well as the off-forward effect through a
simple phenomenological parametrisation. The results are compared to the recent
ZEUS DVCS data.Comment: LaTeX, 6 pages, 5 Figs. Vers 2: Minor modifications. Accepted by EPJ
The DVCS Measurement at HERA
The recent results of the studies of Deeply Virtual Compton Scattering (DVCS)
events at HERA are presented. The possibility offered by this process to gain
information about skewed parton distributions (SPD) is emphasized.Comment: Talk given at New Trends in HERA Physics 2001, Ringberg Castle,
Tegernsee, Germany, 17-22 Jun 2001, 13 pages, 10 figures, recent ZEUS data
discussed, references update
How to detect level crossings without looking at the spectrum
We remind the reader that it is possible to tell if two or more eigenvalues
of a matrix are equal, without calculating the eigenvalues. We then use this
property to detect (avoided) crossings in the spectra of quantum Hamiltonians
representable by matrices. This approach provides a pedagogical introduction to
(avoided) crossings, is capable of handling realistic Hamiltonians
analytically, and offers a way to visualize crossings which is sometimes
superior to that provided by the spectrum. We illustrate the method using the
Breit-Rabi Hamiltonian to describe the hyperfine-Zeeman structure of the ground
state hydrogen atom in a uniform magnetic field.Comment: Accepted for publication in the American Journal of Physic
Diffractive Vector Meson Photoproduction from Dual String Theory
We study diffractive vector meson photoproduction using string theory via
AdS/CFT. The large behavior of the cross sections for the scattering of the
vector meson on a proton is dominated by the soft Pomeron, , where from the string theory model of
\cite{nastase2}, is approximately 1/7 below 10 GeV, and 1/11 for
higher, but still sub-Froissart, energies. This is due to the production of
black holes in the dual gravity. In -photoproduction the mesonic Regge
poles do not contribute, so that we deal with a pure Pomeron contribution. This
allows for an experimental test. At the gauge theory "Planck scale" of about
1-2 GeV, the ratios of the soft Pomeron contributions to the photoproduction
cross-sections of different vector mesons involve not only the obvious quark
model factors, but also the Boltzmann factors , with the
temperature of the dual black hole. The presence of these factors is confirmed
in the experimental data for and
photoproduction and is compatible with the meager photoproduction
data. Throughout, we use vector meson dominance, and from the data we obtain
of about , i.e. the gauge theory "Planck scale," as expected.
The ratio of the experimental soft Pomeron onset scale GeV
and of the gauge theory Planck scale, GeV conforms to the
theoretical prediction of .Comment: 17 pages, 1 figure, late
A detailed QCD analysis of twist-3 effects in DVCS observables
In this paper I present a detailed QCD analysis of twist-3 effects in the
Wandzura-Wilczek (WW) approximation in deeply virtual Compton scattering (DVCS)
observables for various kinematical settings, representing the HERA, HERMES,
CLAS and the planned EIC (electron-ion-collider) experiments. I find that the
twist-3 effects in the WW approximation are almost always negligible at
collider energies but can be large for low Q^2 and smaller x_bj in observables
for the lower energy, fixed target experiments directly sensitive to the real
part of DVCS amplitudes like the charge asymmetry (CA). Conclusions are then
drawn about the reliability of extracting twist-2 generalized parton
distributions (GPDs) from experimental data and a first, phenomenological,
parameterization of the LO and NLO twist-2 GPD , describing all the
currently available DVCS data within the experimental errors is given.Comment: 18 pages, 21 figures, uses Revtex4, final version to be published in
PRD, minor revisions due to referee suggestion
A next-to-leading order analysis of deeply virtual Compton scattering
We present a complete, next-to-leading-order (NLO), leading-twist QCD
analysis of deeply virtual Compton scattering (DVCS) observables, in the scheme, and in the kinematic ranges of the H1, ZEUS and HERMES
experiments. We use a modified form of Radyushkin's ansatz for the input model
for the generalized parton distributions. We present results for leading order
(LO) and NLO for representative observables and find that they compare
favourably to the available data.Comment: 5 pages, 2 figures, revtex, published version, we modify Radyushkin's
ansatz for the GPDs to correct for finite hadronic mass effects, and, using
the latest MRST PDFs, now agree with the H1 data (modified figs). Typo in
Eq.(3) correcte
Screening of charged singularities of random fields
Many types of point singularity have a topological index, or 'charge',
associated with them. For example the phase of a complex field depending on two
variables can either increase or decrease on making a clockwise circuit around
a simple zero, enabling the zeros to be assigned charges of plus or minus one.
In random fields we can define a correlation function for the charge-weighted
density of singularities. In many types of random fields, this correlation
function satisfies an identity which shows that the singularities 'screen' each
other perfectly: a positive singularity is surrounded by an excess of
concentration of negatives which exactly cancel its charge, and vice-versa.
This paper gives a simple and widely applicable derivation of this result. A
counterexample where screening is incomplete is also exhibited.Comment: 12 pages, no figures. Minor revision of manuscript submitted to J.
Phys. A, August 200
Earth Matter Effects in Detection of Supernova Neutrinos
We calculated the matter effect, including both the Earth and supernova, on
the detection of neutrinos from type II supernovae at the proposed Daya Bay
reactor neutrino experiment. It is found that apart from the dependence on the
flip probability P_H inside the supernova and the mass hierarchy of neutrinos,
the amount of the Earth matter effect depends on the direction of the incoming
supernova neutrinos, and reaches the biggest value when the incident angle of
neutrinos is around 93^\circ. In the reaction channel \bar{\nu}_e + p --> e^+ +
n the Earth matter effect can be as big as about 12%. For other detection
processes the amount of the Earth matter effect is a few per cent.Comment: 13 pages, 5 figure
Measurement in Quantum Physics
The conceptual problems in quantum mechanics -- related to the collapse of
the wave function, the particle-wave duality, the meaning of measurement --
arise from the need to ascribe particle character to the wave function. As will
be shown, all these problems dissolve when working instead with quantum fields,
which have both wave and particle character. Otherwise the predictions of
quantum physics, including Bell's inequalities, coincide with those of the
standard treatments. The transfer of the results of the quantum measurement to
the classical realm is also discussed.Comment: 34 pages, in Latex, revised and expanded version with an extra
appendix on decoherenc
Statistical Mechanics of Nonlinear On-line Learning for Ensemble Teachers
We analyze the generalization performance of a student in a model composed of
nonlinear perceptrons: a true teacher, ensemble teachers, and the student. We
calculate the generalization error of the student analytically or numerically
using statistical mechanics in the framework of on-line learning. We treat two
well-known learning rules: Hebbian learning and perceptron learning. As a
result, it is proven that the nonlinear model shows qualitatively different
behaviors from the linear model. Moreover, it is clarified that Hebbian
learning and perceptron learning show qualitatively different behaviors from
each other. In Hebbian learning, we can analytically obtain the solutions. In
this case, the generalization error monotonically decreases. The steady value
of the generalization error is independent of the learning rate. The larger the
number of teachers is and the more variety the ensemble teachers have, the
smaller the generalization error is. In perceptron learning, we have to
numerically obtain the solutions. In this case, the dynamical behaviors of the
generalization error are non-monotonic. The smaller the learning rate is, the
larger the number of teachers is; and the more variety the ensemble teachers
have, the smaller the minimum value of the generalization error is.Comment: 13 pages, 9 figure
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