388 research outputs found
Transverse target spin asymmetry in inclusive DIS with two-photon exchange
We study the transverse target spin dependence of the cross section for
inclusive electron-nucleon scattering with unpolarized beam. Such dependence is
absent in the one-photon exchange approximation (Christ-Lee theorem) and arises
only in higher orders of the QED expansion, from the interference of one-photon
and absorptive two-photon exchange amplitudes as well as from real photon
emission (bremsstrahlung). We demonstrate that the transverse spin-dependent
two-photon exchange cross section is free of QED infrared and collinear
divergences. We argue that in DIS kinematics the transverse spin dependence
should be governed by a "parton-like" mechanism in which the two-photon
exchange couples mainly to a single quark. We calculate the normal spin
asymmetry in an approximation where the dominant contribution arises from quark
helicity flip due to interactions with non-perturbative vacuum fields
(constituent quark picture) and is proportional to the quark transversity
distribution in the nucleon. Such helicity-flip processes are not significantly
Sudakov-suppressed if the infrared scale for gluon emission in the photon-quark
subprocess is of the order of the chiral symmetry breaking scale, mu_chiral^2
>> Lambda_QCD^2. We estimate the asymmetry in the kinematics of the planned
Jefferson Lab Hall A experiment to be of the order 10^{-4}, with different sign
for proton and neutron. We also comment on the spin dependence in the limit of
soft high-energy scattering.Comment: 22 pages, 14 figures; uses revtex
Beam Single-Spin Asymmetry in Semi-Inclusive Deep Inelastic Scattering
We calculate, in a model, the beam spin asymmetry in semi-inclusive jet
production in deep inelastic scattering. This twist-3, -odd observable is
non-zero due to final state strong interactions. With reasonable choices for
the parameters, one finds an asymmetry of several percent, about the size seen
experimentally. We present the result both as an explicit asymmetry calculation
and as a model calculation of the new transverse-momentum dependent
distribution function .Comment: 10 pages, 6 figures; minor changes made in the discussion; version
accepted for publicatio
QED Radiative Corrections in Processes of Exclusive Pion Electroproduction
Formalism for radiative correction (RC) calculation in exclusive pion
electroproduction on the proton is presented. A FORTRAN code EXCLURAD is
developed for the RC procedure. The numerical analysis is done in the
kinematics of current Jefferson Lab experiments.Comment: 13 pages, 9 figures; requires RevTeX
Radiative Correction to the Transferred Polarization in Elastic Electron-Proton Scattering
Model independent radiative correction to the recoil proton polarization for
the elastic electron-proton scattering is calculated within method of electron
structure functions. The explicit expressions for the recoil proton
polarization are represented as a contraction of the electron structure and the
hard part of the polarization dependent contribution into cross-section. The
calculation of the hard part with first order radiative correction is
performed. The obtained representation includes the leading radiative
corrections in all orders of perturbation theory and the main part of the
second order next-to-leading ones. Numerical calculations illustrate our
analytical results.Comment: 14 pages, 4 figure
Theory and Applications of X-ray Standing Waves in Real Crystals
Theoretical aspects of x-ray standing wave method for investigation of the
real structure of crystals are considered in this review paper. Starting from
the general approach of the secondary radiation yield from deformed crystals
this theory is applied to different concreat cases. Various models of deformed
crystals like: bicrystal model, multilayer model, crystals with extended
deformation field are considered in detailes. Peculiarities of x-ray standing
wave behavior in different scattering geometries (Bragg, Laue) are analysed in
detailes. New possibilities to solve the phase problem with x-ray standing wave
method are discussed in the review. General theoretical approaches are
illustrated with a big number of experimental results.Comment: 101 pages, 43 figures, 3 table
Radiative Corrections to Polarized Inelastic Scattering in Coincidence
The coplete analysis of the model-independent leading radiative corrections
to cross-section and polarization observables in semi-inclusive deep-inelastic
electron-nucleus scattering with detection of a proton and scattered electron
in coincidence has been performed. The basis of the calculations consists of
the Drell-Yan like representation in electrodynamics for both spin-independent
and spin-dependent parts of the cross-section in terms of the electron
structure functions. The applications to the polarization transfer effect from
longitudinally polarized electron beam to detected proton as well as to
scattering by the polarized target are considered.Comment: 18p, to be published in JET
Discovery of Damped Lyman-Alpha Systems at Redshifts Less Than 1.65 and Results on their Incidence and Cosmological Mass Density
We report results on the incidence and cosmological mass density of damped
Lyman-alpha (DLA) systems at redshifts less that 1.65. We used HST and an
efficient non-traditional (but unbiased) survey technique to discover DLA
systems at redshifts z<1.65, where we observe the Lyman-alpha line in known
MgII absorption-line systems. We uncovered 14 DLA lines including 2
serendipitously. We find that (1) The DLA absorbers are drawn almost
exclusively from the population of MgII absorbers which have rest equivalent
widths W(2796)>0.6A. (2) The incidence of DLA systems per unit redshift,
n(DLA), is observed to decrease with decreasing redshift. (3) On the other
hand, the cosmological mass density of neutral gas in low-redshift DLA
absorbers, Omega(DLA), is observed to be comparable to that observed at high
redshift. (4) The low-redshift DLA absorbers exhibit a significantly larger
fraction of very high column density systems in comparison to determinations at
both high redshift and locally.Comment: 47 pages in LaTeX - emulateapj style with included tables and
encapsulated postscript figures. Accepted for Publication in Astrophysical
Journal Supplements. Results unchanged, text revise
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