235 research outputs found
Non-universality of transverse Coulomb exchange at small x
Within an explicit scalar QED model we compare, at fixed x << 1, the
leading-twist K_T-dependent `quark' distribution f_q(x, K_T) probed in deep
inelastic scattering and Drell-Yan production, and show that the model is
consistent with the universality of f_q(x, K_T). The extension of the model
from the aligned-jet to the 'symmetric' kinematical regime reveals interesting
properties of the physics of Coulomb rescatterings when comparing DIS and DY
processes. At small x the transverse momentum induced by multiple
scattering on a single centre is process dependent, as well as the transverse
momentum broadening occurring in collisions on a finite size nuclear target.Comment: 28 pages, 3 eps figure
EAS radio detection at large impact parameter: the inverse problem and the design of a giant array
Contribution to the 30th ICRC, July 2007, Merida, MexicoInternational audienceExtensive air shower radio electric fields can be evaluated at large impact parameter with analytical expressions. Such a theoretical tool is most valuable in the present phase where the capabilities of the radio detection of extensive air shower are under investigations. It can help shaping strategies for the analysis of radio detection data. It can also be used to perform non trivial test of much more detailed numerical approaches which are currently under development. The approximation leading to such a formulation will be presented and two applications will be discussed: the "inverse" problem of how to go from a sampling of the radio electric field on a few antennas to the main characteristics of the extensive air shower, and the question of the antenna spacing of a giant array for ultra high energy cosmic rays
Radioelectric fields from cosmic-ray air showers at large impact parameters
International audienceWe discuss electric fields generated by cosmic-ray air showers at large impact parameters b. An approximation relevant to this situation is given. The formulation makes explicit the relationship between the shower profile and the radio pulse shapes at large b, putting forward one important observational consequence, namely the decrease of the high-frequency cutoff Îœcâ1/b when the impact parameter increases. The approximation is also used to give a detailed comparison between two emission models, the geosynchrotron model and the transverse current model
Energy loss of a heavy quark produced in a finite-size quark-gluon plasma
We study the energy loss of an energetic heavy quark produced in a high
temperature quark-gluon plasma and travelling a finite distance before emerging
in the vacuum. While the retardation time of purely collisional energy loss is
found to be of the order of the Debye screening length, we find that the
contributions from transition radiation and the Ter-Mikayelian effect do not
compensate, leading to a reduction of the zeroth order (in an opacity
expansion) energy loss.Comment: QM2006 Proceedings; caption of fig 1 and ref [7] modified in v
Radioelectric Field Features of Extensive Air Showers Observed with CODALEMA
Based on a new approach to the detection of radio transients associated with
extensive air showers induced by ultra high energy cosmic rays, the
experimental apparatus CODALEMA is in operation, measuring about 1 event per
day corresponding to an energy threshold ~ 5. 10^16 eV. Its performance makes
possible for the first time the study of radio-signal features on an
event-by-event basis. The sampling of the magnitude of the electric field along
a 600 meters axis is analyzed. It shows that the electric field lateral spread
is around 250 m (FWHM). The possibility to determine with radio both arrival
directions and shower core positions is discussed.Comment: Accepted for publication in Astroparticle Physic
Shapes of the Proton
A model proton wave function, constructed using Poincare invariance, and
constrained by recent electromagnetic form factor data, is used to study the
shape of the proton. Spin-dependent quark densities are defined as matrix
elements of density operators in proton states of definite spin-polarization,
and shown to have an infinite variety of non-spherical shapes. For high
momentum quarks with spin parallel to that of the proton, the shape resembles
that of a peanut, but for quarks with anti-parallel spin the shape is that of a
bagel.Comment: 8 pages, 5 figures, to be submitted to Phys. Rev. C This corrects a
few typos and explains some further connections with experiment
Retardation Effect for Collisional Energy Loss of Hard Partons Produced in a QGP
We study the collisional energy loss suffered by an energetic parton
travelling the distance L in a high temperature quark-gluon plasma and
initially produced in the medium. We find that the medium-induced collisional
loss -Delta E(L) is strongly suppressed compared to previous estimates which
assumed the collisional energy loss rate -dE/dx to be constant. The large L
linear asymptotic behaviour of -Delta E(L) sets in only after a quite large
retardation time. The suppression of -Delta E(L) is partly due to the fact that
gluon bremsstrahlung arising from the initial acceleration of the energetic
parton is reduced in the medium compared to vacuum. The latter radiation
spectrum is sensitive to the plasmon modes of the quark-gluon plasma and has a
rich angular structure.Comment: 19 pages, 4 figures. Substantial changes have been made in version 2,
especially in section 3. In particular, in section 3.2 the dependence on a
spurious ultraviolet cut-off has been removed. As a consequence numerical
results are quantitatively modified, but the retardation effect is
qualitatively unchanged. The physical origin of the largeness of the
retardation time is also emphasized, and two Appendices have been adde
Charmonium suppression in p-A collisions
The new high precision data on charmonium production in proton-nucleus
collisions by the E866/NuSea collaboration at Fermilab allow - together with
older data at lower energies - to fix a unique set of parameters for the
standard production and absorption scenario of charmonium in a proton-nucleus
reaction. In this scenario the c-cbar pair is formed in an octet state, emits a
gluon and continues its radial expansion in a singlet state until it has
reached the charmonium radius. In all three phases it can interact with the
nuclear environment. We find that the lifetime of the octet state is much
shorter than acceptable on physical grounds. This challenges the physical
reality of the first phase in the standard scenario.Comment: 8 pages, 10 figure
Timelike form factors at high energy
The difference between the timelike and spacelike meson form factors is
analysed in the framework of perturbative QCD with Sudakov effects included. It
is found that integrable singularities appear but that the asymptotic behavior
is the same in the timelike and spacelike regions. The approach to asymptotia
is quite slow and a rather constant enhancement of the timelike value is
expected at measurable large . This is in agreement with the trend
shown by experimental data.Comment: 17 pages, report DAPNIA/SPhN 94 0
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