74 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
Charmonia enhancement in quark-gluon plasma with improved description of c-quarks phase-distribution
We present a dynamical model of heavy quark evolution in the quark-gluon
plasma (QGP) based on the Fokker-Planck equation. We then apply this model to
the case of central ultra-relativistic nucleus-nucleus collisions performed at
RHIC and estimate the component of production (integrated and
differential) stemming from c- pairs that are initially uncorrelated.Comment: contribution presented at SQM0
Tomography of the Quark Gluon Plasma by Heavy Quarks
Using the recently published model \cite{Gossiaux:2008jv,goss2} for the
collisional energy loss of heavy quarks in a Quark Gluon Plasma (QGP), based on
perturbative QCD (pQCD), we study the centrality dependence of and
, %= \frac{dN_{AA}/dp_T}{ dN_{pp}/dp_T}$
measured by the Phenix collaboration, and compare our model with other
approaches based on pQCD and on Anti de Sitter/ Conformal Field Theory
(AdS/CFT)Comment: proceedings for SQM0
Disappearance of Elliptic Flow: A New Probe for the Nuclear Equation of State
Using a relativistic hadron transport model, we investigate the utility of
the elliptic flow excitation function as a probe for the stiffness of nuclear
matter and for the onset of a possible quark-gluon-plasma (QGP)
phase-transition at AGS energies 1 < E_Beam < 11 AGeV. The excitation function
shows a strong dependence on the nuclear equation of state, and exhibits
characteristic signatures which could signal the onset of a phase transition to
the QGP.Comment: 11 pages, 4 Postscript figures, uses epsf.sty, submitted to Physical
Review Letter
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
The importance of initial-final state correlations for the formation of fragments in heavy ion collisions
Using quantum molecular dynamics simulations, we investigate the formation of
fragments in symmetric reactions between beam energies of E=30AMeV and 600AMeV.
After a comparison with existing data we investigate some observables relevant
to tackle equilibration: dsigma/dErat, the double differential cross section
dsigma/pt.dpz.dpt,... Apart maybe from very energetic E>400AMeV and very
central reactions, none of our simulations gives evidence that the system
passes through a state of equilibrium. Later, we address the production
mechanisms and find that, whatever the energy, nucleons finally entrained in a
fragment exhibit strong initial-final state correlations, in coordinate as well
as in momentum space. At high energy those correlations resemble the ones
obtained in the participant-spectator model. At low energy the correlations are
equally strong, but more complicated; they are a consequence of the Pauli
blocking of the nucleon-nucleon collisions, the geometry, and the excitation
energy. Studying a second set of time-dependent variables (radii,
densities,...), we investigate in details how those correlations survive the
reaction especially in central reactions where the nucleons have to pass
through the whole system. It appears that some fragments are made of nucleons
which were initially correlated, whereas others are formed by nucleons
scattered during the reaction into the vicinity of a group of previously
correlated nucleons.Comment: 45 pages text + 20 postscript figures Accepted for publication in
Physical Review
Charmonium suppression in p-A collisions at RHIC
We discuss charmonium production in proton-nucleus collisions at RHIC
energies under the assumption of xF and x2 scaling. We find that all the
ambiguities due to energy loss are gone at this energy and therefore data will
reveal the scaling law, if any. These p-A data will also be crucial to
interpret nucleus-nucleus data with respect to a possible formation of a quark
gluon plasma because the extrapolations for charmonium production from the
present p-A data to RHIC energies, based on the two scaling laws, differ by a
factor of four.Comment: 6 pages, 3 figures. New section on shadowing and energy loss,
References adde
Physics of the Muon Spectrometer of the ALICE Experiment
The main goal of the Muon spectrometer of the ALICE experiment at LHC is the
measurement of heavy quark production in p+p, p+A and A+A collisions at LHC
energies, via the muonic channel. Physics motivations and expected performances
have been presented in this talk.Comment: 10 pages and 4 figures. Talk presented in the ICPAQGP Conference,
February 8-12, 2005, Salt Lake City, Kolkata, India. Web page of the
conference : http://www.veccal.ernet.in/~icpaqgp
Variance of transmitted power in multichannel dissipative ergodic structures invariant under time reversal
We use random matrix theory (RMT) to study the first two moments of the wave
power transmitted in time reversal invariant systems having ergodic motion.
Dissipation is modeled by a number of loss channels of variable coupling
strength. To make a connection with ultrasonic experiments on ergodic
elastodynamic billiards, the channels injecting and collecting the waves are
assumed to be negligibly coupled to the medium, and to contribute essentially
no dissipation. Within the RMT model we calculate the quantities of interest
exactly, employing the supersymmetry technique. This approach is found to be
more accurate than another method based on simplifying naive assumptions for
the statistics of the eigenfrequencies and the eigenfunctions. The results of
the supersymmetric method are confirmed by Monte Carlo numerical simulation and
are used to reveal a possible source of the disagreement between the
predictions of the naive theory and ultrasonic measurements.Comment: 10 pages, 2 figure
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