85 research outputs found
Light from Cascading Partons in Relativistic Heavy-Ion Collisions
We calculate the production of high energy photons from Compton and
annihilation processes as well as fragmentation off quarks in the parton
cascade model. The multiple scattering of partons is seen to lead to a
substantial production of high energy photons, which rises further when parton
multiplication due to final state radiation is included. The photon yield is
found to be proportional to the number of collisions among the cascading
partons.Comment: revised version: 4 pages, 4 figures, uses REVTEX
Coloron Phenomenology
A flavor-universal extension of the strong interactions was recently proposed
in response to the apparent excess of high- jets in the inclusive jet
spectrum measured at the Tevatron. This paper studies the color octet of
massive gauge bosons (`colorons') that is present in the low-energy spectrum of
the model's Higgs phase. Constraints from searches for new particles decaying
to dijets and from measurements of the weak-interaction parameter imply
that the colorons must have masses greater than 870-1000 GeV. The implications
of recent Tevatron data and the prospective input from future experiments are
also discussed.Comment: 13 pages, 4 embedded Postscript figures, LaTeX, full postscript
version also available at http://smyrd.bu.edu/htfigs/htfigs.html rectified
confusing phrase at end of sub-section on 'dijets
Equilibration in Quark Gluon Plasma
The hydrodynamic expansion rate of quark gluon plasma (QGP) is evaluated and
compared with the scattering rate of quarks and gluons within the system.
Partonic scattering rates evaluated within the ambit of perturbative Quantum
Choromodynamics (pQCD) are found to be smaller than the expansion rate
evaluated with ideal equation of state (EoS) for the QGP. This indicate that
during the space-time evolution the system remains out of equilibrium.
Enhancement of pQCD cross sections and a more realistic EoS keep the partons
closer to the equilibrium.Comment: To be published in the Quark Matter 2008 poster proceeding
Inclusive Higgs boson and dijet production via Double Pomeron exchange
We evaluate Higgs boson and dijet cross-sections at the Tevatron collider via
Double Pomeron exchange when accompanying particles in the central region are
taken into account. Such {\it inclusive} processes, normalized to the observed
dijet rate observed at run I, noticeably increase the predictions for tagged
(anti)protons in the run II with respect to {\it exclusive} ones, with the
potentiality of Higgs boson detection.Comment: 6pages, 4 figure
Parton energy loss in an expanding quark-gluon plasma: Radiative vs collisional
We perform a comparison of the radiative and collisional parton energy losses
in an expanding quark-gluon plasma. The radiative energy loss is calculated
within the light-cone path integral approach. The collisional energy loss is
calculated using the Bjorken method with an accurate treatment of the binary
collision kinematics. Our numerical results demonstrate that for RHIC and LHC
conditions the collisional energy loss is relatively small in comparison to the
radiative one. We find an enhancement of the heavy quark radiative energy loss
as compared to that of the light quarks at high energies.Comment: 13 pages, 3 figure
A Poincare-Covariant Parton Cascade Model for Ultrarelativistic Heavy-Ion Reactions
We present a new cascade-type microscopic simulation of nucleus-nucleus
collisions at RHIC energies. The basic elements are partons (quarks and gluons)
moving in 8N-dimensional phase space according to Poincare-covariant dynamics.
The parton-parton scattering cross sections used in the model are computed
within perturbative QCD in the tree-level approximation. The Q^2 dependence of
the structure functions is included by an implementation of the DGLAP mechanism
suitable for a cascade, so that the number of partons is not static, but varies
in space and time as the collision of two nuclei evolves. The resulting parton
distributions are presented, and meaningful comparisons with experimental data
are discussed.Comment: 30 pages. 11 figures. Submitted to Phys.Rev.
Peripheral heavy ion collisions as a probe of the nuclear gluon distribution
At high energies a quark-gluon plasma is expected to be formed in heavy ion
collisions at RHIC and LHC. The theoretical description of these processes is
directly associated to a complete knowledge of the details of medium effects in
the nuclear gluon distribution. In this paper we analyze the possibility to
constraint the behavior of this distribution considering peripheral heavy ion
collisions. We reanalyze the photoproduction of heavy quarks for the deduction
of the in-medium gluon distribution using three current parameterizations for
this parton distribution. Moreover, we show that the elastic photoproduction of
vector mesons is a potential process to probe the nuclear gluon distribution.Comment: 8 figures, accepted for publication in Physicsl review
Rapidity Distributions of Dileptons from a Hadronizing Quark-Gluon Plasma
It has been predicted that dilepton production may be used as a quark-gluon
plasma probe. We calculate the rapidity distributions of thermal dileptons
produced by an evolving quark-gluon plasma assuming a longitudinal scaling
expansion with initial conditions locally determined from the hadronic rapidity
density. These distributions are compared with Drell-Yan production and
semileptonic charm decays at invariant mass , 4, and 6 GeV.Comment: 17 pages (standard LaTeX), 6 figures (available as topdraw files or
printed versions upon request), GSI-93-6
Additional Suppression from High Density Effects
At high energies the saturation effects associated to the high parton density
should modify the behavior of the observables in proton-nucleus and
nucleus-nucleus scattering. In this paper we investigate the saturation effects
in the nuclear production and estimate the modifications in the energy
dependence of the cross section as well as in the length of the nuclear medium.
In particular, we calculate the ratio of to Drell-Yan cross sections
and show that it is strongly modified if the high density effects are included.
Moreover, our results are compared with the data from the NA50 Collaboration
and predictions for the RHIC and LHC kinematic regions are presented. We
predict an additional suppression associated to the high density
effects.Comment: 13 pages, 5 figures, version to be published in Eur. Phys. J.
Hard diffraction in hadron--hadron interactions and in photoproduction
Hard single diffractive processes are studied within the framework of the
triple--Pomeron approximation. Using a Pomeron structure function motivated by
Regge--theory we obtain parton distribution functions which do not obey
momentum sum rule. Based on Regge-- factorization cross sections for hard
diffraction are calculated. Furthermore, the model is applied to hard
diffractive particle production in photoproduction and in
interactions.Comment: 13 pages, Latex, 13 uuencoded figure
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