105 research outputs found
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
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
Anomalous Chromomagnetic Moments of Quarks and Large Transverse Energy Jets
We consider the jet cross sections for gluons coupling to quarks with an
anomalous chromomagnetic moment. We then apply this to the deviation and bounds
from QCD found in the CDF and D0 Fermilab data, respectively, to find a range
of possible values for the anomalous moments. The quadratic and quartic terms
in the anomalous moments can fit to the rise of a deviation with transverse
energy. Since previous analyses have been done on the top quark total cross
section, here we assume the same moment on all quarks except the top and find
the range TeV for the
CDF data. Assuming the anomalous moment is present only on a charm or bottom
quark which is pair produced results in a range TeV. The magnitudes here are compared with anomalous magnetic moments
that could account for and found to be in the same general range, as well
as not inconsistent with LEP and SLD bounds on .Comment: REVTeX, 11 pages, 2 postscript figure
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
Thermal quark production in ultra-relativistic nuclear collisions
We calculate thermal production of u, d, s, c and b quarks in
ultra-relativistic heavy ion collisions. The following processes are taken into
account: thermal gluon decay (g to ibar i), gluon fusion (g g to ibar i), and
quark-antiquark annihilation (jbar j to ibar i), where i and j represent quark
species. We use the thermal quark masses, ,
in all the rates. At small mass (), the production is largely
dominated by the thermal gluon decay channel. We obtain numerical and analytic
solutions of one-dimensional hydrodynamic expansion of an initially pure glue
plasma. Our results show that even in a quite optimistic scenario, all quarks
are far from chemical equilibrium throughout the expansion. Thermal production
of light quarks (u, d and s) is nearly independent of species. Heavy quark (c
and b) production is quite independent of the transition temperature and could
serve as a very good probe of the initial temperature. Thermal quark production
measurements could also be used to determine the gluon damping rate, or
equivalently the magnetic mass.Comment: 14 pages (latex) plus 6 figures (uuencoded postscript files);
CERN-TH.7038/9
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.
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.
Searching for in t \tbar Production
The triple gluon field strength operator represents the only genuinely
gluonic CP conserving term which can appear at dimension-6 within an effective
strong interaction Lagrangian. Previous studies of this operator have revealed
that its effect on gluon scattering is surprisingly difficult to detect. In
this article, we analyze the impact of upon top quark pair production. We
find that it will generate observable cross section deviations from QCD at the
LHC for even relatively small values of its coefficient. Furthermore,
affects the transverse momentum distribution of the produced top quarks more
strongly at high energies than dimension-6 four-quark and chromomagnetic moment
terms in the effective Lagrangian. Top-antitop production at the LHC will
therefore provide a sensitive and clean probe for the elusive triple gluon
field strength operator.Comment: 20 pages, 8 figures (3 updated figures not included but available
upon request), CALT-68-1941, BUHEP-94-18. (Some previously overlooked graphs
are now included. Our conclusions remain unchanged.
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
Viscosities of Quark-Gluon Plasmas
The quark and gluon viscosities are calculated in quark-gluon plasmas to
leading orders in the coupling constant by including screening. For weakly
interaction QCD and QED plasmas dynamical screening of transverse interactions
and Debye screening of longitudinal interactions controls the infrared
divergences. For strongly interacting plasmas other screening mechanisms taken
from lattice calculations are employed. By solving the Boltzmann equation for
quarks and gluons including screening the viscosity is calculated to leading
orders in the coupling constant. The leading logarithmic order is calculated
exactly by a full variational treatment. The next to leading orders are found
to be very important for sizable coupling constants as those relevant for the
transport properties relevant for quark-gluon plasmas created in relativistic
heavy ion collisions and the early universe.Comment: 12 pages + 6 figures, report LBL-3492
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