465 research outputs found
Thermal photon production in high-energy nuclear collisions
We use a boost-invariant one-dimensional (cylindrically symmetric) fluid
dynamics code to calculate thermal photon production in the central rapidity
region of S+Au and Pb+Pb collisions at SPS energy ( GeV/nucleon).
We assume that the hot matter is in thermal equilibrium throughout the
expansion, but consider deviations from chemical equilibrium in the high
temperature (deconfined) phase. We use equations of state with a first-order
phase transition between a massless pion gas and quark gluon plasma, with
transition temperatures in the range MeV.Comment: revised, now includes a_1 contribution. revtex, 10 pages plus 4
figures (uuencoded postscript
The Role of Gluons in Dilepton Production from the Quark-Gluon Plasma
We study high mass dilepton production from gluon-induced processes, , , and ,
in a thermally equilibrated but chemically under-saturated partonic matter that
is expected to be created in the initial stage of ultra-relativistic heavy ion
collisions. Regulating the divergence in these processes by the thermal quark
mass, we find that gluon-induced processes are more important than the
leading-order annihilation process as a result of the larger number
of gluons than quarks in the partonic matter. The dependence of the thermal
dilepton yield from the partonic stage of heavy ion collisions on the initial
conditions for the partonic matter is also studied. We further discuss the
feasibility of observing thermal dileptons from the quark-gluon plasma in heavy
ion experiments.Comment: 23 pages, revtex, 9 figures, added discussion on higher order effect
Electromagnetic probes of dense matter in heavy-ion collisions
Dilepton and photon production in heavy-ion collisions at SPS energies are
studied in the relativistic transport model that incorporates self-consistently
the change of hadron masses in dense matter. It is found that the dilepton
spectra in proton-nucleus reactions can be well described by the conventional
mechanism of Dalitz decay and direct vector meson decay. However, to provide a
quantitative explanation of the observed dilepton spectra in central heavy-ion
collisions requires contributions other than these direct decays and also
various medium effects. Introducing a decrease of vector meson masses in hot
dense medium, we find that these heavy-ion data can be satisfactorily
explained. Furthermore, the single photon spectra in our calculations with
either free or in-medium meson masses do not exceed the upper bound deduced
from the experiments by the WA80 Collaboration.Comment: 8 pages, including 4 postscript figures. Talk presented at Sixth
International Conference on Nucleus-Nucleus Collisions (NN97), Gatlinburg,
June, 1997, to appear in the proceeding
Low-mass dileptons and dropping rho meson mass
Using the transport model, we have studied dilepton production from heavy-ion
collisions at Bevalac energies. It is found that the enhanced production of
low-mass dileptons observed in the experiment by the DLS collaboration cannot
be explained by the dropping of hadron masses, in particular the -meson
mass, in dense matter.Comment: 9 pages, LaTeX, including 1 postscript figure, to appear in Phys.
Lett.
Thermal effects on dilepton production from annihilation
We study finite temperature effects on dilepton production from pion-pion
annihilation in hot hadronic matter. The softening of the pion dispersion
relation in a medium is found to enhance the production rate of dileptons with
invariant masses in the region of . On the other hand,
the reduction of the pion electromagnetic form factor at finite temperature
leads to a suppression of the dilepton production rate. Including both effects,
we have found that the dilepton yield is slightly enhanced in the invariant
mass region MeV but is suppressed around the vector meson
resonance. We further discuss the relevance of our results to recent
experimental data from the S+Au collisions at CERN/SPS energies by the CERES
collaboration.Comment: 11 pages + 3 figures (figures are not included but available on
request), revte
Effects of in-medium vector meson masses on low-mass dileptons from SPS heavy-ion collisions
Using a relativistic transport model to describe the expansion of the
fire-cylinder formed in the initial stage of heavy-ion collisions at SPS/CERN
energies, we study the production of dileptons with mass below about 1 GeV from
these collisions. The initial hadron abundance and their momentum distributions
in the fire-cylinder are determined by following the general features of the
results from microscopic models based on the string dynamics and further
requiring that the final proton and pion spectra and rapidity distributions are
in agreement with available experimental data. For dilepton production, we
include the Dalitz decay of , , , and
mesons, the direct decay of primary , and mesons, and
the pion-pion annihilation that proceeds through the meson, the
pion-rho annihilation that proceeds through the meson, and the
kaon-antikaon annihilation that proceeds through the meson. We find that
the modification of vector meson properties, especially the decrease of their
mass due to the partial restoration of chiral symmetry, in hot and dense
hadronic matter, provides a quantitative explanation of the recently observed
enhancement of low-mass dileptons by the CERES collaboration in central S+Au
collisions and by the HELIOS-3 collaboration in central S+W collisions.Comment: 46 pages, LaTeX, figures available from [email protected], to appear
in Nucl. Phys.
Dilepton production in proton-nucleus and nucleus-nucleus collisions at SPS energies
Dilepton production in proton- and nucleus-induced reactions is studied in
relativistic transport model using initial conditions determined by the string
dynamics from RQMD. It is found that both the CERES and HELIOS-3 data for
dilepton spectra in proton-nucleus reactions can be well described by the
`conventional' mechanism of Dalitz decay and direct vector meson decay.
However, to provide a quantitative explanation of the observed dilepton spectra
in central S+Au and S+W collisions requires contributions other than these
direct decays. Introducing a decrease of vector meson masses in hot and dense
medium, we find that these heavy-ion data can also be satisfactorily explained.
This agrees with our earlier conclusions based on a fire cylinder model. We
also give predictions for Pb+Au collisions at 160 GeV/nucleon using current
CERES mass resolution and acceptance.Comment: RevTeX, 45 pages, including 21 postscript figures, to be published in
Nuclear Physics
Signatures of Thermal Dilepton Radiation at RHIC
The properties of thermal dilepton production from heavy-ion collisions in
the RHIC energy regime are evaluated for invariant masses ranging from 0.5 to 3
GeV. Using an expanding thermal fireball to model the evolution through both
quark-gluon and hadronic phases various features of the spectra are addressed.
In the low-mass region, due to an expected large background, the focus is on
possible medium modifications of the narrow resonance structures from
and mesons, whereas in the intermediate-mass region the old idea of
identifying QGP radiation is reiterated including effects of chemical
under-saturation in the early stages of central Au+Au collisions.Comment: 17 pages ReVTeX including 16 figure
Single Spin Asymmetry in Polarized Proton-Proton Elastic Scattering at GeV
We report a high precision measurement of the transverse single spin
asymmetry at the center of mass energy GeV in elastic
proton-proton scattering by the STAR experiment at RHIC. The was measured
in the four-momentum transfer squared range \GeVcSq, the region of a significant interference between the
electromagnetic and hadronic scattering amplitudes. The measured values of
and its -dependence are consistent with a vanishing hadronic spin-flip
amplitude, thus providing strong constraints on the ratio of the single
spin-flip to the non-flip amplitudes. Since the hadronic amplitude is dominated
by the Pomeron amplitude at this , we conclude that this measurement
addresses the question about the presence of a hadronic spin flip due to the
Pomeron exchange in polarized proton-proton elastic scattering.Comment: 12 pages, 6 figure
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