57 research outputs found
Dynamics of a Disoriented Chiral Condensate
We use the linear model to analyse the dynamics of a disoriented
chiral condensate. For idealized boundary conditions appropriate to high energy
collisions, the problem can be reduced to a one dimensional one. The evolution
of the chiral state is then that of a simple dynamical system and can be
studied analytically.Comment: 14 pages Latex, LPTHE Orsay 94/18 , SPhT T94/01
Hadronization corrections to helicity components of the fragmentation function
In the hadronic decays of Z, gluon emission leads to the appearance of the
longitudinal component of the fragmentation function, F_L. Measurement of F_L
and the transverse component, F_T, could thus provide an insight into the gluon
fragmentation function. However, hadronization corrections at low x can be
significant. Here we present a method of accounting for such corrections, using
the JETSET event generator as illustration.Comment: 11 pages, 5 figure
Properties of an equilibrium hadron gas subjected to the adiabatic longitudinal expansion
We consider an ideal gas of massive hadrons in thermal and chemical
equilibrium. The gas expands longitudinally in an adiabatic way. This evolution
for a baryonless gas reduces to a hydrodynamic expansion. Cooling process is
parametrized by the sound velocity. The sound velocity is temperature dependent
and is strongly influenced by hadron mass spectrum.Comment: 7pages, 7 figures-- uucoded file of figures appended at the end,
separated from the paper by lines with many dashe
Energy Loss of a Heavy Quark Produced in a Finite Size Medium
We study the medium-induced energy loss suffered by a
heavy quark produced at initial time in a quark-gluon plasma, and escaping the
plasma after travelling the distance . The heavy quark is treated
classically, and within the same framework consistently
includes: the loss from standard collisional processes, initial bremsstrahlung
due to the sudden acceleration of the quark, and transition radiation. The
radiative loss {\it induced by rescatterings} is not
included in our study. For a ultrarelativistic heavy quark with momentum p
\gsim 10 {\rm GeV}, and for a finite plasma with L_p \lsim 5 {\rm fm}, the
loss is strongly suppressed compared to the stationary
collisional contribution . Our results
support that is the dominant contribution to the heavy quark
energy loss (at least for L_p \lsim 5 {\rm fm}), as indeed assumed in most of
jet-quenching analyses. However they might raise some question concerning the
RHIC data on large electron spectra.Comment: 18 pages, 3 figures. New version clarified and simplified. A critical
discussion added in section 2, and previous sections 3 and 4 have been merged
together. Main results are unchange
Coulomb Effect: A Possible Probe for the Evolution of Hadronic Matter
Electromagnetic field produced in high-energy heavy-ion collisions contains
much useful information, because the field can be directly related to the
motion of the matter in the whole stage of the reaction. One can divide the
total electromagnetic field into three parts, i.e., the contributions from the
incident nuclei, non-participating nucleons and charged fluid, the latter
consisting of strongly interacting hadrons or quarks. Parametrizing the
space-time evolution of the charged fluid based on hydrodynamic model, we study
the development of the electromagnetic field which accompanies the high-energy
heavy-ion collisions. We found that the incident nuclei bring a rather strong
electromagnetic field to the interaction region of hadrons or quarks over a few
fm after the collision. On the other hand, the observed charged hadrons'
spectra are mostly affected (Coulomb effect) by the field of the charged fluid.
We compare the result of our model with experimental data and found that the
model reproduces them well. The pion yield ratio pi^-/pi+ at a RHIC energy,
Au+Au 100+100 GeV/nucleon, is also predicted.Comment: 23 pages, RevTex, 19 eps figures, revised versio
Imaging the Space-Time Evolution of High Energy Nucleus-Nucleus Collisions with Bremsstrahlung
The bremsstrahlung produced when heavy nuclei collide is estimated for
central collisions at the Relativistic Heavy Ion Collider. Bremsstrahlung
photons with energies below 100 to 200 MeV are sufficient to discern the gross
features of the space-time evolution of electric charge, if they can be
separated from other sources of photons experimentally. This is illustrated
explicitly by considering two very different models, one Bjorken-like, the
other Landau-like, both of which are constructed to give the same final charge
rapidity distribution.Comment: 9 pages revtex style, 9 embedded PS figure
Equation of state and initial temperature of quark gluon plasma at RHIC
In gold-gold collisions of the Relativistic Heavy Ion Collider (RHIC) a
perfect fluid of quarks, sometimes called the strongly interacting quark gluon
plasma (sQGP) is created for an extremely short time. The time evolution of
this fluid can be described by hydrodynamical models. After expansion and
cooling, the freeze-out happens and hadrons are created. Their distribution
reveals information about the final state of the fluid. To investigate the time
evolution one needs to analyze penetrating probes, such as direct photon
observables. Transverse momentum distributions of low energy direct photons
were mesured in 2010 by PHENIX, while azimuthal asymmetry in 2011. These
measurements can be compared to hydrodynamics to determine the equation of
state and the initial temperature of sQGP. In this paper we analyze an 1+3
dimensional solution of relativistic hydrodynamics. We calculate momentum
distribution, azimuthal asymmetry and momentum correlations of direct photons.
Based on earlier fits to hadronic spectra, we compare photon calculations to
measurements to determine the equation of state and the initial temperature of
sQGP. We find that the initial temperature in the center of the fireball is
507+-12 MeV, while for the sound speed we get a speed of sound of 0.36+-0.02.
We also estimate a systematic error of these results. We find that the measured
azimuthal asymmetry is also not incompatible with this model, and predict a
photon source that is significantly larger in the out direction than in the
side direction.Comment: 12 pages, 4 figures. This work was supported by the OTKA grant
NK-73143 and NK-101438 and M. Csanad's Bolyai scholarshi
Strangeness production time and the K+/pi+ horn
We construct a hadronic kinetic model which describes production of strange
particles in ultrarelativistic nuclear collisions in the energy domain of SPS.
We test this model on description of the sharp peak in the excitation function
of multiplicity ratio K+/pi+ and demonstrate that hadronic model reproduces
these data rather well. The model thus must be tested on other types of data in
order to verify the hypothesis that deconfinement sets in at lowest SPS
energies.Comment: proceedings of Hot Quarks 0
Correlations and the Cross Section of Exclusive () Reactions for O
The reduced cross section for exclusive () reactions has been studied
in DWIA for the example of the nucleus O using a spectral function
containing effects of correlations. The spectral function is evaluated directly
for the finite nucleus starting from a realistic nucleon-nucleon interaction
within the framework of the Green's function approach. The emphasis is focused
on the correlations induced by excitation modes at low energies described
within a model-space of shell-model configurations including states up to the
shell. Cross sections for the -wave quasi-hole transitions at low
missing energies are presented and compared with the most recent experimental
data. In the case of the so-called perpendicular kinematics the reduced cross
section derived in DWIA shows an enhancement at high missing momenta as
compared to the PWIA result. Furthermore the cross sections for the - and
-wave quasi-hole transitions are presented and compared to available data at
low missing momenta. Also in these cases, which cannot be described in a model
without correlations, a good agreement with the experiment is obtained.Comment: 12 pages, LaTeX, 4 figures include
Density and expansion effects on pion spectra in relativistic heavy-ion collisions
We compute the pion inclusive momentum distribution in heavy-ion collisions
at AGS energies, assuming thermal equilibrium and accounting for density and
expansion effects at the time of decoupling. We compare to data on mid rapidity
charged pions produced in central Au + Au collisions and find a very good
agreement. The shape of the distribution at low is explained in part as
an effect arising from the high mean pion density achieved in these reactions.
The difference between the positive and negative pion distributions in the same
region is attributed in part to the different average yields of each kind of
charged pions.Comment: Minor changes, typo in Fig. 2b corrected, version to appear in Phys.
Rev.
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