16,885 research outputs found
Interferometry of direct photons in Pb+Pb collisions at 158 AGeV
We present final results from the WA98 experiment which provide first
measurements of Bose-Einstein correlations of direct photons in
ultrarelativistic heavy ion collisions. Invariant interferometric radii were
extracted in the range MeV/c and compared to interferometric
radii of charged pions. The yield of direct photons for MeV/c was
extracted from the correlation strength parameter and compared to the yield of
direct photons measured in WA98 at higher with the statistical
subtraction method, and to predictions of a fireball model.Comment: 4 pages, 3 figures, proceedings for Quark Matter 200
Sustaining supercooled mixed phase via resonant oscillations of the order parameter
We investigate the dynamics of a first order transition when the order
parameter field undergoes resonant oscillations, driven by a periodically
varying parameter of the free energy. This parameter could be a background
oscillating field as in models of pre-heating after inflation. In the context
of condensed matter systems, it could be temperature , or pressure, external
electric/magnetic field etc. We show that with suitable driving frequency and
amplitude, the system remains in a type of mixed phase, without ever completing
transition to the stable phase, even when the oscillating parameter of the free
energy remains below the corresponding critical value (for example, with
oscillating temperature, always remains below the critical temperature
). This phenomenon may have important implications. In cosmology, it will
imply prolonged mixed phase in a first order transition due to coupling with
background oscillating fields. In condensed matter systems, it will imply that
using oscillating temperature (or, more appropriately, pressure waves) one may
be able to sustain liquids in a mixed phase indefinitely at low temperatures,
without making transition to the frozen phase.Comment: 17 pages, 7 figures, Expanded version with more detail
Thermal Particle and Photon Production in Pb+Pb Collisions with Transverse Flow
Particle and photon production is analyzed in the presence of transverse flow
using two approximations to describe the properties of the hadronic medium, one
containing only , and mesons (simplified equation of
state) and the other containing hadrons and resonances from the particle data
table. Both are considered with and without initial quark gluon plasma
formation. In each case the initial temperature is fixed by requiring
550 in the final state. It is shown that most observables are
very sensitive to the equation of state. This is particularly evident when
comparing the results of the simplified equation of state in the scenarios with
and without phase transition. The hadronic gas scenario leads to a
substantially higher rate for the -distribution of all particles. In the
complete equation of state with several hundreds of hadronic resonances, the
difference between the scenarios with and without phase transition is rather
modest. Both photon and particle spectra, in a wide range, show very
similar behavior. It is therefore concluded that from the spectra it will
be hard to disentangle quark gluon plasma formation in the initial state. It is
to be stressed however, that there are conceptual difficulties in applying a
pure hadronic gas equation of state at SPS-energies. The phase transition
scenario with a quark gluon plasma present in the initial state seems to be the
more natural one.Comment: 9 pages RevTeX figures in postscript forma
Uniqueness of nontrivially complete monotonicity for a class of functions involving polygamma functions
For , let
on . In the
present paper, we prove using two methods that, among all for
, only is nontrivially completely monotonic on
. Accurately, the functions and are
completely monotonic on , but the functions for
are not monotonic and does not keep the same sign on
.Comment: 9 page
Velocity of sound in relativistic heavy-ion collisions
We have studied the rapidity distribution of secondary hadrons produced in
nucleus-nucleus collisions at ultra-relativistic energies within the ambit of
the Landau's hydrodynamical model. A reasonable description of the data can
also be obtained by using the Bjorken's hydrodynamical model if the boost
invariance is restricted to a finite rapidity range. The sensitivity of the
hadronic spectra on the equation of state vis- a -vis the velocity of sound has
been discussed. The correlation between the velocity of sound and the
freeze-out temperature has been indicated. The effects of the non-zero widths
of various mesonic and baryonic degrees of freedom up to the mass value ~ 2.5
GeV is seen to be small.Comment: 9 pages and 11 figures. Major changes. To appear in Physical Review
Scaling of hadronic transverse momenta in a hydrodynamic treatment of relativistic heavy ion collisions
The transverse momenta of hadrons in central nucleus-nucleus collisions are
evaluated in a boost invariant hydrodynamics with transverse expansion. Quark
gluon plasma is assumed to be formed in the initial state which expands and
cools via a first order phase transition to a rich hadronic matter and
ultimately undergoes a freeze-out. The average transverse momentum of pions,
kaons, and protons is estimated for a wide range of multiplicity densities and
transverse sizes of the system. For a given system it is found to scale with
the square-root of the particle rapidity density per unit transverse area, and
consistent with the corresponding values seen in experiments at
1800 GeV, suggesting a universal behaviour. The average transverse momentum
shows only an approximate scaling with multiplicity density per nucleon which
is at variance with the data.Comment: 6 pages including 9 figure
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