916 research outputs found
On the dispute between Boltzmann and Gibbs entropy
Very recently, the validity of the concept of negative temperature has been
challenged by several authors since they consider Boltzmann's entropy (that
allows negative temperatures) inconsistent from a mathematical and statistical
point of view, whereas they consider Gibbs' entropy (that does not admit
negative temperatures) the correct definition for microcanonical entropy.
In the present paper we prove that for systems with equivalence of the
statistical ensembles Boltzmann entropy is the correct microcanonical entropy.
Analytical results on two systems supporting negative temperatures, confirm the
scenario we propose. In addition, we corroborate our proof by numeric
simulations on an explicit lattice system showing that negative temperature
equilibrium states are accessible and obey standard statistical mechanics
prevision.Comment: To appear in Annals of Physic
Non-equilibrium effects in steady relativistic winds
We consider an ultra-relativistic wind consisting of electron-positron pairs
and photons with the principal goal of finding the asymptotic Lorentz factor
for zero baryon number. The wind is assumed to originate at
radius where it has a Lorentz factor and a temperature
sufficiently high to maintain pair equilibrium. As increases, decreases
and becomes less than the temperature corresponding to the electron mass ,
after which non-equilibrium effects become important. Further out in the flow
the optical depth drops below one, but the pairs may still be
accelerated by the photons until falls below . Radiative transfer calculations show that only at this point
do the radiation flux and pressure start to deviate significantly from their
blackbody values. The acceleration of the pairs increases by a factor
as compared to its value at the photosphere; it is shown to approach
\gamma_{\infty} \sim 1.4\times 10^3 (r_i/10^6\mbox{cm})^{1/4} \gamma_{i}^{3/4}
T_i/m_e.Comment: 41 pages, 9 figures. Submitted to MNRA
Medium effects in high energy heavy-ion collisions
The change of hadron properties in dense matter based on various theoretical
approaches are reviewed. Incorporating these medium effects in the relativistic
transport model, which treats consistently the change of hadron masses and
energies in dense matter via the scalar and vector fields, heavy-ion collisions
at energies available from SIS/GSI, AGS/BNL, and SPS/CERN are studied. This
model is seen to provide satisfactory explanations for the observed enhancement
of kaon, antikaon, and antiproton yields as well as soft pions in the
transverse direction from the SIS experiments. In the AGS heavy-ion
experiments, it can account for the enhanced ratio, the difference
in the slope parameters of the and transverse kinetic energy
spectra, and the lower apparent temperature of antiprotons than that of
protons. This model also provides possible explanations for the observed
enhancement of low-mass dileptons, phi mesons, and antilambdas in heavy-ion
collisions at SPS energies. Furthermore, the change of hadron properties in hot
dense matter leads to new signatures of the quark-gluon plasma to hadronic
matter transition in future ultrarelativistic heavy-ion collisions at RHIC/BNL.Comment: RevTeX, 65 pages, including 25 postscript figures, invited topical
review for Journal of Physics G: Nuclear and Particle Physic
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