439 research outputs found
The quark condensate at finite temperature
The temperature evolution of the quark condensate is studied using three
different methods. In the spirit of a many-body approach we make an expansion
in the scalar density up to second order. Our result is consistent chiral
perturbation theory to two-loop order.Comment: Latex 14 pages + 1 figure (postscript
Rho Meson Propagation and Dilepton Enhancement in Hot Hadronic Matter
A realistic model for the free rho meson with coupling to two-pion states is
employed to calculate the rho propagator in a hot and dense hadron gas. The
medium modifications are based on hadronic rescattering processes: intermediate
two-pion states are renormalized through interactions with surrounding nucleons
and deltas, and rho meson scattering is considered off nucleons, deltas, pions
and kaons. Constraints from gauge invariance as well as the full off-shell
dynamics of the interactions are accounted for. Within the vector dominance
model we apply the resulting in-medium rho spectral function to compute
production rates from annihilation. The calculation of
corresponding spectra as recently measured in central collisions of
heavy-ions at CERN/SpS energies gives reasonable agreement with the
experimental data.Comment: 27 pages RevTeX, 9 eps-figures, submitted to Nucl. Phys.
Medium Dependence of the Vector-Meson Mass: Dynamical and/or Brown-Rho Scaling?
We discuss the similarities and differences for the theories of Rapp, Wambach
and collaborators (called R/W in short) and those based on Brown-Rho scaling
(called B/R), as applied to reproduce the dileptons measured by the CERES
collaboration in the CERN experiments. In both theories the large number of
dileptons at invariant masses ~ are shown to be chiefly
produced by a density-dependent -meson mass. In R/W the medium dependence
is dynamically calculated using hadronic variables defined in the matter-free
vacuum. In B/R scaling it follows from movement towards chiral symmetry
restoration due to medium-induced vacuum change, and is described in terms of
constituent (or quasiparticle) quarks. We argue that the R/W description should
be reliable up to densities somewhat beyond nuclear density, where hadrons are
the effective variables. At higher density there should be a crossover to
constituent quarks as effective variables scaling according to B/R. In the
crossover region, the two descriptions must be ``dual''.Comment: 13 pages LaTeX, incl. 5 eps-figures and appb.sty; Talk given at the
Workshop on 'The Structure of Mesons, Baryons and Nuclei', Cracow, May 1998,
in honor of J. Speth's 60th birthday, to be published in Acta Physica
Polonica
In-medium Pion-Pion Interaction and Chiral Symmetry Restoration
We discuss medium modifications of the unitarized pion-pion interaction in
the nuclear medium. We incorporate both the effects of chiral symmetry
restoration and the influence of collective nuclear pionic modes, originating
from the p-wave coupling of the pion to delta-hole configurations. We show how
the resulting strong enhancement of the sigma-meson spectral function is
related to large fluctuations of the condensate associated with the partial
restoration of chiral symmetry.Comment: 8 pages, 3 Postscript figures, contribution to the `International
Workshop XXVIII on Gross Properties of Nuclei and Nuclear Excitations',
Hirschegg, Austria, January 16-22 200
Thermal QRPA with Skyrme interactions and supernova neutral-current neutrino-nucleus reactions
The Thermal Quasiparticle Random-Phase Approximation is combined with the
Skyrme energy density functional method (Skyrme-TQRPA) to study the response of
a hot nucleus to an external perturbation. For the sample nuclei, Fe and
Ge, the Skyrme-TQRPA is applied to analyze thermal effects on the
strength function of charge-neutral Gamow-Teller transitions which dominate
neutrino-nucleus reactions at ~MeV. For the relevant
supernova temperatures we calculate the cross sections for inelastic neutrino
scattering. We also apply the method to examine the rate of
neutrino-antineutrino pair emission by hot nuclei. The cross sections and rates
are compared with those obtained earlier from the TQRPA calculations based on
the phenomenological Quasiparticle-Phonon Model Hamiltonian. For inelastic
neutrino scattering on Fe we also compare the Skyrme-TQRPA results to
those obtained earlier from a hybrid approach that combines shell-model and RPA
calculations.Comment: Minor revisions according to referee's recomendation
Low-Mass Pairs from in-Medium Meson Propagation
Based on a realistic model for the rho meson in free space we investigate
it's medium modifications in a hot hadron gas generated by hadronic
rescattering processes, i.e. renormalization of intermediate two-pion states as
well as direct rho meson scattering off hadrons. Within the vector dominance
model the resulting in-medium rho spectral function is applied to calculate
spectra as recently measured in heavy-ion collisions at CERN-SpS
energies in the CERES experiment.Comment: 9 pages LaTeX, postscript figures included using epsf.sty,
epsfig.sty; Proc. of the Int. Workshop XXV, Hirschegg '97, on 'QCD Phase
Transitions
Gamow-Teller strength distributions at finite temperatures and electron capture in stellar environments
We propose a new method to calculate stellar weak-interaction rates. It is
based on the Thermo-Field-Dynamics formalism and allows the calculation of the
weak-interaction response of nuclei at finite temperatures. The thermal
evolution of the GT distributions is presented for the sample nuclei Fe and ~Ge. For Ge we also calculate the strength distribution
of first-forbidden transitions. We show that thermal effects shift the GT
centroid to lower excitation energies and make possible negative- and
low-energy transitions. In our model we demonstrate that the unblocking effect
for GT transitions in neutron-rich nuclei is sensitive to increasing
temperature. The results are used to calculate electron capture rates and are
compared to those obtained from the shell model.Comment: 16 pages, 9 figure
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