381 research outputs found
Hydrodynamics near the QCD Phase Transition: Looking for the Longest-Lived Fireball
We propose a new strategy for the experimental search of the QCD phase
transition in heavy ion collisions: One may tune collision energy around the
point where the lifetime of the fireball is expected to be longest. We
demonstrate that the hydrodynamic evolution of excited nuclear matter does
change dramatically as the initial energy density goes through the "softest
point" (where the pressure to energy density ratio reaches its minimum). For
our choice of equation of state, this corresponds to epsilon_i approx. = 1.5
GeV/fm^3 and collision energy E_lab/A approx. = 30 GeV (for Au+Au). Various
observables seem to show distinct changes near the softest point.Comment: 7 pages, 3 Postscript figures (tar compressed and uuencoded)
submitte
Many-Body Corrections to Charged-Current Neutrino Absorption Rates in Nuclear Matter
Including nucleon--nucleon correlations due to both Fermi statistics and
nuclear forces, we have developed a general formalism for calculating the
charged--current neutrino--nucleon absorption rates in nuclear matter. We find
that at one half nuclear density many--body effects alone suppress the rates by
a factor of two and that the suppression factors increase to 5 at
g cm. The associated increase in the neutrino--matter
mean--free--paths parallels that found for neutral--current interactions and
opens up interesting possibilities in the context of the delayed supernova
mechanism and protoneutron star cooling.Comment: 11 pages, APS REVTeX format, 1 PostScript figure, uuencoded
compressed, and tarred, submitted to Physical Review Letter
Concentration Dependence of the Effective Mass of He-3 Atoms in He-3/He-4 Mixtures
Recent measurements by Yorozu et al. (S. Yorozu, H. Fukuyama, and H.
Ishimoto, Phys. Rev. B 48, 9660 (1993)) as well as by Simons and Mueller (R.
Simons and R. M. Mueller, Czhechoslowak Journal of Physics Suppl. 46, 201
(1976)) have determined the effective mass of He-3 atoms in a He-3/He-4 mixture
with great accuracy. We here report theoretical calculations for the dependence
of that effective mass on the He-3 concentration. Using correlated basis
functions perturbation theory to infinite order to compute effective
interactions in the appropriate channels, we obtain good agreement between
theory and experiment.Comment: 4 pages, 1 figur
QCD sum rule analysis for light vector and axial-vector mesons in vacuum and nuclear matter
Extending previous work we study the constraints of QCD sum rules on mass and
width of light vector and axial-vector mesons in vacuum and in a medium with
finite nuclear density. For the latter case especially the effect of nuclear
pions leading to vector-axial-vector mixing is included in the analysis.Comment: RevTeX, 32 pages, 10 eps figure
Neutrino Opacities in Neutron Stars with Kaon Condensates
The neutrino mean free paths in hot neutron-star matter are obtained in the
presence of kaon condensates. The kaon-induced neutrino absorption process,
which is allowed only in the presence of kaon condensates, is considered for
both nondegenerate and degenerate neutrinos. The neutrino mean free path due to
this process is compared with that for the neutrino-nucleon scattering. While
the mean free path for the kaon-induced neutrino absorption process is shown to
be shorter than the ordinary two-nucleon absorption process by several orders
of magnitude when temperature is not very high, the neutrino-nucleon scattering
process has still a dominant contribution to the neutrino opacity. Thus, the
kaon-induced neutrino absorption process has a minor effect on the thermal and
dynamical evolution of protoneutron stars.Comment: 35 pages, 4 figure
In-medium pion weak decay constants
In nuclear matter, the pion weak decay constant is separated into the two
components corresponding to the time and space components of the
axial-vector current. Using QCD sum rules, we compute the two decay constants
from the pseudoscalar-axial vector correlation function in the matter . It is found that the sum rule for
satisfies the in-medium Gell-Mann--Oakes--Renner (GOR) relation precisely while
the sum rule does not. The sum rule contains the non-negligible
contribution from the dimension 5 condensate in addition to the
in-medium quark condensate. Using standard set of QCD parameters and ignoring
the in-medium change of the pion mass, we obtain MeV at the nuclear
saturation density. The prediction for depends on values of the dimension
5 condensate and on the Borel mass. However, the OPE constrains that , which does not agree with the prediction from the in-medium chiral
perturbation theory. Depending on the value of the dimension 5 condensate,
at the saturation density is found to be in the range MeV
at the Borel mass GeV.Comment: 19 pages including two postscript figures, substantially revise
Nuclear Saturation with in-Medium Meson Exchange Interactions
We show that the assumption of dropping meson masses together with
conventional many-body effects, implemented in the relativistic Dirac-Brueckner
formalism, explains nuclear saturation. We use a microscopic model for
correlated exchange and include the standard many-body effects on the
in-medium pion propagation, which initially increase the attractive
nucleon-nucleon () potential with density. For the vector meson exchanges
in both the and sector, we assume Brown-Rho scaling which---in
concert with `chiral' contact interactions---reduces the attraction at
higher densities.Comment: 5 pages REVTeX, 2 eps-figures included, submitted to Phys. Rev. Let
Contribution of the massive photon decay channel to neutrino cooling of neutron stars
We consider massive photon decay reactions via intermediate states of
electron-electron-holes and proton-proton-holes into neutrino-antineutrino
pairs in the course of neutron star cooling. These reactions may become
operative in hot neutron stars in the region of proton pairing where the photon
due to the Higgs-Meissner effect acquires an effective mass that
is small compared to the corresponding plasma frequency. The contribution of
these reactions to neutrino emissivity is calculated; it varies with the
temperature and the photon mass as
for . Estimates show that these processes appear as extra
efficient cooling channels of neutron stars at temperatures K.Comment: accepted to publication in Zh. Eksp. Teor. Fiz. (JETP
Reduction of Weak Interaction Rates in Neutron Stars by Nucleon Spin Fluctuations: Degenerate Case
Nucleon spin fluctuations in a dense medium reduce the ``naive'' values of
weak interaction rates (neutrino opacities, neutrino emissivities). We extend
previous studies of this effect to the degenerate case which is appropriate for
neutron stars a few ten seconds after formation. If neutron-neutron
interactions by a one-pion exchange potential are the dominant cause of neutron
spin fluctuations, a perturbative calculation of weak interaction rates is
justified for T\alt 3m/(4\pi\alpha_\pi^2)\approx 1 MeV, where is the
neutron mass and the pion fine-structure constant. At
higher temperatures, the application of Landau's theory of Fermi liquids is no
longer justified, i.e. the neutrons cannot be viewed as simple quasiparticles
in any obvious sense.Comment: 5 pages, RevTex, no figures, to be published in PR
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