317,350 research outputs found
Elliptic flow in heavy ion collisions near the balance energy
The proton elliptic flow in collisions of Ca on Ca at energies from 30 to 100
MeV/nucleon is studied in an isospin-dependent transport model. With increasing
incident energy, the elliptic flow shows a transition from positive to negative
flow. Its magnitude depends on both the nuclear equation of state (EOS) and the
nucleon-nucleon scattering cross section. Different elliptic flows are obtained
for a stiff EOS with free nucleon-nucleon cross sections and a soft EOS with
reduced nucleon-nucleon cross sections, although both lead to vanishing
in-plane transverse flow at the same balance energy. The study of both in-plane
and elliptic flows at intermediate energies thus provides a means to extract
simultaneously the information on the nuclear equation of state and the
nucleon-nucleon scattering cross section in medium.Comment: 6 pages, 2 figure
Resonance model study of kaon production in baryon baryon reactions for heavy ion collisions
The energy dependence of the total kaon production cross sections in baryon
baryon ( and ) collisions are studied in the resonance model, which
is a relativistic, tree-level treatment. This study is the first attempt to
complete a systematic, consistent investigation of the elementary kaon
production reactions for both the pion baryon and baryon baryon reactions. Our
model suggests that the magnitudes of the isospin-averaged total cross sections
for the and ( or )
reactions are almost equal at energies up to about 200 MeV above threshold.
However, the magnitudes for the reactions become about 6 times
larger than those for the reactions at energies about 1 GeV above
threshold. Furthermore, the magnitudes of the isospin-averaged total cross
sections for the reactions turn out to be comparable to
those for the reactions at invariant collision energies
about 3.1 GeV, and about 5 to 10 times larger at invariant collision
energies about 3.5 GeV. The microscopic cross sections are parametrized in all
isospin channels necessary for the transport model studies of kaon production
in heavy ion collisions. These cross sections are then applied in the
relativistic transport model to study the sensitivity to the underlying
elementary kaon production cross sections.Comment: Latex, 47 pages, 23 postscript figures. Typos in the published
version, which informed as errata to the editor, are corrected for the use of
simulation cod
Hot Nuclear Matter Equation of State with a Three-body Force
The finite temperature Brueckner-Hartree-Fock approach is extended by
introducing a microscopic three-body force. In the framework of the extended
model, the equation of state of hot asymmetric nuclear matter and its isospin
dependence have been investigated. The critical temperature of liquid-gas phase
transition for symmetric nuclear matter has been calculated and compared with
other predictions. It turns out that the three-body force gives a repulsive
contribution to the equation of state which is stronger at higher density and
as a consequence reduces the critical temperature of liquid-gas phase
transition. The calculated energy per nucleon of hot asymmetric nuclear matter
is shown to satisfy a simple quadratic dependence on asymmetric parameter
as in the zero-temperature case. The symmetry energy and its density
dependence have been obtained and discussed. Our results show that the
three-body force affects strongly the high-density behavior of the symmetry
energy and makes the symmetry energy more sensitive to the variation of
temperature. The temperature dependence and the isospin dependence of other
physical quantities, such as the proton and neutron single particle potentials
and effective masses are also studied. Due to the additional repulsion produced
by the three-body force contribution, the proton and neutron single particle
potentials are correspondingly enhanced as similar to the zero-temperature
case.Comment: 16 pages, 8 figure
Difficulties in probing density dependent symmetry potential with the HBT interferometry
Based on the updated UrQMD transport model, the effect of the symmetry
potential energy on the two-nucleon HBT correlation is investigated with the
help of the coalescence program for constructing clusters, and the CRAB
analyzing program of the two-particle HBT correlation. An obvious non-linear
dependence of the neutron-proton (or neutron-neutron) HBT correlation function
() at small relative momenta on the stiffness factor of the
symmetry potential energy is found: when , the
increases rapidly with increasing , while it starts to saturate if
. It is also found that both the symmetry potential energy
at low densities and the conditions of constructing clusters at the late stage
of the whole process influence the two-nucleon HBT correlation with the same
power.Comment: 11 pages, 4 figure
Kaon differential flow in relativistic heavy-ion collisions
Using a relativistic transport model, we study the azimuthal momentum
asymmetry of kaons with fixed transverse momentum, i.e., the differential flow,
in heavy-ion collisions at beam momentum of 6 GeV/c per nucleon, available from
the Alternating Gradient Synchrotron (AGS) at the Brookhaven National
Laboratory (BNL). We find that in the absence of kaon potential the kaon
differential flow is positive and increases with transverse momentum as that of
nucleons. The repulsive kaon potential as predicted by theoretical models,
however, reduces the kaon differetnial flow, changing it to negative for kaons
with low momenta. Cancellation between the negative differential flow at low
mementa and the positive one at high momenta is then responsible for the
experimentally observed nearly vanishing in-plane transverse flow of kaons in
heavy ion experiments.Comment: Phys. Rev. C in pres
Differential flow in heavy-ion collisions at balance energies
A strong differential transverse collective flow is predicted for the first
time to occur in heavy-ion collisions at balance energies. We also give a novel
explanation for the disappearance of the total transverse collective flow at
the balance energies. It is further shown that the differential flow especially
at high transverse momenta is a useful microscope capable of resolving the
balance energy's dual sensitivity to both the nuclear equation of state and
in-medium nucleon-nucleon cross sections in the reaction dynamics.Comment: Phys. Rev. Lett. (1999) in pres
Field-induced suppression of the pi-band superconductivity and magnetic hysteresis in the microwave surface resistance of MgB_2 at temperatures near T_c
We report on the magnetic-field-induced variations of the microwave surface
resistance, R_s, in a polycrystalline MgB_2 sample, at different values of
temperature. We have detected a magnetic hysteresis in R_s, which exhibits an
unexpected plateau on decreasing the DC magnetic field below a certain value.
In particular, at temperatures near T_c the hysteresis manifests itself only
through the presence of the plateau. Although we do not quantitatively justify
the anomalous shape of the magnetic hysteresis, we show that the results
obtained in the reversible region of the R_s(H) curve can be quite well
accounted for by supposing that, in this range of magnetic field, the pi-gap is
almost suppressed by the applied field and, consequently, all the pi-band
charge carriers are quasiparticles. On this hypothesis, we have calculated
R_s(H) supposing that fluxons assume a conventional (single core) structure and
the flux dynamics can be described in the framework of conventional models.
From the fitting of the experimental results, we determine the values of
H_{c2}^pi(T) at temperatures near T_c. In our opinion, the most important
result of our investigation is that, at least at temperatures near T_c, the
value of the applied field that separates the reversible and irreversible
regions of the R_s(H) curves is just H_{c2}^pi(T); a qualitative discussion of
the possible reason of this finding is given.Comment: 20 pages, 8 embedded figures, 2 Appendices, accepted for publication
in Supercond. Sci. Techno
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