682 research outputs found
Can Momentum Correlations Proof Kinetic Equilibration in Heavy Ion Collisions at 160 AGeV?
We perform an event-by-event analysis of the transverse momentum distribution
of final state particles in central Pb(160AGeV)+Pb collisions within a
microscopic non-equilibrium transport model (UrQMD). Strong influence of
rescattering is found. The extracted momentum distributions show less
fluctuations in A+A collisions than in p+p reactions. This is in contrast to
simplified p+p extrapolations and random walk models.Comment: 9 pages, 3 eps figures, submitted to Phys. Lett.
Quantum Stability of Accelerated Black Holes
We study quantum aspects of the accelerated black holes in some detail.
Explicitly shown is the fact that a uniform acceleration stabilizes certain
charged black holes against the well-known thermal evaporation. Furthermore, a
close inspection of the geometry reveals that this is possible only for
near-extremal black holes and that most nonextremal varieties continue to
evaporate with a modified spectrum under the acceleration. We also introduce a
two-dimensional toy model where the energy-momentum flow is easily obtained for
general accelerations, and find the behavior to be in accordance with the
four-dimensional results. After a brief comparison to the classical system of a
uniformly accelerated charge, we close by pointing out the importance of this
result in the WKB expansion of the black hole pair-creation rate.Comment: LaTeX, 22 pages, 5 uuencoded figures (minor errors corrected, more
discussions on the case with black holes formed by gravitational collapse.
The Origin of Transverse Flow at the SPS
We study the transverse expansion in central Pb+Pb collisions at the CERN
SPS. Strong collective motion of hadrons can be created. This flow is mainly
due to meson baryon rescattering. It allows to study the angular distribution
of intermediate mass meson baryon interactions.Comment: submitted to Phys. Lett.
Non-vanishing Magnetic Flux through the Slightly-charged Kerr Black Hole
In association with the Blanford-Znajek mechanism for rotational energy
extraction from Kerr black holes, it is of some interest to explore how much of
magnetic flux can actually penetrate the horizon at least in idealized
situations. For completely uncharged Kerr hole case, it has been known for some
time that the magnetic flux gets entirely expelled when the hole is
maximally-rotating. In the mean time, it is known that when the rotating hole
is immersed in an originally uniform magnetic field surrounded by an ionized
interstellar medium (plasma), which is a more realistic situation, the hole
accretes certain amount of electric charge. In the present work, it is
demonstrated that as a result of this accretion charge small enough not to
disturb the geometry, the magnetic flux through this slightly charged Kerr hole
depends not only on the hole's angular momentum but on the hole's charge as
well such that it never vanishes for any value of the hole's angular momentum.Comment: 33pages, 1 figure, Revtex, some comments added, typos correcte
Coherent photon bremsstrahlung and dynamics of heavy-ion collisions: comparison of different models
Differential spectra of coherent photon bremsstrahlung in relativistic heavy
ion collisions are calculated within various schematic models of the
projectile-target stopping. Two versions of the degradation length model, based
on a phenomenological deceleration law, are considered. The simple shock wave
model is studied analytically. The predictions of these models agree in the
soft photon limit, where the spectrum is determined only by the final velocity
distribution of charged particles. The results of these models in the case of
central Au+Au collisions at various bombarding energies are compared with the
predictions of the microscopic transport model UrQMD. It is shown that at the
AGS energy the coherent photon bremsstrahlung exceeds the photon yield from
-decays at photon energies \omega\loo 50 MeV.Comment: 23 pages RevTeX, 9 eps Figure
Excitation Function of Energy Density and Partonic Degrees of Freedom in Relativistic Heavy Ion Collisions
We estimate the energy density pile-up at mid-rapidity in central Pb+Pb
collisions from 2 - 200 GeV/nucleon. The energy density is decomposed into
hadronic and partonic contributions. A detailed analysis of the collision
dynamics in the framework of a microscopic transport model shows the importance
of partonic degrees of freedom and rescattering of leading (di)quarks in the
early phase of the reaction for lab-energies > 30 GeV/nucleon. In Pb+Pb
collisions at 160 GeV/nucleon the energy density reaches up to 4 GeV/fm^3, 95%
of which are contained in partonic degrees of freedom.Comment: 10 pages, 4 figure
Microscopic calculations of stopping and flow from 160AMeV to 160AGeV
The behavior of hadronic matter at high baryon densities is studied within
Ultrarelativistic Quantum Molecular Dynamics (URQMD). Baryonic stopping is
observed for Au+Au collisions from SIS up to SPS energies. The excitation
function of flow shows strong sensitivities to the underlying equation of state
(EOS), allowing for systematic studies of the EOS. Effects of a density
dependent pole of the -meson propagator on dilepton spectra are studied
for different systems and centralities at CERN energies.Comment: Proceedings of the Quark Matter '96 Conference, Heidelberg, German
Reaction dynamics in Pb+Pb at the CERN/SPS: from partonic degrees of freedom to freeze-out
We analyze the reaction dynamics of central Pb+Pb collisions at 160
GeV/nucleon. First we estimate the energy density pile-up at mid-rapidity and
calculate its excitation function: The energy density is decomposed into
hadronic and partonic contributions. A detailed analysis of the collision
dynamics in the framework of a microscopic transport model shows the importance
of partonic degrees of freedom and rescattering of leading (di)quarks in the
early phase of the reaction for E > 30 GeV/nucleon. The energy density reaches
up to 4 GeV/fm^3, 95% of which are contained in partonic degrees of freedom. It
is shown that cells of hadronic matter, after the early reaction phase, can be
viewed as nearly chemically equilibrated. This matter never exceeds energy
densities of 0.4 GeV/fm^3, i.e. a density above which the notion of separated
hadrons loses its meaning. The final reaction stage is analyzed in terms of
hadron ratios, freeze-out distributions and a source analysis for final state
pions.Comment: 10 pages, 7 figures, Proceedings of the Erice School on Nuclear
Physics in Erice, Sicily, Italy, September 17 -25 1998; to be published in
Progress in Particle and Nuclear Physics Vol. 4
Photodisintegration of three- and four- nucleon systems
Three- and four-nucleon photodisintegration processes are quite efficiently
treated by means of effective two-body integral equations in momentum space. We
recall some aspects of their derivation, present previous and most recent
results obtained within this framework, and discuss general features, trends
and effects observed in these investigations: At low energies final-state
interaction plays an important role. Even more pronounced is the effect of
meson exchange currents. A considerable potential dependence shows up in the
low-energy peak region. The different peak heights are found to be closely
correlated with the corresponding binding energies. Above the peak region only
the difference between potentials with or without p-wave contributions remains
relevant. In the differential cross sections the electric quadrupole
contributions have to be taken into account. The remarkable agreement between
theory and experiment in - radiative capture is achieved only when
incorporating this contribution, together with most of the above-mentioned
effects. In the final part of this report we briefly review also methods
developed, and results achieved in three- and four- nucleon
electrodisintegration. We, in particular, compare them with a recent access to
this problem, based on the construction of nucleon-nucleus potentials via
Marchenko inversion theory.Comment: 20 pages LaTeX and 22 postscript figures included, uses epsfig.sty
and espcrc1.sty. Invited talk at the XVth International Conference on
Few-Body Problems in Physics (22-26 July, 1997, Groningen, The Netherlands).
To be published in the conference proceedings in Nucl. Phys.
Deceleration and trapping of heavy diatomic molecules using a ring-decelerator
We present an analysis of the deceleration and trapping of heavy diatomic
molecules in low-field seeking states by a moving electric potential. This
moving potential is created by a 'ring-decelerator', which consists of a series
of ring-shaped electrodes to which oscillating high voltages are applied.
Particle trajectory simulations have been used to analyze the deceleration and
trapping efficiency for a group of molecules that is of special interest for
precision measurements of fundamental discrete symmetries. For the typical case
of the SrF molecule in the (N,M) = (2, 0) state, the ring-decelerator is shown
to outperform traditional and alternate-gradient Stark decelerators by at least
an order of magnitude. If further cooled by a stage of laser cooling, the
decelerated molecules allow for a sensitivity gain in a parity violation
measurement, compared to a cryogenic molecular beam experiment, of almost two
orders of magnitude
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