230 research outputs found
Correlation studies of fission fragment neutron multiplicities
We calculate neutron multiplicities from fission fragments with specified
mass numbers for events having a specified total fragment kinetic energy. The
shape evolution from the initial compound nucleus to the scission
configurations is obtained with the Metropolis walk method on the
five-dimensional potential-energy landscape, calculated with the
macroscopic-microscopic method for the three-quadratic-surface shape family.
Shape-dependent microscopic level densities are used to guide the random walk,
to partition the intrinsic excitation energy between the two proto-fragments at
scission, and to determine the spectrum of the neutrons evaporated from the
fragments. The contributions to the total excitation energy of the resulting
fragments from statistical excitation and shape distortion at scission is
studied. Good agreement is obtained with available experimental data on neutron
multiplicities in correlation with fission fragments from U(n,f). At higher neutron energies a superlong fission mode appears which
affects the dependence of the observables on the total fragment kinetic energy.Comment: 12 pages, 10 figure
Enhanced Event-by-Event Fluctuations in Pion Multiplicity as a Signal of Disoriented Chiral Condensates at RHIC
The factorial moments of the pion multiplicity distributions are calculated
with HIJING and UrQMD and found to be independent of the pT range included, in
contrast to recent simulations with the linear sigma model which leads to large
enhancements for pions with transverse kinetic energies below 200 MeV. This
supports the use of the ratio of the factorial moments of low and high pT pions
as a signal of ``new'' physics at low momentum scales, such as the formation of
disoriented chiral condensates.Comment: 4 pages total, incl 4 eps figures ([email protected]
Angular momentum sharing in dissipative collisions
Light charged particles emitted by the projectile-like fragment were measured
in the direct and reverse collision of Nb and Sn at 25 AMeV. The
experimental multiplicities of Hydrogen and Helium particles as a function of
the primary mass of the emitting fragment show evidence for a correlation with
net mass transfer. The ratio of Hydrogen and Helium multiplicities points to a
dependence of the angular momentum sharing on the net mass transfer.Comment: 8 pages, 2 figure
Larger Domains from Resonant Decay of Disoriented Chiral Condensates
The decay of disoriented chiral condensates into soft pions is considered
within the context of a linear sigma model. Unlike earlier analytic studies,
which focused on the production of pions as the sigma field rolled down toward
its new equilibrium value, here we focus on the amplification of
long-wavelength pion modes due to parametric resonance as the sigma field
oscillates around the minimum of its potential. This process can create larger
domains of pion fluctuations than the usual spinodal decomposition process, and
hence may provide a viable experimental signature for chiral symmetry breaking
in relativistic heavy ion collisions; it may also better explain physically the
large growth of domains found in several numerical simulations.Comment: 4pp, 2 figs, Revtex. Minor revisions, conclusions unchange
Chiral Fluid Dynamics and Collapse of Vacuum Bubbles
We study the expansion dynamics of a quark-antiquark plasma droplet from an
initial state with restored chiral symmetry. The calculations are made within
the linear model scaled with an additional scalar field representing
the gluon condensate. We solve numerically the classical equations of motion
for the meson fields coupled to the fluid-dynamical equations for the plasma.
Strong space-time oscillations of the meson fields are observed in the course
of the chiral transition. A new phenomenon, the formation and collapse of
vacuum bubbles, is also predicted. The particle production due to the
bremsstrahlung of the meson fields is estimated.Comment: 12 pages Revtex,5 figures, Figures modified, minor changes in text.
To be published in Phys. Rev. Let
Lambda flow in heavy-ion collisions: the role of final-state interactions
Lambda flow in Ni+Ni collisions at SIS energies is studied in the
relativistic transport model (RVUU 1.0). It is found that for primordial
lambdas the flow is considerably weaker than proton flow. The inclusion of
final-state interactions, especially the propagation of lambdas in mean-field
potential, brings the lambda flow close to that of protons. An accurate
determination of lambda flow in heavy-ion experiments is shown to be very
useful for studying lambda properties in dense matter.Comment: 14 pages, LaTeX, figures available from [email protected], to appear
in Phys. Rev.
From QFT to DCC
A quantum field theoretical model for the dynamics of the disoriented chiral
condensate is presented. A unified approach to relate the quantum field theory
directly to the formation, decay and signals of the DCC and its evolution is
taken. We use a background field analysis of the O(4) sigma model keeping
one-loop quantum corrections (quadratic order in the fluctuations). An
evolution of the quantum fluctuations in an external, expanding metric which
simulates the expansion of the plasma, is carried out. We examine, in detail,
the amplification of the low momentum pion modes with two competing effects,
the expansion rate of the plasma and the transition rate of the vacuum
configuration from a metastable state into a stable state.We show the effect of
DCC formation on the multiplicity distributions and the Bose-Einstein
correlations.Comment: 34 pages, 10 figure
Comparative Analysis of the Mechanisms of Fast Light Particle Formation in Nucleus-Nucleus Collisions at Low and Intermediate Energies
The dynamics and the mechanisms of preequilibrium-light-particle formation in
nucleus-nucleus collisions at low and intermediate energies are studied on the
basis of a classical four-body model. The angular and energy distributions of
light particles from such processes are calculated. It is found that, at
energies below 50 MeV per nucleon, the hardest section of the energy spectrum
is formed owing to the acceleration of light particles from the target by the
mean field of the projectile nucleus. Good agreement with available
experimental data is obtained.Comment: 23 pages, 10 figures, LaTeX, published in Physics of Atomic Nuclei
v.65, No. 8, 2002, pp. 1459 - 1473 translated from Yadernaya Fizika v. 65,
No. 8, 2002, pp. 1494 - 150
Upper bounds for number of removed edges in the Erased Configuration Model
Models for generating simple graphs are important in the study of real-world
complex networks. A well established example of such a model is the erased
configuration model, where each node receives a number of half-edges that are
connected to half-edges of other nodes at random, and then self-loops are
removed and multiple edges are concatenated to make the graph simple. Although
asymptotic results for many properties of this model, such as the limiting
degree distribution, are known, the exact speed of convergence in terms of the
graph sizes remains an open question. We provide a first answer by analyzing
the size dependence of the average number of removed edges in the erased
configuration model. By combining known upper bounds with a Tauberian Theorem
we obtain upper bounds for the number of removed edges, in terms of the size of
the graph. Remarkably, when the degree distribution follows a power-law, we
observe three scaling regimes, depending on the power law exponent. Our results
provide a strong theoretical basis for evaluating finite-size effects in
networks
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