300,867 research outputs found

### Chiral expansion of the $\pi^0\rightarrow\gamma\gamma$ decay width

A chiral field theory of mesons has been applied to study the contribution of
the current quark masses to the $\pi^0\rightarrow\gamma\gamma$ decay width at
the next leading order. $2\%$ enhancement has been predicted and there is no
new parameter.Comment: 9 page

### Excitation function of nucleon and pion elliptic flow in relativistic heavy-ion collisions

Within a relativistic transport (ART) model for heavy-ion collisions, we show
that the recently observed characteristic change from out-of-plane to in-plane
elliptic flow of protons in mid-central Au+Au collisions as the incident energy
increases is consistent with the calculated results using a stiff nuclear
equation of state (K=380 MeV). We have also studied the elliptic flow of pions
and the transverse momentum dependence of both the nucleon and pion elliptic
flow in order to gain further insight about the collision dynamics.Comment: 8 pages, 2 figure

### Correlations of chaotic eigenfunctions: a semiclassical analysis

We derive a semiclassical expression for an energy smoothed autocorrelation
function defined on a group of eigenstates of the Schr\"odinger equation. The
system we considered is an energy-conserved Hamiltonian system possessing
time-invariant symmetry. The energy smoothed autocorrelation function is
expressed as a sum of three terms. The first one is analogous to Berry's
conjecture, which is a Bessel function of the zeroth order. The second and the
third terms are trace formulae made from special trajectories. The second term
is found to be direction dependent in the case of spacing averaging, which
agrees qualitatively with previous numerical observations in high-lying
eigenstates of a chaotic billiard.Comment: Revtex, 13 pages, 1 postscript figur

### The influence of reconstruction criteria on the sensitive probes of the symmetry potential

Different criteria of constructing clusters and tracing back $\Delta$
resonances from the intermediate-energy neutron-rich HICs are discussed by
employing the updated UrQMD transport model. It is found that both the
phase-space and the coordinate-density criteria affect the single and the
double neutron/proton ratios of free nucleons at small transverse momenta, but
the influence becomes invisible at large transverse momenta. The effect of
different methods of reconstructing freeze-out $\Delta$s on the
$\Delta^0/\Delta^{++}$ ratio is strong in a large kinetic energy region.Comment: 8 pages, 7 fig

### 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

### $\tau\to\rho\pi\pi\nu$ decays

Effective chiral theory of mesons is applied to study the four decay modes of
$\tau\to\rho\pi\pi\nu$. Theoretical values of the branching ratios are in
agreement with the data. The theory predicts that the $a_{1}$ resonance plays a
dominant role in these decays. There is no new parameter in this study.Comment: 12 pages and one figur

### Spreading Speed, Traveling Waves, and Minimal Domain Size in\ud Impulsive Reaction-diffusion Models

How growth, mortality, and dispersal in a species affect the species’ spread and persistence constitutes a central problem in spatial ecology. We propose impulsive reaction-diffusion equation models for species with distinct reproductive and dispersal stages. These models can describe a seasonal birth pulse plus nonlinear mortality and dispersal throughout the year. Alternatively they can describe seasonal harvesting, plus nonlinear birth and mortality as well as dispersal throughout the year. The population dynamics in the seasonal pulse is described by a discrete map that gives the density of the populationat the end stage as a possibly nonmonotone function of the density of the population at the beginning of the stage. The dynamics in the dispersal stage is governed by a nonlinear reaction-diffusion equation in a bounded or unbounded domain. We develop a spatially explicit theoretical framework that links species vital rates (mortality or fecundity) and dispersal characteristics with species’ spreading speeds, traveling wave speeds, as well as and minimal domain size for species persistence. We provide an explicit formula for the spreading speed in terms of model parameters, and show that the spreading speed can be characterized as the slowest speed of a class of traveling wave solutions. We also determine an explicit formula for the minimal domain size using model parameters. Our results show how the diffusion coefficient, and the combination of discrete- and continuous-time growth and mortality determine the spread and persistence dynamics of the population in a wide variety of ecological scenarios. Numerical simulations are presented to demonstrate the theoretical results

### Uranium on uranium collisions at relativistic energies

Deformation and orientation effects on compression, elliptic flow and
particle production in uranium on uranium collisions (UU) at relativistic
energies are studied within the transport model ART. The density compression in
tip-tip UU collisions is found to be about 30% higher and lasts approximately
50% longer than in body-body or spherical UU reactions. The body-body UU
collisions have the unique feature that the nucleon elliptic flow is the
highest in the most central collisions and remain a constant throughout the
reaction. We point out that the tip-tip UU collisions are more probable to
create the QGP at AGS and SPS energies while the body-body UU collisions are
more useful for studying properties of the QGP at higher energies.Comment: 8 pages + 4 figure

### Directed flow of neutral strange particles at AGS

Directed flow of neutral strange particles in heavy ion collisions at AGS is
studied in the ART transport model. Using a lambda mean-field potential which
is 2/3 of that for a nucleon as predicted by the constituent quark model,
lambdas are found to flow with protons but with a smaller flow parameter as
observed in experiments. For kaons, their repulsive potential, which is
calculated from the impulse approximation using the measured kaon-nucleon
scattering length, leads to a smaller anti-flow than that shown in the
preliminary E895 data. Implications of this discrepancy are discussed.Comment: 6 pages, 2 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
($C_{np,nn}$) at small relative momenta on the stiffness factor $\gamma$ of the
symmetry potential energy is found: when $\gamma \lesssim 0.8$, the $C_{np,nn}$
increases rapidly with increasing $\gamma$, while it starts to saturate if
$\gamma \gtrsim 0.8$. 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

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