Nucleon-nucleon Elastic Cross Sections Research Articles

Accessing the in-medium effects on nucleon-nucleon elastic cross section with collective flows and nuclear stopping

A systematic study of the in-medium correction factor (F) on nucleon-nucleon elastic cross section is performed within the Ultra-relativistic Quantum Molecular Dynamics (UrQMD) model. The effects of the beam energy dependence of F on the directed, elliptic flow and nuclear stopping in 197Au+197Au collisions with energy ranging from 0.09 to 0.8A GeV are explored. It is found that the directed, elliptic flow and nuclear stopping at relatively low energies are very sensitive to F, and the sensitivity gradually weakens with increasing beam energy. The beam energy dependent in-medium correction factor F is deduced from the comparison of the excitation functions of the directed, elliptic flow and nuclear stopping between the calculated results and the FOPI experimental data.

Nucleon-nucleon bremsstrahlung of dark gauge bosons and revised supernova constraints

We calculate the rate of production of hypothetical light vector bosons (LVBs) from nucleon-nucleon bremsstrahlung reactions in the soft radiation limit directly in terms of the measured nucleon-nucleon elastic cross sections. We use these results and the observation of neutrinos from supernova SN1987a to deduce constraints on the couplings of vector bosons with masses $\lesssim 200$ MeV to either electric charge (dark photons) or to baryon number. We establish for the first time strong constraints on LVB that couple only to baryon number, and revise earlier constraints on the dark photon. For the latter, we find that the excluded region of parameter space is diminished by about a factor of 10.

Collective flow of light particles in Au + Au collisions at intermediate energies

The Skyrme potential energy density functional is introduced into the Ultrarelativistic Quantum Molecular Dynamics (UrQMD) model and the updated version is applied to studying the directed and elliptic flows of light particles (protons, neutrons, deuterons, tritons, $^3$He and $^4$He) in $^{197}$Au+$^{197}$Au collisions at beam energies 150, 250 and 400 MeV/nucleon. The results are compared with the recent FOPI experimental data. It is found that the yields and collective flows of light particles can be described quite well. The influence of the equation of state (EoS), medium-modified nucleon-nucleon elastic cross sections (NNECS) and cluster recognition criteria on the directed and elliptic flows is studied in detail. It is found that the flows of light particles are sensitive to the medium-modified NNECS, but not sensitive to the isospin dependent cluster recognition criteria. It seems difficult, however, even with the new data and calculations, to obtain a more accurate constraint on the nuclear incompressibility $K_0$ than the interval 200-260 MeV.

Study of nuclear stopping in Au+Au collisions at alternating gradient synchrotron energies by the ultra-relativistic quantum molecular dynamic model

Nuclear stopping in Au+Au collisions at alternating gradient synchrotron energies is studied in the framework of the modified ultra-relativistic quantum molecular dynamics transport model, in which mean field potentials of both formed and “preformed” hadrons (from string fragmentation), medium-modified nucleon-nucleon elastic cross sections, and cluster recognition criteria are considered. It is found that nuclear stopping is influenced by both the mean field potentials of formed and “preformed” hadrons and the medium modification of nucleon-nucleon elastic cross section. The free proton number is higher than that from the experimental rapidity distribution in the central region of rapidity distribution, which can be understood by considering the new criteria of judging fragments.

Nonequilibrium dynamics in heavy-ion collisions at low energies available at the GSI Schwerionen Synchrotron

The updated ultrarelativistic quantum molecular dynamics (UrQMD) model, a microscopic transport model, is used to study the directed and elliptic collective flows and the nuclear stopping in Au$+$Au collisions at incident energies covered by INDRA and lower-energy FOPI experiments. It is seen clearly that these observables are sensitive to both the potential terms (including isoscalar and isovector parts as well as the momentum dependent term) in the equation of state (EoS) and the collision term [including the Pauli blocking and the medium-modified nucleon-nucleon elastic cross section (NNECS)]. The momentum modifications of both, the mean-field potentials and the density dependent NNECS, are found to affect the collectivity of heavy-ion collisions. At INDRA energies ($\ensuremath{\leqslant}$$150$ MeV/nucleon) the dynamic transport with a soft EoS with momentum dependence and with the momentum-modified density-dependent NNECS describes the directed flow exhibited by hydrogen isotopes ($Z=1$) emitted at midrapidity fairly well.

Transport model study of nuclear stopping in heavy-ion collisions over the energy range from0.09Ato160A GeV

Nuclear stopping in heavy-ion collisions over a beam energy range from SIS and AGS up to SPS is studied in the framework of the modified Ultrarelativistic Quantum Molecular Dynamics transport model, in which mean field potentials of both formed and ``preformed'' hadrons (from string fragmentation) and medium-modified nucleon-nucleon elastic cross sections are considered. It is found that nuclear stopping is influenced by both the stiffness of the equation of state and medium modifications of nucleon-nucleon cross sections at SIS energies. At high SPS energies, a two-bump structure is shown in the experimental rapidity distribution of free protons, which can be understood by considering the preformed hadron potentials.

Density and temperature dependence of nucleon-nucleon elastic cross section

The in-medium neutron-proton, proton-proton (neutron-neutron) elastic scattering cross sections ($\sigma_{np}^{*}$, $\sigma_{pp(nn)}^{*}$) are studied based on the effective Lagrangian of density dependent relativistic hadron theory in which the $\delta$[$a_0(980)$] meson is included. Our study shows that at low densities the $\sigma_{np}^*$ is about 3-4 times larger than $\sigma_{pp(nn)}^*$ and at densities higher than the normal density the isospin effect is almost washed out. Because of coupling to $\delta$ meson the $\sigma_{nn}^*$ and $\sigma_{pp}^*$ are different in isospin asymmetric medium following the splitting of the proton and neutron mass. The isospin effect on the density dependence of the in-medium nucleon elastic cross section is dominantly contributed by the isovector $\delta$ and $\rho$ mesons. The temperature effect on the $\sigma_{np}^*$ and $\sigma_{pp(nn)}^*$ is studied. It is shown that the temperature effect is weaker compared with the density effect but it becomes obvious as density increases.

Screening effects on the elastic nucleon-nucleon cross section in relativistic nuclear matter

We investigate the screening effects on the nucleon-nucleon elastic cross section inside nuclear matter at zero and finite temperature. The N - N interaction is described phenomenologically via meson exchanges (σ, π and ω) in the framework of a relativistic lagrangian model. The expressions for the in-medium meson propagators, which take into account the effects of matter polarization and renormalized vacuum polarization, are used in the calculation of the various meson contributions to the total cross section in the one-boson exchange approximation. The final expressions allow a satisfactory fit of the Coulomb substracted proton-proton cross section data in vacuum. At finite density or temperature the cross section is reduced as compared to the vacuum values and compatible with the existing experimental constraints. At high density the p- p differential cross section gets increasingly forward and backward peaked. The σ-ω mixing plays a key role in the understanding of the results.