Search

We study the cross section of ϕ meson production in proton-nucleus collisions. The decay width of the ϕ meson is affected by the change of the masses of the ϕ, K + and K − mesons in the medium. A strong attractive K − potential leads to a broadening of the ϕ meson, which results in a measurable change of the behavior of the cross section as a function of the target mass.

Small-angle correlations of pairs of nonidentical light charged particles produced in central collisions of heavy ions in the A= 100 mass region at a beam energy of 400 A· MeV are investigated with the FOPI detector system at GSI Darmstadt. The difference of longitudinal correlation functions with the relative velocity parallel and anti-parallel to the center-of-mass velocity of the pair in the central source frame is studied. This method allows extracting the apparent space-time differences of the emission of the charged particles. Comparing the correlations with results of a final-state interaction model delivers quantitative estimates of these asymmetries. Time delays as short as 1~fm/c or – alternatively – source radius differences of a few tenth fm are resolved. The strong collective expansion of the participant zone introduces not only an apparent reduction of the source radius but also a modification of the emission times. After correcting for both effects a complete sequence of the space-time emission of p, d, t, 3He, α particles is presented for the first time.

In a transport model calculation we study some of the basic assumptions of thermal models, We find that at SIS energies the particles are mainly produced in a highly non-equilibrium situation, At freeze-out only nucleons are thermalized.

Differential production cross sections of ${K}^{\ifmmode\pm\else\textpm\fi{}}$ mesons have been measured in $p+\mathrm{C}$ and $p+\mathrm{Au}$ collisions at 1.6, 2.5, and 3.5 GeV proton beam energy. At beam energies close to the production threshold, the ${K}^{\ensuremath{-}}$ multiplicity is strongly enhanced with respect to proton-proton collisions. According to microscopic transport calculations, this enhancement is caused by two effects: the strangeness exchange reaction $NY\ensuremath{\rightarrow}{K}^{\ensuremath{-}}NN$ and an attractive in-medium ${K}^{\ensuremath{-}}N$ potential at saturation density.

We compare multiplicities as well as rapidity and transverse momentum distributions of protons, pions and kaons calculated within presently available transport approaches for heavy ion collisions around 1 AGeV. For this purpose, three reactions have been selected: Au+Au at 1 and 1.48 AGeV and Ni+Ni at 1.93 AGeV., 15 pages, 13 figures, proceedings of Strange Quark Matter 2004, Cape Town, South Africa

The cross section for production of Phi mesons in proton-nucleus reactions is calculated as a function of the target mass. The decay width of the Phi meson is affected by the change of the masses of the Phi, K+ and K- mesons in the medium. A strong attractive K- potential leads to a measurable change of the behavior of the cross section as a function of of the target mass. Comparison between the kaon and electron decay modes are made., 4 pages, 1figure, new figure, new references

The cross sections for producing ${K}^{\ensuremath{-}}$ mesons in nucleon-hyperon elementary processes are estimated assuming one-pion exchange and using the experimentally known pion-hyperon cross sections. The results are implemented in a transport model which is applied to calculation of proton-nucleus collisions. In significant difference to earlier estimates for heavy-ion collisions the inclusion of the nucleon-hyperon cross section roughly doubles the ${K}^{\ensuremath{-}}$ production in near-threshold proton-nucleus collisions.

Correlations of two like charged pions emitted from a hot and charged spherically expanding nuclear system are investigated. The motion of the pions is described quantum mechanically using the Klein-Gordon equation which includes Coulomb field and pion absorption. Flow modifies the radial distribution of the source function and rescales the pion wave functions. The radii extracted from the correlation functions are calculated in sideward and outward direction as a function of the pair momentum. Comparison with recent measurements at SIS and AGS energies is made., 13 pages and 4 figures

Analysis of hard two-photon correlations measured in heavy-ion reactions at intermediate energies H.W.Barz1,2, B.Kämpfer1,2, GY. Wolf3,4, W. Bauer5 1Institut für Kern- und Hadronenphysik, FZ Rossendorf, PF 510119, 01314 Dresden, Germany 1Institut für Theoretische Physik, TU Dresden, Momrnsenstr. 13, 01062 Dresden, Germany 3GSI Darmstadt, PF 110552, 64220 Darmstadt, Germany 4KFKI RMKI Budapest, POB. 49, H-1525 Budapest, Hungary MSU Michigan, East Lansing, MI 48824, USA Abstract Recently reported measurements of hard photon correlations in the reactions 36Ar On 27Al at 95 A*MeV, 86Kr on natNi at 60 A*MeV, and 181Ta on 197Au at 39.5 A*MeV are analyzed. A Boltzmann-Ühling-Uhlenbeck transport model is used to describe the photon production by individual nucleon-nucleon collisions. In the lighter systems we find the best agreement with data when taking into account only the photons from first chance collisions of nucleons or photons produced during the Passage of the nuclei, while the model predicts also a considerable late-time emission of photons which leads to a depletion of the calculated correlation function. The accuracy of the present data daas not allow firm conclusions on the reliability of this late time evolution. Our inveatigatiions do not support a recently reported interference pattern in the heavy Ta + Au system.

Cross sections for production of pions, kaons and antikaons in heavy ion reactions are calculated in the framework of the hadrochemical model. For the reaction Ne on NaF satisfactory agreement to the measured data is reached in a wide range of energies. The importance of consecutive hadron-hadron collisions for the mass dependence is shown.

Taking into account a finite conversion rate the transition of nuclear matter to quark-gluon matter and vice versa is treated in a hydrodynamical model for heavy ion collisions. For fast conversion pure quark matter is formed for laboratory energies between 4 and 10 GeV per nucleon while below 4 GeV per nucleon matter stays in a mixed phase. Due to the large amount of latent heat the temperature does not exceed k T = 160 MeV.

A two-fluid model is presented which describes the interpenetration of nuclei colliding at ultra-relativistic energies. The two fluids are coupled by friction resulting from hadron-hadron collisions. The model calculations predict the deconfinement transition in the baryon-rich fragmentation region. The degree of the conversion and the maximum temperature depend sensitively on the characteristic time for the rearrangement of nuclear matter into the quark-gluon plasma.

Proton and pion spectra from relativistic heavy ion collisions are calculated in the framework of a hydrochemical model. An explanation of the striking absence of the delta decay pion peak in the observed pion spectra is suggested.

It is shown that subthreshold pion production in a collision between two nuclei at intermediate energy has to be attributed to an early stage of the collision and cannot take place together with a multifragmentation break-up at a later stage of the collision.

Strange particle production in heavy ion collisions is calculated in the framework of a hadrochemical model. It is compared with the results of cascade and fireball model calculations and experimental data. The importance of πN and πΔ collisions in producing kaons and hyperons is pointed out.

The advantage of the continuum shell-model in describing nuclear reactions producing hypernuclei is discussed. Numerical results obtained within this model for the (K−,π−) reaction on9Be,12C,16O and40Ca are presented.

It is shown that the differential cross section of (p,n) reactions with 1 GeV protons corresponding to a Fermi-type transition between mirror nuclei is sensitive to the isovector component of the nuclear quadrupole moment because of the nuclear spin-re-orientation effect.

In a fluid-dynamical model, the extra entropy production which arises from a nonequilibrium phase transition from nuclear to quark-gluon matter is calculated.

Coulomb effects on $\pi^\pm$ and $K^\pm$ spectra in relativistic nuclear collisions are investigated. At collision energies around 1 GeV the ratio of $\pi^-$ to $\pi^+$ is enhanced several times at low transverse momenta but less at ultrarelativistic energies. We describe the ratios at SIS, AGS and SPS energies with simple analytic models as well as more elaborate numerical models incorporating the expansion dynamics. The Coulomb effect depends on the properties of the source after the violent collision phase and provides information on source sizes, freeze-out times, and expansion velocities. Comparison with results from HBT analyses are made. Predictions for $\pi^\pm$ and $K^\pm$ at RHIC and LHC energies are given., Comment: 22 pages including 12 figures. Added figure and revisions

The influence of the nuclear Coulomb field on transverse spectra of $\pi^+$ and $\pi^-$ measured in $Pb+Pb$ reactions at 158 A GeV has been investigated. Pion trajectories are calculated in the field of an expanding fireball. The observed enhancement of the $\pi^-/\pi^+$ ratio at small momenta depends on the temperature and transverse expansion velocity of the source, the rapidity distribution of the net positive charge, and mainly the time of the freeze-out., Comment: 11 pages including 2 figures

Within a transport code of BUU type the production of phi-mesons in the reactions Ni+Ni at 1.93 AGeV and Ru+Ru at 1.69 AGeV is studied. New elementary reaction channels rho+N(Delta) to phi+N and pi+N(1520) to phi+N are included. In spite of a substantial increase of the \phi multiplicities by these channels the results stay below the tentative numbers extracted from experimental data., Comment: 17 pages(LaTeX), two new figures added

Describing the expanding fireballs in a relativistic gas dynamical approach the formation of deuterons is calculated under consideration of inmedium corrections. The theoretical results are in good agreement with experimental data.