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H. W. Barz

Publications and source records attributed to H. W. Barz.

15 recordsLinked to original sources
arXiv2009

Propagation of vector-meson spectral-functions in a BUU type transport model: Application to di-electron production

The time evolution of vector-meson spectral-functions is studied within a kinetic theory approach. We implement this formalism in a BUU type transport model. Applications focus on rho and omega mesons being important pieces for the interpretation of the di-electron invariant mass spectrum measured by the HADES collaboration for the reaction C + C at 2 AGeV bombarding energy. Since the evolution of the spectral functions is driven by the local density, the in-medium modifications are tiny for small collision systems within this approach.

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arXiv2006

First observation of in-medium effects on phase space distributions of antikaons measured in proton-nucleus collisions

Differential production cross sections of $K^{\pm}$ mesons have been measured in $p$ + C and $p$ + Au collisions at 1.6, 2.5 and 3.5 GeV proton beam energy. At beam energies close to the production threshold, the $K^-$ 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 $NY \to K^- NN$ and an attractive in-medium $K^-N$ potential at saturation density.

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arXiv2005

Transport Theories for Heavy Ion Collisions in the 1 AGeV Regime

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.

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arXiv2004

Angular distribution and azimuthal asymmetry for pentaquark production in proton-proton collisions

Angular distributions for production of the $Θ^+$ pentaquark are calculated for the collisions of polarized protons with polarized target protons. We compare calculations based on different assumptions concerning spin and parity ($J=1/2^\pm,3/2^\pm$) of the $Θ^+$ state. For a wide class of interactions the spin correlation parameters describing the asymmetric angular distributions are calculated up to 250 MeV above production threshold. The deviations from the near threshold behavior are investigated.

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arXiv2004

Calculations of K+, K- and phi Production in Near-Threshold Proton-Nucleus Collisions

K+, K- and ϕmeson production in proton-nucleus (pA) collisions has been calculated within a BUU transport model. It is shown that the nucleon-hyperon strangeness transfer channel is essential. The role of three-body reactions has been investigated within the medium. The targetmass dependence of $ϕ$ production is predicted to give important information on the in-medium properties of all three mesons.

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arXiv2004

Phi meson production in near threshold proton-nucleus collisions

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.

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arXiv2003

Contribution of the nucleon-hyperon reaction channels to K$^-$ production in proton-nucleus collisions

The cross sections for producing K$^-$ 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$^-$ production in near-threshold proton-nucleus collisions.

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arXiv2002

Production of phi mesons in subthreshold heavy-ion collisions

Within a BUU type transport model we study $ϕ$ meson production in subthreshold Ni+Ni and Ru+Ru reactions. For the first time we included in our model the elementary reaction channels $ρ+N,Δ\to ϕ+N$, $π+N(1520) \to ϕ+N$ and $πρ$ $\to$ $ϕ$. In spite of a substantial increase of the $ϕ$ multiplicities by these channels our results stay significantly below the preliminary experimental data.

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arXiv2002

Subthreshold phi-meson production in heavy-ion collisions

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 ϕmultiplicities by these channels the results stay below the tentative numbers extracted from experimental data.

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arXiv2000

The role of three-body collisions in phi-meson production processes near threshold

The amplitude of subthreshold phi-meson production is calculated using dominant tree-level diagrams for three-body collisions. It is shown that the production can overwhelmingly be described by two-step processes. The effect of the genuine three-body contribution (i.e. the contribution which cannot be factorized) is discussed. The production rate of phi-mesons is presented for proton induced reactions on carbon.

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arXiv1999

On the Space-Time Difference of Proton and Composite Particle Emission in Central Heavy-Ion Reactions at 400 AMeV

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 AMeV 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 space-time emission points of p, d, t, 3He, alpha particles is presented for the first time.

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arXiv1998

Combined effects of nuclear Coulomb field, radial flow, and opaqueness on two-pion correlations

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.

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arXiv1997

Coulomb Effects on Particle Spectra in Relativistic Nuclear Collisions

Coulomb effects on $π^\pm$ and $K^\pm$ spectra in relativistic nuclear collisions are investigated. At collision energies around 1 GeV the ratio of $π^-$ to $π^+$ 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 $π^\pm$ and $K^\pm$ at RHIC and LHC energies are given.

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arXiv1997

Freeze-Out Time in Ultrarelativistic Heavy Ion Collisions from Coulomb Effects in Transverse Pion Spectra

The influence of the nuclear Coulomb field on transverse spectra of $π^+$ and $π^-$ 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 $π^-/π^+$ 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.

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