Beyond mean field confrontation of different models with high transverse momentum proton spectra

Abstract

Several models have been proposed to simulate heavy ion reactions beyond the mean field level. The lack of data in phase space regions which may be sensitive to different treatments of fluctuations made it difficult to judge these approaches. The recently published high energy proton spectra, measured in the reaction 94 AMeV Ar + Ta, allow for the first time for a comparison of the models with data. We find that these spectra are reproduced by Quantum Molecular Dynamics (QMD) and BoltzmannÜhling Uhlenbeck (BUU) calculations. Models like Boltzmann Langevin (BL) in which additional fluctuations in momentum space are introduced overpredict the proton yield at very high energies. The BL approach has been successfully used to describe the recently measured very subthreshold kaon production assuming that the fluctuations provide the necessary energy to overcome the threshold in two body collisions. Our new findings suggest that the very subthreshold kaon production cannot be due to two body scattering and thus remains a open problem.

Semiclassical transport of particles with dynamical spectral functions

“…The dashed line is practically identical to the BUU calculations from Ref. [53] and is limited to collisions far below the kaon production threshold of √ s 0 ≈ 2.54 GeV. Thus the kaon production yield of (2.9±1.6) • 10 −9 b claimed in Ref.…”

“…9) only extend up to 200 MeV due to statistics, however, match with the experimental data within the errorbars. A similar comparison has been performed by Germain et al [53] where the traditional BUU and QMD calculations seem to be compatible with the experimental data within the statistics achieved whereas the Boltzmann Langevin (BL) calculations incorporating fluctuations in momentum space [34] overestimate the proton spectra by more than three orders of magnitude [53]. This rules out the latter BL approach but does not imply that our off-shell transport approach will properly describe the experimental spectra up to E kin = 350 MeV.…”

“…Our first applications we devote to nuclear reactions at GANIL (92 -95 A MeV) since here the more recent measurements have lead to conflicting results between different transport approaches [53]. We start with the reaction Ar + T a at 92 A MeV.…”

Isospin effects on the mass dependence of the balance energy

“…It is worth mentioning here that the isospin-dependent quantum molecular dynamics (IQMD) model has also been able to reproduce the other data (e.g. high-energy proton spectra, gamma production) in the incident energies relevant in this article [21,22]. The present aim is atleast twofold.…”

Strong Enhancement of Extremely Energetic Proton Production in Central Heavy Ion Collisions at Intermediate Energy

“…We have found that multistep two-body collisions, which play an important role in the production of protons with energy between 100 and 120 MeV, do not reproduce the trend of the most energetic protons ͑E NN p $ 130 MeV͒ which exhibits a stronger dependence on A part ͑b͒. Dynamical fluctuations [23] are not expected to lead to the A part quadratic behavior observed experimentally. Other effects, such as high momentum tails, are weakly dependent on density and, at the energy considered, density variation from central to peripheral impact parameters are small [24].…”