120 research outputs found

    Isospin dependence of relative yields of K+K^+ and K0K^0 mesons at 1.528 AGeV

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    Results on K+K^+ and K0K^0 meson production in 4496^{96}_{44}Ru + 4496^{96}_{44}Ru and 4096^{96}_{40}Zr + 4096^{96}_{40}Zr collisions at a beam kinetic energy of 1.528AA GeV, measured with the FOPI detector at GSI-Darmstadt, are investigated as a possible probe of isospin effects in high density nuclear matter. The measured double ratio (K+/K0K^+/K^0)Ru_{Ru}/(K+/K0K^+/K^0)Zr_{Zr} is compared to the predictions of a thermal model and a Relativistic Mean Field transport model using two different collision scenarios and under different assumptions on the stiffness of the symmetry energy. We find a good agreement with the thermal model prediction and the assumption of a soft symmetry energy for infinite nuclear matter while more realistic transport simulations of the collisions show a similar agreement with the data but also exhibit a reduced sensitivity to the symmetry term.Comment: 5 pages, 3 figures. accepted for publication in Phys. Rev.

    Two-proton small-angle correlations in central heavy-ion collisions: a beam-energy and system-size dependent study

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    Small-angle correlations of pairs of protons emitted in central collisions of Ca + Ca, Ru + Ru and Au + Au at beam energies from 400 to 1500 MeV per nucleon are investigated with the FOPI detector system at SIS/GSI Darmstadt. Dependences on system size and beam energy are presented which extend the experimental data basis of pp correlations in the SIS energy range substantially. The size of the proton-emitting source is estimated by comparing the experimental data with the output of a final-state interaction model which utilizes either static Gaussian sources or the one-body phase-space distribution of protons provided by the BUU transport approach. The trends in the experimental data, i.e. system-size and beam energy dependences, are well reproduced by this hybrid model. However, the pp correlation function is found rather insensitive to the stiffness of the equation of state entering the transport model calculations.Comment: 9 pages, 8 figures, accepted at Eur. Phys. Journ.

    Centrality dependence of subthreshold ϕ\phi meson production in Ni+Ni collisions at 1.9A GeV

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    We analysed the ϕ\phi meson production in central Ni+Ni collisions at the beam kinetic energy of 1.93A GeV with the FOPI spectrometer and found the production probability per event of [8.6 ± 1.6 (stat)±1.5 (syst)]×104[8.6 ~\pm~ 1.6 ~(\text{stat}) \pm 1.5 ~(\text{syst})] \times 10^{-4}. This new data point allows for the first time to inspect the centrality dependence of the subthreshold ϕ\phi meson production in heavy-ion collisions. The rise of ϕ\phi meson multiplicity per event with mean number of participants can be parameterized by the power function with exponent α=1.8±0.6\alpha = 1.8 \pm 0.6. The ratio of ϕ\phi to K\text{K}^- production yields seems not to depend within the experimental uncertainties on the collision centrality, and the average of measured values was found to be 0.36±0.050.36 \pm 0.05.Comment: 9 pages, 5 figure

    Charged pion production in 4496^{96}_{44}Ru+4496^{96}_{44}Ru collisions at 400A and 1528A MeV

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    We present transverse momentum and rapidity spectra of charged pions in central Ru + Ru collisions at 400AA and 1528AA MeV. The data exhibit enhanced production at low transverse momenta compared to the expectations from the thermal model that includes the decay of Δ(1232)\Delta(1232)-resonances and thermal pions. Modification of the Δ\Delta-spectral function and the Coulomb interaction are necessary to describe the detailed shape of the transverse momentum spectra. Within the framework of the thermal model, the freeze-out radii of pions are similar at both beam energies. The IQMD model reproduces the shapes of the transverse momentum and rapidity spectra of pions, but the predicted absolute yields are larger than in the measurements, especially at lower beam energy.Comment: 12 pages, 11 figure

    Strange meson production in Al+Al collisions at 1.9A GeV

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    The production of K+^+, K^- and φ\varphi(1020) mesons is studied in Al+Al collisions at a beam energy of 1.9A GeV which is close or below the production threshold in NN reactions. Inverse slopes, anisotropy parameters, and total emission yields of K±^{\pm} mesons are obtained. A comparison of the ratio of kinetic energy distributions of K^- and K+^+ mesons to the HSD transport model calculations suggests that the inclusion of the in-medium modifications of kaon properties is necessary to reproduce the ratio. The inverse slope and total yield of ϕ\phi mesons are deduced. The contribution to K^- production from ϕ\phi meson decays is found to be [17 ±\pm 3 (stat) 7+2^{+2}_{-7} (syst)] %. The results are in line with previous K±^{\pm} and ϕ\phi data obtained for different colliding systems at similar incident beam energies.Comment: 16 pages, 11 figure

    Measurement of K(892)0K^*(892)^0 and K0K^0 mesons in Al+Al collisions at 1.9AA GeV

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    New measurement of sub-threshold K(892)0K^*(892)^0 and K0K^0 production is presented. The experimental data complete the measurement of strange particles produced in Al+Al collisions at 1.9AA GeV measured with the FOPI detector at SIS/GSI. The K(892)0K^*(892)^0 / K0K^0 yield ratio is found to be 0.0315±0.006(stat.)±0.012(syst.)0.0315\pm 0.006 (\mathrm{stat.})\pm 0.012 (\mathrm{syst.}) and is in good agreement with the UrQMD model prediction. These measurements provide information on in-medium cross section of K+K^+ - π\pi^- fusion which is the dominant process on sub-threshold K(892)0K^*(892)^0 production.Comment: 5 pages, 4 figures, accepted for publication in Phys. Rev.

    Identification of baryon resonances in central heavy-ion collisions at energies between 1 and 2 AGeV

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    The mass distributions of baryon resonances populated in near-central collisions of Au on Au and Ni on Ni are deduced by defolding the ptp_t spectra of charged pions by a method which does not depend on a specific resonance shape. In addition the mass distributions of resonances are obtained from the invariant masses of (p,π±)(p, \pi^{\pm}) pairs. With both methods the deduced mass distributions are shifted by an average value of -60 MeV/c2^2 relative to the mass distribution of the free Δ(1232)\Delta(1232) resonance, the distributions descent almost exponentially towards mass values of 2000 MeV/c^2. The observed differences between (p,π)(p, \pi^-) and (p,π+)(p, \pi^+) pairs indicate a contribution of isospin I=1/2I = 1/2 resonances. The attempt to consistently describe the deduced mass distributions and the reconstructed kinetic energy spectra of the resonances leads to new insights about the freeze out conditions, i.e. to rather low temperatures and large expansion velocities.Comment: 30 pages, 13 figures, Latex using documentstyle[12pt,a4,epsfig], to appear in Eur. Phys. J.
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