Subthreshold Particle Production in Heavy-Ion Collisions (低および中高エネルギ-重イオン核反応機構)
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概要
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We have analysed subthreshold pion production on the basis of the VUU transport theory and the incoherent production mechanism with free production rates. Within the semi classical VUU formulation we have taken into account the high momentum components as described by HF ground state momentum distributions on the mean-field level by introducing a Gaussian discretization in momentum space for the one-body phase-space distribution f(r,p;t). It should be noticed that the high momentum components described above are not a direct result of short range correlations nor of many-body correlations beyond the mean field level, but a consequene of the many-body effects through the mean field and the effective interaction. Therefore their inclusion does not contradict the VUU description and the incoherent treatment of particle production. The present results with the high momentum components show a remarkable agreement with the data for and the angle integrated energy spectra of π^<0'>s in the laboratory frame down to 44 MeV/u. They also reproduce well the recent data of the invariant energy spectrum of π^^- at 95° in the NN center-of-mass frame from ^<14>N + ^<12>C at 42 MeV/u incident beam energy. Furthermore, the exponential slope parameter of π^- spectra at 90° in the center-of-mass frame is analyzed over a wide range of beam energy. Our results follow well the anomalous behaviour of the experimenta data, i.e. the slope parameter decreases monotonically with beam energy down to around 100 MeV/u and turns to be flat below 100 MeV/u. Since this anomalous behaviour can be easily understood in our model based on the basic dynamical effects of nucleus-nucleus collisions in this energy region, the general trend of the slope parameter obtained in the present study may not be wiped out by the more detailed analysis including the dynamical treatment of the pion reabsoption, the medium effects, or the momentum dependent force. Though the experimental trend is not so clear due to the large error bars for T_<0'>, this anomalous slope parameter might be an additional clear evidence for the incoherent production mechanism for pions in nucleus-nucleus collisions above 40 MeV/u: The slope parameter for π^0 angle-integrated energy spectrum in the laboratory frame as obtained in the cooperative model does not show the anomaly but monotonically decreases below 100 MeV/u. The experimetal situation will become more clear by forthcoming new measurements.
- 素粒子論グループ 素粒子研究編集部の論文
- 1991-08-20
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関連論文
- Effective Interaction in VUU
- 中高エネルギー重イオン反応(原子核物理の将来-核理論の課題-,各論シリーズ 第8回)
- Subthreshold Particle Production in Heavy-Ion Collisions (低および中高エネルギ-重イオン核反応機構)