arXiv 1301.07862013-01nucl-thhep-phnucl-ex

SIS能重离子碰撞中双轻子产生的系统尺寸和能量依赖性

System Size and Energy Dependence of Dilepton Production in Heavy-Ion Collisions at SIS Energies

E. L. Bratkovskaya, J. Aichelin, M. Thomere, S. Vogel, M. Bleicher

备注：: 46 pages, 48 figures, version to be published in Phys. Rev. C
发表信息：: Phys. Rev. C 87, 064907 (2013)
DOI：: 10.1103/PhysRevC.87.064907

摘要：

我们研究了能量为1-2 AGeV的重离子碰撞以及能量为1 - 3.5 GeV的质子诱发pp、pn、pd和p+A反应中的双轻子产生。对于分析，我们采用三种不同的输运模型-微观离壳强子弦动力学（HSD）输运方法，同位旋量子分子动力学（IQMD）方法以及超相对论量子分子动力学（UrQMD）方法。我们证实了实验观察到的双轻子产率（归一化为中性π介子$N_{π^0}$的多重性）在重离子碰撞中的增强相对于在$NN = (pp+pn)/2$碰撞中测量。我们确定了两个贡献，这种增强：a）$pN$韧致辐射的规模与碰撞的数量，而不是与参与者的数量，即π介子; b）从中间$Δ$的双轻子发射的反应周期的一部分，$Δ\to πN ; πN \to Δ$和$NN\to NΔ; NΔ\to NN$。随着系统规模的增加，产生了更多代的中间$Δ$。如果这样的$Δ$衰变为π介子，π介子可以被重吸收，然而，如果它衰变为双轻子，双轻子会从系统中逃逸。因此，在实验中，人们只能观察到一个π介子（来自最后产生的$Δ$），而双轻子产额则累积了循环中所有$Δ$的贡献。我们还表明，费米运动以同样的方式增强了π介子和双轻子的产生。此外，采用离壳HSD方法，我们探讨了介质效应的影响，如修改的自能和矢量介子的谱函数，由于它们与强子环境的相互作用。

英文摘要：

We study the dilepton production in heavy-ion collisions at energies of 1-2 AGeV as well as in proton induced pp, pn, pd and p+A reactions from 1 GeV up to 3.5 GeV. For the analysis we employ three different transport models - the microscopic off-shell Hadron-String-Dynamics (HSD) transport approach, the Isospin Quantum Molecular Dynamics (IQMD) approach as well as the Ultra-relativistic Quantum Molecular Dynamics (UrQMD) approach. We confirm the experimentally observed enhancement of the dilepton yield (normalized to the multiplicity of neutral pions $N_{π^0}$) in heavy-ion collisions with respect to that measured in $NN = (pp+pn)/2$ collisions. We identify two contributions to this enhancement: a) the $pN$ bremsstrahlung which scales with the number of collisions and not with the number of participants, i.e. pions; b) the dilepton emission from intermediate $Δ$'s which are part of the reaction cycles $Δ\to πN ; πN \to Δ$ and $NN\to NΔ; NΔ\to NN$. With increasing system size more generations of intermediate $Δ$'s are created. If such $Δ$ decays into a pion, the pion can be reabsorbed, however, if it decays into a dilepton, the dilepton escapes from the system. Thus, experimentally one observes only one pion (from the last produced $Δ$) whereas the dilepton yield accumulates the contributions from all $Δ$'s of the cycle. We show as well that the Fermi motion enhances the production of pions and dileptons in the same way. Furthermore, employing the off-shell HSD approach, we explore the influence of in-medium effects like the modification of self-energies and spectral functions of the vector mesons due to their interactions with the hadronic environment.

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