arXiv 1809.070492018-09-19nucl-thhep-ph

小横动量重离子碰撞中的双轻子辐射

Dilepton Radiation in Heavy-Ion Collisions at Small Transverse Momentum

Mariola Klusek-Gawenda, Ralf Rapp, Wolfgang Schäfer, Antoni Szczurek

备注：: 10 pages, 4 figures
DOI：: 10.1016/j.physletb.2019.01.035

摘要：

研究了极低对横动量$P_T\leq 0.15$ GeV的超相对论重离子碰撞中产生的双轻子的不变质量分布。具体来说，我们调查的相互作用的热辐射与初始光子湮灭过程，$γγ\to l^+ l^-$，引发的相干电磁场的传入的核。对于热辐射，我们采用的发射从QGP和强子阶段与介质中的矢量谱函数，描述了包容性的过剩辐射在很宽的碰撞能量范围内观察到的。对于相干光子聚变过程，其光谱比热辐射软得多，我们采用初始通量从碰撞核的电荷分布的傅立叶变换中的等效光子近似。我们首先验证，光子聚变，热辐射和末态强子衰变的组合给出了一个公平的描述低$P_T$双轻子质谱最近测量的STAR合作在$\sqrt{s_{NN}}$ =200 GeV金+金碰撞不同的中心性类，包括实验验收削减。相干贡献占主导地位的周边碰撞，而热辐射显示出显着较强的增加与中心。我们在较低的碰撞能量（$\sqrt{s_{NN}}$ =17.3 GeV）进行类似的计算，并比较接受校正的dimuon过量光谱测量的NA 60实验在欧洲核子研究中心SPS，在这里，从光生的贡献是subleading。我们还提供了ALICE实验在LHC的预测;从SPS到LHC能量的相关激发函数揭示了光生和热辐射的非平凡的相互作用。

英文摘要：

We study the invariant-mass distributions of dileptons produced in ultrarelativistic heavy-ion collisions at very low pair transverse momenta, $P_T\leq 0.15$ GeV. Specifically, we investigate the interplay of thermal radiation with initial photon annihilation processes, $γγ\to l^+ l^-$, triggered by the coherent electromagnetic fields of the incoming nuclei. For the thermal radiation, we employ the emission from the QGP and hadronic phases with in-medium vector spectral functions which describes the inclusive excess radiation observed over a wide range of collision energies. For the coherent photon fusion processes, whose spectrum is much softer than for thermal radiation, we employ initial fluxes from the Fourier transform of charge distributions of the colliding nuclei in the equivalent-photon approximation. We first verify that the combination of photon fusion, thermal radiation and final-state hadron decays gives a fair description of the low-$P_T$ dilepton mass spectra as recently measured by the STAR collaboration in $\sqrt{s_{NN}}$=200 GeV Au+Au collisions for different centrality classes, including experimental acceptance cuts. The coherent contribution dominates in peripheral collisions, while thermal radiation shows a markedly stronger increase with centrality. We perform similar calculations at lower collision energies ($\sqrt{s_{NN}}$=17.3 GeV) and compare to the acceptance-corrected dimuon excess spectra measured by the NA60 experiment at the CERN SPS; here, the contribution from photoproduction is subleading. We also provide predictions for the ALICE experiment at the LHC; the pertinent excitation function from SPS to LHC energies reveals a nontrivial interplay of photoproduction and thermal radiation.

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