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arXiv 1404.6232nucl-ex
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夸克和胶子如何在QGP中损失能量？

How do quarks and gluons lose energy in the QGP?

M. J. Tannenbaum

备注：
16 pages, 20 figures, to appear in the Proceedings of the 9th International Workshop on High-pT physics at LHC (25th-28th September, 2013,LPSC, Grenoble, France)
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摘要：

在RHIC实验中，在$\sqrt{s_{NN}}=200$ GeV的中心Au+Au碰撞中发现了$π^0$与$5\leq p_T\leq 20$ GeV/c的抑制，相对于二进制标度的$R_{AA}\approx 0.2$，令人惊讶的是，对于重夸克衰变产生的单电子也是如此。这两个结果都已在LHC $\sqrt{s_{NN}}=2.76$ TeV的Pb+Pb碰撞中得到证实。在这个$p_T$范围内，π介子的LHC结果几乎与RHIC结果重叠，但LHC的平坦光谱意味着介质中的能量损失必须比RHIC大$\sim 40$ %。在大型强子对撞机上，相对于p-p碰撞，Pb+Pb碰撞中双喷流的分数横向动量不平衡$1-\langle{\hat{p}_{T_2}/\hat{p}_{T_1}}\rangle$的独特而美丽的测量显示了具有$120\leq\hat{p}_{T_1}\leq 360$ GeV/c的喷流的$\approx 15\%$。这是一个小得多的分数射流不平衡比$\approx 45\%$来自双射流碎片在RHIC对应于射流$\hat{p}_T\approx 10-20$ GeV/c的两粒子相关。这对理论和实验都提出了挑战，以提高理解。还有许多其他类似的未解决的问题，例如，缺乏通过观察由外向部分子转移到介质的动量来证明$\hat{q}$效应的证据。基于一个流行的公式${dN_{\rm ch}^{AA}/dη}= [(1-x) \langle{N_{\rm part}}\rangle {dN_{\rm ch}^{pp}/dη}/2 + x\, \langle{N_{\rm coll}}\rangle {dN_{\rm ch}^{pp}/dη}]$，在A+A碰撞的软物理多重性分布中隐含的硬散射分量，似乎是一种非物理的方式来理解$N_{\rm part}$标度的偏差。基于最近的p-p和d+A测量，提出了一种更物理的方法，以及最近的d+Au（p+Pb）测量的其他一些令人兴奋的结果和想法。

英文摘要：

At RHIC, a suppression, $R_{AA}\approx 0.2$ relative to binary-scaling, for $π^0$ with $5\leq p_T\leq 20$ GeV/c was discovered in central Au+Au collisions at $\sqrt{s_{NN}}=200$ GeV, and surprisingly also for single-electrons from the decay of heavy quarks. Both these results have been confirmed in Pb+Pb collisions at the LHC at $\sqrt{s_{NN}}=2.76$ TeV. In this $p_T$ range, the LHC results for pions nearly overlap the RHIC results but the flatter spectrum at LHC implies that the energy loss in the medium must be $\sim 40$% larger than at RHIC. At LHC, the unique and beautiful measurement of the fractional transverse momentum imbalance $1-\langle{\hat{p}_{T_2}/\hat{p}_{T_1}}\rangle$ of di-jets in Pb+Pb collisions relative to p-p collisions, shows $\approx 15\%$ for jets with $120\leq\hat{p}_{T_1}\leq 360$ GeV/c. This is a much smaller fractional jet imbalance than the $\approx 45\%$ derived from two-particle correlations of di-jet fragments at RHIC corresponding to jet $\hat{p}_T\approx 10-20$ GeV/c. This presents a challenge to both theory and experiment for improved understanding. There are many other such unresolved issues, for instance, the absence of evidence for a $\hat{q}$ effect by observation of momentum transferred to the medium by outgoing partons. An implied hard scattering component for the soft physics multiplicity distributions in A+A collisions based on a popular formula, ${dN_{\rm ch}^{AA}/dη}= [(1-x) \langle{N_{\rm part}}\rangle {dN_{\rm ch}^{pp}/dη}/2 + x\, \langle{N_{\rm coll}}\rangle {dN_{\rm ch}^{pp}/dη}]$, seems to be an unphysical way to understand the deviation from $N_{\rm part}$ scaling. Based on recent p-p and d+A measurements, a more physical way is presented along with several other stimulating results and ideas from recent d+Au (p+Pb) measurements.

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