大型强子对撞机Pb+Pb碰撞中约化喷流-介质耦合的研究
Examining a reduced jet-medium coupling in Pb+Pb collisions at the Large Hadron Collider
摘要：
最近在大型强子对撞机（LHC）上获得的2.76 AeV Pb+Pb碰撞中喷流碎片的核修正因子$R_{AA}$的数据表明，在LHC能量下，夸克-胶子等离子体（QGP）中的喷流-介质耦合被减弱，这与相对论性强子对撞机（RHIC）上的数据不一致。我们估计的减少因子从一个综合的适合现有的数据上$R_{AA}(\sqrt{s},p_T,b)$和椭圆流$v_2(\sqrt{s},p_T,b)$在$\sqrt{s}=0.2,2.76$ ATEV的横向动量范围$p_T$ 10-100 GeV和广泛的影响参数，b，范围。我们使用一个简单的分析“多面体”模型（$dE/dx=- κE^{a} x^z T^{c}$），调查的动力学射流能量损失模型的依赖性。改变a=0-1在喷流能量依赖的弱耦合和强耦合模型之间插值，而z=0-2涵盖了从弹性和辐射到全息弦机制的广泛的可能的喷流路径依赖性。我们对LHC数据的拟合表明，从RHIC到LHC的耦合$κ$减少了大约40%，并且排除了以喷流能量指数a>1/3为特征的能量损失模型。特别是，$R_{AA}$的快速上升与$p_T$ >10 GeV结合的渐近$v_2(p_T)$在大型强子对撞机的缓慢变化排除了流行的指数几何光学模型（a=1）。LHC的数据与$0\leq a\leq 1/3$ pQCD类能量损失模型兼容，其中RHIC和LHC之间的喷流-介质耦合减少了约10%。
英文摘要：
Recent data on the nuclear modification factor $R_{AA}$ of jet fragments in 2.76 ATeV Pb+Pb collisions at the Large Hadron Collider (LHC) indicate that the jet-medium coupling in a Quark-Gluon Plasma (QGP) is reduced at LHC energies and not compatible with the coupling deduced from data at the Relativistic Hadron Collider (RHIC). We estimate the reduction factor from a combined fit to the available data on $R_{AA}(\sqrt{s},p_T,b)$ and the elliptic flow $v_2(\sqrt{s},p_T,b)$ at $\sqrt{s}=0.2,2.76$ ATeV over a transverse momentum range $p_T$ 10-100 GeV and a broad impact parameter, b, range. We use a simple analytic "polytrope" model ($dE/dx=- κE^{a} x^z T^{c}$) to investigate the dynamical jet-energy loss model dependence. Varying a=0-1 interpolates between weakly-coupled and strongly-coupled models of jet-energy dependence while z=0-2 covers a wide range of possible jet-path dependencies from elastic and radiative to holographic string mechanisms. Our fit to LHC data indicates an approximate 40% reduction of the coupling $κ$ from RHIC to LHC and excludes energy-loss models characterized by a jet-energy exponent with a>1/3. In particular, the rapid rise of $R_{AA}$ with $p_T$>10 GeV combined with the slow variation of the asymptotic $v_2(p_T)$ at the LHC rules out popular exponential geometric optics models (a=1). The LHC data are compatible with $0\leq a\leq 1/3$ pQCD-like energy-loss models where the jet-medium coupling is reduced by approximately 10% between RHIC and LHC.
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