Yerevan Physics Institute, Yerevan, Armenia.
Institut für Hochenergiephysik, Wien, Austria.
Phys Rev Lett. 2018 Dec 14;121(24):242301. doi: 10.1103/PhysRevLett.121.242301.
Measurements of fragmentation functions for jets associated with an isolated photon are presented for the first time in pp and Pb-Pb collisions. The analysis uses data collected with the CMS detector at the CERN LHC at a nucleon-nucleon center-of-mass energy of 5.02 TeV. Fragmentation functions are obtained for jets with p_{T}^{jet}>30 GeV/c in events containing an isolated photon with p_{T}^{γ}>60 GeV/c, using charged tracks with transverse momentum p_{T}^{trk}>1 GeV/c in a cone around the jet axis. The association with an isolated photon constrains the initial p_{T} and azimuthal angle of the parton whose shower produced the jet. For central Pb-Pb collisions, modifications of the jet fragmentation functions are observed when compared to those measured in pp collisions, while no significant differences are found in the 50% most peripheral collisions. Jets in central Pb-Pb events show an excess (depletion) of low (high) p_{T} particles, with a transition around 3 GeV/c. This measurement shows for the first time the in-medium shower modifications of partons (quark dominated) with well-defined initial kinematics. It constitutes a new well-controlled reference for testing theoretical models of the parton passage through the quark-gluon plasma.
首次在 pp 和 Pb-Pb 碰撞中呈现了与孤立光子相关的喷注碎裂函数的测量结果。该分析使用 CMS 探测器在 CERN LHC 上以核子-核子质心能量为 5.02 TeV 收集的数据进行。在含有 p_{T}^{γ}>60 GeV/c 的孤立光子的事件中，获得了 p_{T}^{jet}>30 GeV/c 的喷注的碎裂函数，使用在喷注轴周围的圆锥内具有横向动量 p_{T}^{trk}>1 GeV/c 的带电迹。与孤立光子的关联限制了产生喷注的部分子的初始 p_{T}和方位角。与 pp 碰撞中测量的碎裂函数相比，在中心 Pb-Pb 碰撞中观察到喷注碎裂函数的修正，而在 50%最边缘的碰撞中则没有发现显著差异。在中心 Pb-Pb 事件中的喷注显示出低（高）p_{T}粒子的过剩（耗尽），在 3 GeV/c 左右有一个转变。该测量首次展示了具有明确定义初始运动学的部分子（夸克主导）在介质中的喷注修正。它为测试夸克-胶子等离子体中部分子通过的理论模型提供了一个新的、受良好控制的参考。
