相对论重离子碰撞中整体喷注级联簇射和强子化过程的研究

Currently, using full jets to probe the properties of the quark-gluon plasma (QGP) created in the relativistic heavy-ion collisions at RHIC and the LHC, is one of the core activities both in theory and experiment. With abundant experimental data from full jets measurements, it is important for theoretical studies to analyze and understand them in a unified framework, in order to determine the jet-QGP interaction mechanisms and the properties of QGP more quantitatively. Based on our previous studies on full jets, this project aims to develop a Monte Carlo program to study full jet shower evolution and hadronization in heavy-ion collisions. The parton shower process includes the splitting in vacuum and the splitting induced by jet-medium interactions. In QGP medium, shower partons also lose energy due to elastic scatterings with QGP partons, and finally partonic jets hadronize into hadronic jets. Since abundant partons are available in the QGP medium and could be combined with jet shower partons into hadrons, the process of jet hadronization needs to take into account both fragmentation and combination mechanisms. The accomplishment of this project enables us to study various experimental data systematically in a unified framework, and to make detailed comparison with other theoretical results, which can deepen our understanding on the energy loss and the nuclear modification of internal structure of full jets via different jet-QGP interaction mechanisms. Especially, the mechanism and the role of initial parton shower and final hadronization could be clarified further.

用整体喷注研究RHIC和LHC上相对论重离子碰撞中形成的夸克胶子等离子体（QGP）的性质，是这个领域理论和实验上的热点。整体喷注提供了丰富的实验数据，理论上对它们进行统一分析和解释，才能更好地确定喷注与QGP相互作用机制和QGP性质。申请人计划在已有整体喷注研究工作的基础上，发展出一套研究整体喷注级联簇射和强子化的蒙特卡洛模拟程序。部分子喷注在QGP中的级联簇射包含真空的劈裂过程和QGP介质诱发的的劈裂过程。喷注在簇射过程中一直与QGP发生弹性散射并损失能量，最后强子化为强子喷注。由于QGP提供了大量可供组合的热部分子，喷注强子化要考虑碎裂强子化和组合强子化两种机制。本项目的开展使我们可以在统一框架下对各种观测量进行系统分析，并与其他理论结果进行对照，深入理解整体喷注在QGP中通过不同相互作用机制损失能量并改变内部结构的详细过程，尤其是喷注的级联簇射和强子化过程。

喷注淬火是研究相对论重离子碰撞中形成的夸克胶子等离子体（QGP）的性质的重要工具，2010年后对整体喷注核修正的测量已积累了丰富的实验数据，理论上对它们进行统一分析和解释，才能更好地确定喷注与QGP相互作用机制和QGP性质。我们用与流体耦合的微分输运方程研究了单举喷注和光子-喷注在QGP中的能量损失和喷注形状函数的核修正，并研究了介质响应对喷注能量和喷注形状函数的影响。我们发现CMS实验组观察到的单举喷注和光子-喷注事例中喷注形状函数核修正的不同主要来自于喷注横动量的不同，而与喷注初始部分子的味道不同关系不大。喷注在穿过QGP时转移到QGP中的能量，会随QGP一起做流体力学演化，导致QGP强子化后喷注周围的强子产额增高，这就是介质响应。介质响应会弥补一部分喷注在部分子阶段损失的能量，它对喷注形状函数的贡献随着半径的增大而增大，也使单举喷注横动量谱的核修正因子随着喷注半径变大而变大。计划与微分输运方程做对比研究写出的蒙特卡洛程序得到的结果不如微分输运方程，在合理输运参数内不能同时描述单举喷注横动量谱的压低和喷注形状函数的核修正，我们转而改进解微分输运方程的算法，使它能稳定求解更高能量的喷注在更大QGP介质中长时间的演化。针对喷注内三维能动量分布的特点，我们使用了非均匀的变量变换，使稳定求解的范围从原来的200GeV扩大到1TeV。这使我们获得了在广泛能量区间分析现有实验数据的能力。我们已经可以同时描述ALICE、ATLAS和CMS实验组单举喷注的横动量谱在不同横动量范围内的核修正因子数据,正在分析近几年出现的其他极高横动量的实验数据。

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