邀请人 (Invited by): 林树

时间 (Time): 星期一，2023/10/16-10:30-11:30

地点 (Location): 珠海校区瀚林3号B331会议室

摘要 (Abstract): 

Gluons are found to become increasingly dominant constituents of nuclear matter when being probed at higher energies or smaller Bjorken-x. This has led to the question of the ultimate fate of nuclear gluonic structure and its interaction with external probes at extreme density regimes when approaching the limit allowed by unitarity. In ultraperipheral collisions (UPCs) of relativistic heavy ions, the coherent J/ψ production via photon-nuclear interactions is of particular interest, since its cross section is directly sensitive to the nuclear gluon density at leading order. However, in experimental measurements, because each of the two nuclei in symmetric UPCs can serve both as a photon-emitter projectile and a target, this two-way ambiguity has prevented us from disentangling contributions involving high- and low-energy photon-nucleus interactions, thus limiting our capability of probing the extremely small-x regime, where nonlinear QCD effects are expected to emerge. In this talk, we will present a new measurement of coherent J/ψ photoproduction, where the two-way ambiguity is solved by implementing for the first time a forward neutron tagging technique in UPC Pb+Pb collisions in 2018 at 5.02 TeV. The coherent J/ψ photoproduction cross section will be presented, for the first time, as a function of the photon-Pb center-of-mass energy in UPCs up to about 400 GeV, corresponding to an extremely low J/ψ of ∼5×10−5. We will discuss the physics implications of this new result, as well as exciting opportunities in future LHC heavy ion runs.

主讲人简介 (Speaker's CV): 

杨帅，华南师范大学物理学院教授，博士毕业于中国科学技术大学，先后在美国布鲁克海文国家实验室、莱斯大学做博士后。研究方向：夸克胶子等离子体（QGP）、相对论重离子碰撞。