Impact Parameter Dependence of Photon-Induced Processes in Heavy-Ion Collisions

The Lorentz boosted electromagnetic fields shrouding relativistic heavy ions can be treated as a flux of linearly polarized quasi-real photons. Therefore, ions can interact when their impact parameter is greater than twice the nuclear radius, the so-called ultraperipheral collisions (UPCs), via photon-photon process. This photon-photon process has also been observed in hadronic heavy-ion collisions. The intriguing thing is that the mean transverse momentum (p_T) of lepton pairs from photon-photon scatterings in hadronic collisions is found to be larger than that from UPCs. This phenomenon sparked an intense discussion in the field that whether the observed broadening is caused by final-state electromagnetic modifications of lepton pairs in a Quark Gluon Plasma (QGP) medium or p_T hardening of initial-state photons as the impact parameter decreases toward central hadronic collisions. 

In this talk, I will firstly summarize the achievements of photon-photon interactions based on experimental results in hadronic collisions. Then a novel experimental tool for controlling the impact parameter in UPCs will be presented. This novel tool helps to unambiguously study the role of initial photon p_T in the observed broadening in hadronic collisions. Finally, I will discuss the roadmap and opportunities of using photon-photon interactions to quantitatively probe the electromagnetic properties of QGP medium with precise heavy-ion data in future.