Speaker
Description
Azimuthal modulations in electron-positron pair production in ultraperipheral collisions (UPCs) of gold ions have been observed by the STAR collaboration at RHIC. The modulations arise from the collisions of unpolarized and polarized photons emanating from the highly charged ions. In this talk the theoretical description in terms of Generalized Transverse Momentum Dependent parton distributions (GTMDs) of photons inside ions will be discussed. From this description it can be understood that in principle harmonics of all (even) orders will arise. From a model that is able to describe the RHIC data, it furthermore follows that the results depend considerably on the mass of produced particles. The most promising channels to experimentally demonstrate this in UPCs at RHIC and LHC will be discussed. This talk is based on work done together with Cristian Pisano and Luca Maxia, published in JHEP01(2025)076.