Speaker
Description
The ultra-peripheral collisions (UPCs) of relativistic ion beams lead to a prevalent and diverse set of photon-nucleus interactions in the case of Pb+Pb beams and photon-proton interactions in the case of $p$+Pb beams. The measurements of particles and their interactions produced in photo-nucleon reactions can shed light on the QCD dynamics of EM-induced, extremely asymmetric colliding systems, with energies between those available at RHIC and the LHC. Understanding the hadronic fluctuation spectrum of the photon in this fashion is also critical for maximizing the precision of measurements at a future Electron Ion Collider facility.
This talk presents measurements of two-particle long-range azimuthal correlations in photo-nuclear collisions using 1.73 nb$^{-1}$ of 5.02 TeV Pb+Pb data collected by the ATLAS experiment as well as long-range azimuthal correlations in photon-proton collisions using 68.8 nb$^{-1}$ of 8.16 TeV $p$Pb data collected by the CMS experiment. Candidate photo-nuclear and photo-proton events are selected using a combination of the zero-degree calorimeter and reconstructed pseudorapidity gaps.
Correlation functions are constructed using charged-particle tracks, separated in pseudorapidity, and binned in trigger-particle $p_{T}$ as well as charged-particle event multiplicity. The ATLAS experiment performs a template fitting procedure to subtract the non-flow contribution. After subtraction, significant elliptic and triangular flow-like correlations are observed in photo-nuclear collisions and are presented as a function of charged-particle multiplicity and $p_{T}$. The results are compared to flow coefficients obtained in proton-proton and proton-lead collisions in similar multiplicity ranges.
