Event Selection in Extremely Peripheral Pb+Pb Collisions with ATLAS

In order to investigate the quark gluon plasma through jet quenching, one wants to understand the continuum from Pb+Pb to pp collisions. Thus, it is useful to investigate peripheral Pb+Pb collisions, where jet quenching effects are expected to smoothly turn off. Ultra-peripheral collisions (UPCs) are a background for these collisions, and the pT spectra of charged hadrons in these events are significantly different from hadronic Pb+Pb collisions for reasons unrelated to jet quenching. They therefore should be removed in order to get an unbiased measurement of the effect of jet quenching. Recent measurements have shown that the sum of the pseudorapidity gaps between the particles in the event can be used to efficiently separate the UPCs from the hadronic Pb+Pb collisions. Using these pseudorapidity gap variables in low-multiplicity 5.02 TeV Pb+Pb data from the ATLAS experiment at the Large Hadron Collider, as well as those in simulated events from HIJING and DPMJET, we estimate the fraction of peripheral and ultra-peripheral Pb+Pb collisions in ATLAS Pb+Pb data and investigate the effectiveness of using gap variables to separate them.