Abstract
Two-particle long-range azimuthal correlations are measured in photonuclear collisions using 1.7 nb$^{-1}$ of 5.02 TeV Pb+Pb collision data collected by the ATLAS experiment at the LHC. Candidate events are selected using a dedicated high-multiplicity photonuclear event trigger, a combination of information from the zero-degree calorimeters and forward calorimeters, and from pseudorapidity gaps constructed using calorimeter energy clusters and charged-particle tracks. Distributions of event properties are compared between data and Monte Carlo simulations of photonuclear processes. Two-particle correlation functions are formed using charged-particle tracks in the selected events, and a template-fitting method is employed to subtract the non-flow contribution to the correlation. Significant nonzero values of the second- and third-order flow coefficients are observed and presented as a function of charged-particle multiplicity and transverse momentum. The results are compared with flow coefficients obtained in proton-proton and proton-lead collisions in similar multiplicity ranges, and with theoretical expectations. The unique initial conditions present in this measurement provide a new way to probe the origin of the collective signatures previously observed only in hadronic collisions.
Highlights
In ultrarelativistic collisions of lead nuclei at the CERN Large Hadron Collider (LHC), the typical processes studied are those for which the nuclei have an impact parameter less than twice their radius (b 2RA)
All highmultiplicity triggers (HMT) had an additional highlevel trigger (HLT) requirement of less than 5 GeV of transverse energy in the photon-going forward calorimeter (FCal), which rejected a large fraction of the 0nXn peripheral Pb+Pb background events
Monte Carlo (MC) simulations of the relevant physics processes were used to understand the performance of the detector, and provide distributions to be compared with the data
Summary
In ultrarelativistic collisions of lead nuclei at the CERN Large Hadron Collider (LHC), the typical processes studied are those for which the nuclei have an impact parameter less than twice their radius (b 2RA) Such lead-lead (Pb+Pb) collisions are understood to create a quark-gluon plasma and result in a large number of particles in the final state which participate in collective motion as a consequence of the plasma evolution [1,2,3]. In addition to the particles produced in Pb+Pb collisions, those produced in high-energy proton-proton (pp) and proton-lead (p + Pb) collisions exhibit a collective behavior which manifests as an event-wide azimuthal variation persisting broadly in pseudorapidity, initially observed as a “ridge” [4,5,6,7].
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