Measurement of photon–jet transverse momentum correlations in 5.02 TeV Pb + Pb and pp collisions with ATLAS

Abstract

Jets created in association with a photon can be used as a calibrated probe to study energy loss in the medium created in nuclear collisions. Measurements of the transverse momentum balance between isolated photons and inclusive jets are presented using integrated luminosities of 0.49nb−1of Pb+Pb collision data at √sNN=5.02TeVand 25pb⁻¹of ppcollision data at √s=5.02TeVrecorded with the ATLAS detector at the LHC. Photons with transverse momentum 63.1 <pγT<200GeV and ηγ <2.37are paired with all jets in the event that have pjetT>31.6GeVand pseudorapidity ηjet <2.8. The transverse momentum balance given by the jet-to-photon pTratio, xJγ, is measured for pairs with azimuthal opening angle Δφ>7π/8. Distributions of the per-photon jet yield as a function of xJγ, (1/Nγ)(dN/dxJγ), are corrected for detector effects via a two-dimensional unfolding procedure and reported at the particle level. In pp collisions, the distributions are well described by Monte Carlo event generators. In Pb+Pb collisions, the xJγ distribution is modified from that observed in pp collisions with increasing centrality, consistent with the picture of parton energy loss in the hot nuclear medium. The data are compared with a suite of energy-loss models and calculations.