Measurement of photon–jet transverse momentum correlations in 5.02 TeV Pb + Pb and $pp$ collisions with ATLAS
Phys.Lett. B789 (2019) 167-190
The collaboration

Abstract (data abstract)
CERN-LHC. Measurements of the transverse momentum balance between isolated photons and inclusive jets are presented using 0.49 nb^{-1} of Pb+Pb collision data at 5.02 TeV and 25 pb^{-1} of pp collision data at 5.02 TeV recorded with the ATLAS detector at the LHC. Photons with pT(γ)>63.1 GeV and |eta(γ)|<2.37 are paired inclusively with all jets that have pT>31.6GeV and |eta|<2.8 in the event. The transverse momentum balance given by the jet-to-photon pT ratio, xJγ, are measured for pairs with azimuthal opening angl dphi > 7pi/8. Distributions of the per-photon jet yield (1/Nγ)(dN/dxJγ) are corrected via a two-dimensional unfolding procedure for detector effects and reported at the particle level for the first time. In pp collisions, the distributions are well described by Monte Carlo event generators. In Pb+Pb collisions, the xJγ distribution is modified with increasing centrality, consistent with the picture of parton-energy loss in the hot nuclear medium. The data are compared to a suite of energy loss models and calculations.