Measurement of isolated photon plus two-jet correlations in Pb+Pb and pp collisions at 5.02 TeV with ATLAS

This paper presents a measurement of photon plus two-jet events in pp and Pb+Pb collisions, i.e. events in which the transverse momentum of a single photon is balanced by two distinct jets. The measurement was performed using pp data taken in 2017 with an integrated luminosity 260 pb − 1 , and Pb+Pb data taken in 2018 with an integrated luminosity 1.72 nb − 1 , both at s NN =5.02 TeV, as recorded by the ATLAS detector. Events with photons in the transverse momentum range 90–180 GeV and at least two anti- k t R = 0.2 jets with a p T > 30 GeV are selected, and three observables are measured: x JJ γ , A JJ γ , and Δ R JJ . These observables characterise the overall energy loss of the multiparton system from medium interactions (x JJ γ ), the relative energy loss between the two colour-charge carriers (A JJ γ ), and medium-induced modifications to their opening angle (Δ R JJ ). The observables are corrected for uncorrelated combinatoric background contributions using a novel multijet mixing technique, for photon purity, and for detector resolution effects via iterative unfolding. Final results are presented per photon, and the ratio ( I AA ) is taken between measurements in Pb+Pb and pp collisions, for Pb+Pb centrality intervals of 30–80%, 10–30%, and 0–10%. Significant suppression of per photon two-jet yields in all three observables, I AA < 1, is observed as a result of parton-medium interactions. The experimental measurements are compared to three different jet quenching models: Jewel , Jetscape , and Lbt .