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

Abstract

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 $\sqrt{s_\mathrm{NN}}$ =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 $p_\mathrm{T}$ > 30 GeV are selected, and three observables are measured: ${\mathrm{x}}_{\mathrm{JJ}\gamma}$, $\mathrm{A}_{\mathrm{JJ}\gamma}$, and $\Delta R_{\mathrm{JJ}}$. These observables characterise the overall energy loss of the multiparton system from medium interactions (${\mathrm{x}}_{\mathrm{JJ}\gamma}$), the relative energy loss between the two colour-charge carriers ($\mathrm{A}_{\mathrm{JJ}\gamma}$), and medium-induced modifications to their opening angle ($\Delta R_{\mathrm{JJ}}$). 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 ($I_\mathrm{AA}$) 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, $I_\mathrm{AA} < 1$, is observed as a result of parton-medium interactions. The experimental measurements are compared to three different jet quenching models: JEWEL, JETSCAPE, and LBT.