Comparison of inclusive and photon-tagged jet suppression in 5.02 TeV Pb+Pb collisions with ATLAS

This note presents a measurement of photon-tagged jet energy loss in the quark-gluon plasma, using 1.7 nb$^{-1}$ of Pb+Pb data and 260 pb$^{-1}$ of $pp$ data, both at 5.02 TeV, with the ATLAS detector. The process $pp \to \gamma$+jet+$X$ and its analogue in Pb+Pb events is measured in events with a $p_\mathrm{T}^\gamma > 50$ GeV isolated photon and reported as a function of jet $p_\mathrm{T}$. This selection results in a sample of jets with a steeply falling $p_\mathrm{T}$ distribution, similar to that for inclusive jets but with a different fraction of quark and gluon-initiated jets. The nuclear modification factor, $R_\mathrm{AA}$, is reported as a function of jet $p_\mathrm{T}$ and Pb+Pb event centrality, and compared to theoretical predictions. The $R_\mathrm{AA}$ for photon-tagged jets is measured to be significantly higher than that previously measured for inclusive jets, demonstrating that energy loss in the QGP is sensitive to the color-charge of the initiating parton.