摘要
Parton energy loss in the quark-gluon plasma (QGP) is studied with a measurement of photon-tagged jet production in 1.7 nb $^{-1}$of Pb+Pb data and 260 pb $^{-1}$of$pp$data, both at$\sqrt{s_\mathrm{NN}} = 5.02$TeV, with the ATLAS detector. The process$pp \to \gamma$ +jet+ $X$and its analogue in Pb+Pb collisions is measured in events containing an isolated photon with transverse momentum ( $p_\mathrm{T}$ ) above$50$GeV 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 that are mostly initiated by the showering of quarks. The$pp$and Pb+Pb measurements are used to report the nuclear modification factor,$R_\mathrm{AA}$ , and the fractional energy loss,$S_\mathrm{loss}$ , for photon-tagged jets. In addition, the results are compared with the analogous ones for inclusive jets, which have a significantly smaller quark-initiated fraction. The$R_\mathrm{AA}$and$S_\mathrm{loss}$values are found to be significantly different between those for photon-tagged jets and inclusive jets, demonstrating that energy loss in the QGP is sensitive to the colour-charge of the initiating parton. The results are also compared with a variety of theoretical models of colour-charge-dependent energy loss.