Abstract
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 √sNN = 5.02 TeV, with
the ATLAS detector. The process pp → γ +jet+X and its analogue in Pb+Pb collisions is measured in
events containing an isolated photon with transverse momentum (pT) above 50 GeV and reported as
a function of jet pT. This selection results in a sample of jets with a steeply falling pT distribution that
are mostly initiated by the showering of quarks. The pp and Pb+Pb measurements are used to report the
nuclear modification factor, RAA, and the fractional energy loss, Sloss, 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 RAA and Sloss 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.
