Measurement of the fragmentation function for photon-tagged jets in $\sqrt{s_\mathrm{NN}} = 5.02$ TeV Pb+Pb and $pp$ collisions with the ATLAS detector

This note presents a measurement of the charged-particle fragmentation function (FF) for jets azimuthally balanced by a high transverse momentum ($p_\mathrm{T}$) prompt, isolated photon. FFs as a function of particle-$p_\mathrm{T}$ and fraction of the jet $p_\mathrm{T}$ carried by the particle, $z$, are measured in 26 pb$^{−1}$ of $pp$ and 0.49 nb$^{−1}$ of Pb+Pb collision data at $\sqrt{s_\mathrm{NN}} = 5.02$ TeV recorded with the ATLAS detector at the Large Hadron Collider. In pp collisions, a comparison of the photon-tagged jet FF to that for inclusively selected jets is sensitive to the difference in fragmentation between quark– and gluon–initiated jets in vacuum. The ratio of the FF in Pb+Pb events to those in $pp$ events with the same selection on the photon kinematic variables provides information on the modification of the parton shower for quark-initiated jets as they traverse the hot nuclear medium. In comparison to the modified FF for inclusively-selected jets, photon-tagged jets show a similar modification in particle-$p_\mathrm{T}$ or $z$ in semi-central and peripheral events, but a stronger one in central events