Measurement of jet fragmentation in 5.02 TeV lead-lead and proton-proton collisions with the ATLAS detector

This note presents a measurement of charged particle jet fragmentation functions at 5.02 TeV in Pb+Pb collisions. The data were collected in 2015 by the ATLAS detector. These measurements should be sensitive to the strength and mechanism for jet quenching in the quark-gluon plasma created in the aftermath of ultra-relativistic collisions between two nuclei. The jet fragmentation functions are compared to fragmentation functions measured in pp collisions at the same collision energy as a function of the transverse momentum of the jet and the centrality of the collision using charged tracks with transverse momentum greater than 4 GeV. Jets are measured over the rapidity interval $|y|<$ 2.1 and with transverse momentum range 126--501 GeV. In central Pb+Pb collisions there is a suppression of the jet fragmentation functions compared to pp collisions for tracks that have a longitudinal momentum fraction smaller than approximately 0.2; for tracks with a higher longitudinal momentum fraction there is an enhancement. The strength of the modification decreases in peripheral collisions. The no dependence on the jet transverse momentum is observed in the measured interval.