Jet fragmentation in p+Pb and Pb+Pb with ATLAS

The modification of jets in heavy ion collisions is a central tool to study the properties of QCD matter at high temperature and its interaction with hard probes. Jet production in heavy ion collisions is observed to be suppressed relative to expectations from $pp$ collisions and the measurement of the longitudinal fragmentation functions in Pb+Pb\ are expected to provide insight into mechanism of the modification of the parton showering by the QCD medium. In proton-nucleus collisions, a large amount of hot QCD matter is not expected to be made and no evidence of jet suppression is observed. However, there is an unexpected excess of high momentum charged particles relative to expectations from $pp$ collisions that is observed in these collisions. The mechanism for this excess is not understood. The $p$+Pb fragmentation functions are expected to provide insight into the high momentum charged particle excess and into a possible influence of Pb+Pb jet fragmentation by effects due to the nuclear initial state. The latest ATLAS measurements of charged particle fragmentation functions in $pp$, $p$+Pb and Pb+Pb collisions are presented.