  Jet quenching in relativistic heavy ion collisions can have multiple
phenomenological consequences: jet energy loss, modification of jet
substructure, and induced acoplanarity. In these proceedings, we report a
measurement of the jet fragmentation function, which is one of the jet
substructure observables, in peripheral Au+Au collisions at sqrt(s_NN) = 200
GeV by the STAR experiment at RHIC. In particular, we use a semi-inclusive
population of jets recoiling from a high transverse momentum trigger hadron.
The fragmentation function is constructed from the fraction of the transverse
momentum of charged particles projected onto the jet axis over the transverse
momentum of the jet. In a previous STAR publication of the semi-inclusive
charged-jet spectra, the Mixed-Event technique was used along with the
semi-inclusive approach to remove the uncorrelated background contributions,
which enables the measurement of jet distributions at low transverse momentum
and large jet radius (R) in heavy ion collisions. In this analysis we extend
this approach to the measurement of jet fragmentation functions. The reported
fragmentation functions are corrected for uncorrelated background and
instrumental effects via unfolding. The results are compared to those in PYTHIA
simulations for pp collisions.
