  Data from heavy ion collisions suggest that the evolution of a parton shower
is modified by interactions with the color charges in the dense partonic medium
created in these collisions, but it is not known where in the shower evolution
the modifications occur. The momentum ratio of the two leading partons,
resolved as subjets, provides information about the parton shower evolution.
This substructure observable, known as the splitting function, reflects the
process of a parton splitting into two other partons and has been measured for
jets with transverse momentum between 140 and 500 GeV, in pp and PbPb
collisions at a center-of-mass energy of 5.02 GeV per nucleon pair. In central
PbPb collisions, the splitting function indicates a more unbalanced momentum
ratio, compared to peripheral PbPb and pp collisions. The measurements are
compared to various predictions from event generators and analytical
calculations.
