Another question in regards to the underlying event would be how does it change with trigger energy and with trigger species (gamma-rich vs. pi0).  To investigate this, I took the off-axis jets (jets falling in the delta-phi region UE[0] as defined in the link below) to be a measure of the underlying event and plotted the pTreco = pTjet - rho*Ajet distributions and their ratio as a function of transverse trigger energy.

https://drupal.star.bnl.gov/STAR/blog/dmawxc/update-01232018-run-9-pp-underlying-event-study

For comparison, I included the recoil jet distributions and their ratio as well.  Both the off-axis and recoil distributions are normalized per trigger and corrected for bin-width and eta acceptance.  No additional normalization has been applied to the off-axis distributions.  I forgot to remove the titles which say 'Recoil Jet pTreco', so please ignore those.