PHOTOS

The decay process Upsilon->MuMu also produces final state radiation (FSR), off of the final state muon. This process is important in determining the mass resultion, before detector smearing effects. In this analysis, we get this distribution from a MC simulation where we run UpsilonGun + PHOTOS, where PHOTOS is the process that simulates the FSR. Unfortunately, PHOTOS has a minimum photon energy cutoff (XPHCUT) set to 1% of the decaying particle mass over two (so for Y(1S) this would correspond to about 47MeV). The shape of the radiative mass distribution increases sharply near Mupsilon, consequently, this 47MeV is substantial.

Since the interface between particle gun and PHOTOS in CMSSW is difficult to modify (this may happen at some point, though it's not clear when), we decided to implement a work around.

By reading the PHOTOS documentation, we found the expression for the final state radiation probability.

Results

The figure shown below shows the generator leve mass distribution (points), the fit (black) and the extrapolated curve (red). The blue point represents the correction to the number of events in the peak.