I'm looking for events where a W and a Z boson is produced from collision of two protons, with backgrounds coming from top-antitop production, production of 2 Z bosons and Drell-Yann processes (production of 1 Z boson). Selections have been made to reduce these backgrounds.
Plotted here is the transverse mass of the missing transverse momentum (i.e. the neutrino from W decay) and the lepton associated with the W boson.
I'm unsure how to explain the shape of the WZ component. Why is there a flat, somewhat constant part in the beginning and a 'normal' but fairly wide resonance at the W boson mass? The transverse mass is an intuitively difficult concept for me so it's a hard to draw conclusions myself... I've tried looking it up online but I mostly find plots with a Jacobian peak, which looks nothing what I have. I also see a very slight increase of the ttbar background around 80 GeV (which I've confirmed on another plot). Is this an anomaly or can it be explained?