Question

In this (attached) Summer 2011 plot from CMS (twiki page), they have a plot of the dimuon invariant mass spectrum across 3 orders of magnitude in energy. There seems to be a 'bump' near M???30 GeV which I have indicated in the plot. Does anyone know where the gentle but distinct rise is coming from?

enter image description here

Answer 1

I would say that this is not physics but rather a detector/instrumentation effect. More specifically this could be due to the fact that the grey distribution is one that is collected with a high-pT muon trigger. These triggers kick in at around 15-20 GeV. Depending on the topology of the event (angles) the invariant mass would of this range as well.

It is instructive to have a look at plots using different triggers.

1) in fact in the plot you reference there a a number of different trigger configurations (setup specifically for the resonances plus two general low/high pt trigger paths) superimposed over each other. CMS dimuon with different triggers

2) here's a similar plot with muons trigger that do not have a pt threshold. notice the lack of a bump in the region you specified. CMS dimuon no threshold

3) here a similar plot by ATLAS (from 2010) where only events passing the high pt muon threshold are plotted. Notice how the bump seems to reappear.