What is the 'bump' near $M_{\mu\mu}\approx 30\text{ GeV}$

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 $M_{\mu\mu}\approx 30\text{ GeV}$ which I have indicated in the plot. Does anyone know where the gentle but distinct rise is coming from?

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Hmmm...nothing in my rather outdated home copy of the Particle Physics Booklet. Be aware that not every bump is a particle, but I haven't a clue. – dmckee♦ Dec 10 '12 at 5:38
Yeah, I know its not a particle. I just wanted to know what's going on in that region in terms of the dynamics. Or maybe is it because they are counting EVENTS and not absolute cross section? So there's a little bit of spectral luminosity dependence? – QuantumDot Dec 10 '12 at 6:29

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One needs a Monte Carlo to be sure, but I think it is dimuons made up by a muon from a Z or W decay and the second one a random from some other process, even another Z or W . W and Z open at 80 and 90, GeV 30 GeV seems a reasonable sort of threshold for misidentified pairs to appear in bulk. – anna v Dec 10 '12 at 7:13

2 Answers

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-$p_\mathrm{T}$ 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.

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.

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.

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If you go to the preprint of CMS at this link the uncorrected dimuon plot is shown in fig1 on the left. The bump is there. The ratio of data to Monte Carlo is correctly around 1. From the monte carlo it is seen that the 30GeV bump comes from collective QCD related effects.

Figure 1: The observed dimuon (left) and dielectron (right) invariant mass spectra. No corrections are applied to the distributions. The points with error bars represent the data, while the various contributions from simulated events are shown as stacked histograms. By "EWK" we denote W to lepton neutrino and diboson production. The "QCD" contribution results from processes associated with QCD and could be genuine or misidentied leptons. The lower panels show the ratios between the measured and the simulated distributions including the statistical uncertainties from both.

The dielectron distribution does not show the bump, a good reason to accept that it is misidentification and kinematic balance effect.

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regarding the arxiv paper: given that this is a log scale, the QCD contribution in the muon plot seems not too large, maybe 10% at most. – luksen Dec 10 '12 at 16:51
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