A Search for Collinear Muons
The D0 Collaboration recently posted a brief paper describing a search for Higgs bosons in the NMSSM (arXiv:0905.3381). The model supposes that the scalar Higgs boson, h, decays predominantly to a pair of very light pseudo-scalar bosons, a. If the a bosons are very light, then they may be expected to decay predominantly to a pair of muons or tau leptons. So the signature would include events with four muons (very distinctive!) or two muons and the decay products of two tau leptons (also distinctive).
I like the idea of finding a light particle produced in high-energy collisions through its decay to muons (or photons or electrons) since it helps underscore the fact that hadron collider experiments are also a good place to look for very rare processes, not just heavy particles. I wrote earlier this year about looking for a bosons at a hadron collider and it is very nice to see this analysis by D0.
The physicists who conducted this analysis were faced with at least one interesting challenge: reconstructing two high-energy muons which come close together in space. (The muons are close together because they come, hypothetically, from a fairly light particle which itself comes from the decay of a fairly heavy particle. So the muons have a large Lorentz boost in the laboratory frame.) Most muon detectors are designed to register a single muon well, or perhaps two muons that land far apart. Their granularity is poor, compared to tracking devices or even the calorimetry. There can be problems with producing a valid muon trigger, and also with reconstructing the muon tracks themselves, even offline. Finally, one has to be careful when demanding that the muons be isolated, since they are not isolated, strictly speaking.
For the four-muon channel, the D0 physicists approach this challenge by asking only that one muon out of each pair be reconstructed, and then they pair each of two reconstructed muons with a “companion” track, meant to be the muons that were not reconstructed successfully. This may sound like it should lead to a large background, but remarkably, it does not, thanks to the isolation criteria they applied. Only two events are selected in over 4 fb-1, consistent with expected backgrounds, and neither of these has more than two muons.
The reconstruction of nearly-collinear tau decays is much harder. In fact, the D0 group did not try to reconstruct the tau’s directly, but rather leaned on the fairly large missing energy coming from the neutrinos emitted in the tau decays. (For a nice simulated event, see the analysis web page.) With two muons, significant missing energy, and then evidence of tau decays and vetos against jets, again the expected background is quite small. The two muons from the a decay (remember, for this channel one a decays to a muon pair, and the other to a tau pair) can be used to reconstruct the mass of the a boson. So the natural strategy is to look for a bump in the di-muon spectrum for this sample of events.
Here is the D0 result:
The eye catches the “peak” at 4 GeV, but of course this peak is not statistically significant. It is nice that the D0 physicists have produced a smooth background prediction from data (blue dashed histogram) – the selected events appear quite consistent with that. Narrow peaks are also drawn on the graph, indicating hypothetical signals, as black curves and black histograms. These are meant for illustration, to show the narrowness of the peak that might have been reconstructed
with the D0 detector, if a real signal had been present.
The D0 paper contains some limits placed on this production channel, which require a few not unreasonable assumptions. I’m not so interested in those limits, which generally are higher than the theory predicts. What pleases me about this piece of work is the ability of the D0 detector and event reconstruction to look for such events.
Entry filed under: Particle Physics.