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Ken Bloom | USLHC | USA

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The road to the Higgs boson

Everyone knows that there isn’t enough LHC data yet to learn anything new about the Higgs. But that doesn’t mean that you shouldn’t try, and if you do try you might learn some interesting things along the way. Consider for instance this recent paper from CMS. It is a very nice representation of one of the trajectories that the LHC can follow to a discovery of the Higgs boson (should it actually exist).

The analysis makes use of events that have two high-momentum leptons with opposite electric charge, where here we define leptons as electrons or muons. In proton-proton collisions, the production of even one high-momentum lepton is already unusual, and two is quite interesting. There are a variety of physics processes that can lead to this. The most common, by far, is the decay of a Z boson; this was easily observed last summer. Another process is the decay of a pair of top quarks; a few percent of the time both will decay to a lepton. Top quarks are produced only about one sixth as often as Z’s, so that takes a bit more data to find. [Experts will note that I’m only quoting total production rates and not accounting for branching fractions, but stay with me here.]

The next process that can lead to two leptons is the direct production of a pair of W bosons, which happens about four times less frequently than top-pair production. This process is what is observed in the paper; there are a total of thirteen candidate WW events observed, and the estimated number of background events in the sample is about three. It isn’t too hard to separate the WW events from the top background — top decays also tend to include jets of hadrons, whereas the WW events generally don’t. The events observed have the properties expected for WW pairs. In particular, the momenta of the leptons are consistent with what’s expected from the standard model, as opposed to what would be predicted from theories that include different ways for W’s to interact with other particles.

And, finally, another process that can lead to two leptons is the production of a Higgs boson that would be heavy enough to decay to a pair of W’s. Don’t forget that we don’t know what the mass of the Higgs boson is; from other measurements, we have been able to bound the range, but there is no theory that predicts a value, so how you might look for a Higgs depends what mass you might think it has. But should the Higgs be sufficiently heavy, a decay to WW is quite common and the two-lepton signature is quite clean. Separating the Higgs production from the more common direct WW production is more of a challenge, requiring a more careful examination of subtle features of the events, and really we would need a factor of ten more data to have a hope of seeing a Higgs boson this way. But it’s worth making an effort, and thus the paper sets upper limits on the production rate of a standard model Higgs. It’s not competitive with the limits that have been set by the Tevatron experiments, but it establishes that it is possible to do this analysis at the LHC.

There is one more trick that the paper pulls out. We are used to thinking of having three generations of quarks and leptons in our world. There is nothing to suggest that this isn’t so, but if there were a fourth generation of particles that were very, very heavy, it would be very hard to know about it because they would be out of the reach of our current experiments. But if such a scenario were true, it turns out that Higgs particles would be produced at a much greater rate at the LHC. The fact that no Higgs signal is observed in this paper tells us that this scenario is unlikely.

This is not an unusual path to follow in experimental particle physics: observe a high-rate process, then take more data, then look for a lower-rate process with a similar signature, then repeat until you see something truly new. But this is a path that might soon lead to a major discovery.

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