No more Higgs-like, Higgs-ish or even Higgsy boson. The CMS and ATLAS collaborations, the two large experiments operating at the Large Hadron Collider (LHC) at CERN, have now gathered sufficient evidence to say that the new boson discovered last summer is almost certainly “a” Higgs boson. Note that we are going to call it “a” Higgs boson and not “the” Higgs boson since we still need more data to determine what type of Higgs boson we have found. But all the analysis conducted so far strongly indicates that we are indeed dealing with a type of Higgs boson.
The Standard Model predicts there should be only one Higgs boson and so far, our Higgs boson is compatible with being the Standard Model Higgs boson. But it could still be one of the five types of Higgs bosons postulated by supersymmetry, a theory that would build on the Standard Model and complete it in a way that it would not only be able to explain the world made of matter that we know, but also provide a possible explanation for something still completely unknown called dark matter.
Both ATLAS and CMS checked not only the mass but also the couplings of the new boson. In all cases where the experiments have sensitivity, the couplings are consistent with the Standard Model. But the truth may lie in the tiniest detail. Take for example the signal strength, a quantity that measures how many signal events are found in different decay channels compared with the numbers expected from the Standard Model. The Standard Model boson would then come out with signal strength of one in all decay channels. But if other, yet undiscovered particles exist, then they would provide more options in the ways the Higgs boson could decay, and we should start seeing more signal events or if additional Higgs bosons exist we might see less signal strength in some channels.
Among new results shown this week at the Moriond QCD conference, CMS reported updated results for a Higgs decaying into two photons and ATLAS had an update on the Higgs decaying to a pair of W bosons. CMS presented their main result and a result from a cross-check analysis using a different analysis approach. The two results, of 0.78±0.27 for the main analysis and 1.11±0.31 for the cross-check, are consistent within uncertainties. ATLAS measured a signal strength of 1.0±0.3 in the WW channel and, 1.30±0.21 for all channels combined. These results are so far in reasonable agreement with a value of one predicted by the Standard Model. Values different from one can come from statistical fluctuations as well as from new physics as mentioned earlier. Only more data and more study will allow us to tell.
The latest results presented at Moriond mark an important step forward in the Higgs analysis, but also serve as a reminder that we still have a long way to go. It looks very much as though we have “a” Higgs boson, the question now is what kind?
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