The special CERN seminar on recent Higgs boson results held yesterday was one of the most exciting presentations I ever attended. The ambiance was electricifying and the room was packed more than two hours before it even started.
Members of each collaboration working on this, namely CMS and ATLAS, both knew their half of the story. But the two teams had worked independently and all the crucial details of the final results were not known outside the collaborations. Everybody wanted to see if the small excesses observed in their team coincided with similar findings from the other collaboration.
Physicists are notoriously cautious for good reason. To claim a discovery, we ask that if there is only background (and no Higgs), the odds of seeing an excess of event as large as the one observed be less than 0.00003% or 5-sigma.
In the case of the Higgs boson, if we find some signs of its possible presence, we will want it to do much more than just ”look like” a Higgs but also behave like one, smell like one, dance and sing like only that particle can do. As it is, it may look like a Higgs with a mass somewhere around 124-126 GeV but the level of confidence is way too low to draw conclusions. Each experiment has small signals at the 2-3 sigma level, which is what is expected if there is a Higgs boson given current data size. To reach the unambiguous 5 sigma-deviation level will require adding new data.
The higher the number of sigma, the more incompatible the data are with having only background and no Higgs.
Of course, it is encouraging that both groups find similar results, not only in one decay mode, but in multiple channels. A decay channel represents one of the many ways the Higgs boson can decay. As one of my colleagues put it, if the Higgs boson was a large coin, each decay channel would represent one way to break this coin to make small change. CMS and ATLAS collected all events corresponding to specific decay channels. The fact that they all point somewhere to roughly the same mass value is an indication they could all be coming from the same particle.
ATLAS spokesperson, Fabiola Gianotti, presented the ATLAS findings first.
Two separate decay channels both favour a mass value around 126 GeV: Higgs decaying into two photons and Higgs into two Z bosons, with each Z going into a pair of electrons or muons. A third channel with Higgs decaying into two W bosons, each W decaying into an electron or muon plus a neutrino is also consistent with this hypothesis but at a lesser level.
Guido Tonelli, CMS spokesperson, showed the combination of five different channels, adding the Higgs to two taus and Higgs to pairs of heavy quarks to those investigated by ATLAS. The combined results are compatible with a Higgs signal, the highest probability being found at 124 GeV, but not enough data were available to draw any definitive conclusions. The observed excess of events could be a statistical fluctuation of the known background processes, either with or without the existence of the Standard Model Higgs boson in this mass range.
The probability of obtaining an upward fluctuation as large or larger than that is observed if there is only background, prior to accounting for the look-elsewhere effect. As one can see, the excess falls in the same position for two different search channels and is also compatible with a much smaller excess in the third channel. The statistical significance is still modest but having three channels, especially two robust ones, is an indication this could be real. Nevertheless, this is a stronger signal than what was expected from a Standard Model Higgs boson with a mass of 126 GeV, which is shown by the black dashed curved.
The small excess of events observed by CMS in five different decay channels. The dotted line shows what was expected in the absence of a Higgs boson. The green and yellow bands represent the 1-sigma and 2-sigma error margin on this prediction. The black curve is the observed data. Excursions beyond the yellow band indicate where a Higgs signal is the strongest. The most significant value is found for a Higgs mass around 124 GeV.
When all their channels are combined, ATLAS obtains an excess of 2.3 sigma over background, while CMS gets 1.9 sigma, after taking into account the “look-elsewhere effect”, namely how often when looking at all the possible mass points under study would one point fluctuate that much. The chance of obtaining an upward fluctuation this large or larger if there is only background is 1% for ATLAS and about 2.9% for CMS.
Without the “look-elsewhere” correction, the ATLAS probability of such an excess of events if there is only background 3.6 sigma. This value can be compared to the 2.4 sigma deviation one would expect if the excess was due to a Higgs boson. So ATLAS sees slightly more events than what is expected from a Standard Model Higgs boson. Statistical fluctuations can happen in both directions, which is why caution is required until more data is analyzed.
Having already combined all data for 2010 and 2011 from more channels, CMS showed they now exclude all possible Higgs masses from 127 to 600 GeV with a 95% confidence level, leaving only a narrow window open between 114-127 GeV. ATLAS excludes masses above 131 GeV up to 453 GeV with the same confidence level, but also between 114-115.5 GeV.
The exclusion limits presented by the CMS collaboration. The dotted curve shows what was expected while the black line with dots indicates what is observed. Whenever this curve falls below the red line is excluded. All masses above 127 GeV are now excluded at 95% confidence level.
Of course, everybody would love to be able to say: that’s it! We found it. But it is still premature despite encouraging signs. More data will be collected in 2012. The answer will then become unambiguous: we will either discover the Higgs or rule it out completely. If the small effects presented today keep growing, we will then see the Higgs do its little song and dance.
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