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At the Hadron Collider Physics symposium in Kyoto, Japan, last month (November 12-16), physicists from the ATLAS and CMS detectors on the LHC >presented data about the different ways in which the Higgs boson decayed. Ominously, however, data about the boson's decay into two photons (diphoton decay) was not presented.
The Higgs boson is a heavy particle and quickly decays in a low-energy environment to ligher particles, and being the Higgs boson, there are specific combinations of particles it can decay into each time. To be sure, there are eight such combinations, and the chances of decaying into each combination are fixed, rather predicted, by the Standard Model, a theoretical framework of principles guiding the behaviour of fundamental particles.
There was initial surprise in Kyoto about the diphoton-decay data being suppressed. However, the experimental data of the Higgs' decay into the other combinations of particles coincided (fairly) perfectly with the theoretical predictions, easing physicists toward the conclusion that the Standard Model had done a good job... again.
And now, the reason for suppression has been revealed! On December 13 (exactly a year to the day the first tentative spottings were announced), a new round of data from the ATLAS detector was >revealed at CERN , showing that the Higgs boson seemed to be decaying into two photons twice as often as allowed by the Standard Model. Not just that: the elusive particle also seemed to exist at two different masses - almost as if there are two kinds of Higgs bosons, not one.
The diphoton-decay result as such is directly affected by the exact mass of the Higgs boson assumed by physicists: on December 13, the results were declared in the context of the mass being 126.6 GeV because that's what the photons' energy accounted for. The July 4, 2012, announcement, on the other hand, provided evidence for a Higgs-like particle at 125.3 GeV with an uncertainty of 0.6 GeV.
At 126.6 GeV, the observed likelihood of a Higgs decaying into two photons was 1.8+/- 0.3 at 6.1 standard-deviations (a confidence level of 99.99966 per cent).
The "second mass" observation was derived from results of the particle's decay into four leptons. More specifically, at 4.1 standard-deviations (a confidence level of 99.534 per cent), the mass of the Higgs boson from this channel was calculated to be 123.5 +/- 0.9 GeV, which is conspicuously lower than the mass observed from the diphoton-decay channel (by about 3 GeV).
Before we start jumping up and down: The ATLAS collaboration has also announced that the two masses are compatible only at 2.7 standard deviations, which represents a lower statistical confidence than is required to assert evidence. Moreover, results from the CMS collaboration will also have to be factored in before anything is confirmed, and the next batch of data-release from CERN is expected in March, 2013. And last, it's quite unlikely that the Higgs would have a twin so close in mass - if you've already walked 122.6 m, would you consider another 3 m as requiring a lot more energy than what you've already spent?
Notably, more than anything else, physicists are open to the possibility of the data being just consequences of technical anomalies, human error, etc.
So, till they get things sorted out, we wait...
