Ending the year with a cliffhanger

A couple of years ago ‘LHC’ made a list of the top ten words of the year. This year, it should be the turn of ‘sigma’ to feature on such a list. It’s been a year in which the importance of statistics in particle physics has really come to the fore, along with the caution and rigour necessary in statistics-based analyses.

In particle physics, the significance of an observation is measured in terms of standard deviations - sigma for short. The standard deviation is a measure of how likely it is that an observation is due to chance rather than being something new.  Two sigma effects come and go roughly as often as an unloaded die will come up with two sixes in a row. A three-sigma effect corresponds to a few parts per thousand chance that the observation is a statistical fluke, and is generally accepted as the point at which an observation becomes intriguing. A discovery, however, requires a five-sigma effect, for which the chance of a statistical fluke is less than one in a million.

This week, in open seminars from ATLAS and CMS, we’ve seen a number of relatively low significance effects for a possible Standard Model Higgs boson, and the particle physics world is taking them very seriously. What’s interesting is that there are multiple measurements coming from two independent experiments, all of which point to the same conclusion: possible evidence for a Standard Model Higgs boson with a mass in the region of 124-126 GeV. It’s far too early to say that ATLAS and CMS have discovered the Higgs boson, but the coincidence of these measurements is certainly raising eyebrows.

The Higgs boson, if it exists, is relatively short-lived and can decay in many different ways. Discovery relies on observing the decay products rather than the Higgs itself. Both ATLAS and CMS have analysed several decay channels, and both experiments see small excesses in their data in two separate decay channels and at the same energy. More data will be needed to turn these excesses into a discovery, or to definitively rule out the existence of a Standard Model Higgs boson, so we can’t expect any announcement on that front until next summer at the earliest. Nevertheless, it’s safe to say that both experiments’ analyses over the coming months will be focusing on this mass, and that whatever the weather, next year’s winter conferences will be the hot ticket for particle physicists.

Rolf Heuer