Was that a Higgs I just saw?

Well, CERN was certainly twittering away last night, though, to be fair, I’m glad I didn’t stay up for the press conference. Some things are worth trading in your sleep for, such as an eclipse of the moon (occasionally) or other astronomical event, an Ashes test, a Royal Wedding (just about), but, I’m afraid, not a seminar on particle physics, not even one about glimpses of the Higgs Boson.

So, it transpires that the much-hyped seminar about the ‘discovery’ of the Higgs turned into something more like a status report in the search, though, perhaps, with a some tantalising hints.

We have not yet found or disproved the Higgs – Rolf Heuer

At the Large Hadron Collider, there are two experiments, ATLAS and CMS, each with vast teams of people, looking for the Higgs. (It’s worth emphasizing that this isn’t the only thing that these teams are doing – the data from the two experiments is used for other things as well.)

ATLAS reports that, if it exists (oh those words), the Higgs is now corned in a patch of dense forest lying between 116 and 130 GeV in energy.  In the coming months they’ll be making further forays into this territory to try to flush it out. The CMS group agrees with them, though reckons the area of interest is between 115 and 127 GeV.

Who’ll be the first to bag it? It appears that both groups have made reasonable progress here, though maybe ATLAS is slightly ahead. They’re looking carefully at about 125 GeV, reporting a 2.4 sigma result, compared to CMS’s 1.9 sigma result. 

What does ‘sigma’ mean here? It’s a measure of how certain (in a statistical sense) you are about something. The experiments have collected a raft of data, and the question that has to be asked is: What is the chance of this dataset having occurred by chance alone, if there is no Higgs? What is the likelihood that that rustling you heard in the bush was just a bit of localized wind, rather than due to the moving beastie? A high sigma result tells us the probability that it was just a chance occurrence is very low; in other words gives you some confidence that you’ve actually found something. A low sigma result means there’s a fair likelihood it was just chance, so nothing to get terribly excited about, though perhaps something to take a closer look at.

Now, 2-sigma is often used in science as a threshold for identifying some effect. If my PhD students were to identify something at a 2-sigma level, I’d be quite happy. I’ve seen lots of presentations at conferences where people proudly state that because group X and group Y have results that differ at a 2-sigma level, where group X is a control group and group Y has received some intervention, then the intervention is ‘proved’ to be a success. (One really should be more careful making such blanket statements, there is more to it than that.) For important things, such as when we’re talking medical interventions of some form, a 3-sigma level is often used – more confidence is required that the result isn’t due to chance.  For particle physics, the standard is a 5-sigma result – which means that it’s very unlikely indeed that the results are just a statistical fluctuation.

It’s worth remembering that these thresholds are entirely arbitrary. There is no hard scientific reason why particle physics has to use a 5-sigma threshold. It just has, by convention, chosen to do so. As science goes, this is pretty high, denoting a very cautious approach to claiming any discoveries. To get there, ATLAS and CMS are going to need to collect more data.

Assuming, of course, that this Higgs boson beastie actually exists.



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