Physicists chatter excitedly at CERN. (Courtesy: Georges Boixader)
By Hamish Johnston
The particle-physics rumour mill is going into overdrive as physicists look forward to next week’s meeting of the CERN’s Scientific Policy Committee – which will include Higgs updates from the LHC’s ATLAS and CMS experiments.
If various blogs are to be believed – and a trusted source assures us that the claims are credible – the two experiments are closing in on the Higgs boson. This undiscovered particle and its associated field explain how electroweak symmetry broke after the Big Bang and why some fundamental particles are blessed with the property of mass.
The latest rumour is that both ATLAS and CMS have evidence that the Higgs mass is about 125 GeV/C2 at confidence levels of 3.5σ and 2.5σ respectively. At 3.5σ, the measurement could be the result of a random fluke just 0.1% of the time whereas at 2.5σ the fluke factor is about 1%.
If you are really optimistic, I believe you can add these two results together in quadrature to get an overall result with a significance of 4.3σ.
While these might sound like fantastic odds to you and me, particle physicists normally wait until they have a confidence of 5σ or greater before they call it a “discovery”. Anything over 3σ is described as “evidence”.
Blogger Lubos Motl has reproduced what he says is an e-mail from CERN director general Rolf-Dieter Heuer inviting CERN staff to a briefing on 13 December to hear about “significant progress in the search for the Higgs boson, but not enough to make any conclusive statement on the existence or non-existence of the Higgs”.
This seems to tie in nicely with the rumoured ATLAS and CMS results, which together are strong evidence for the Higgs at about 125 GeV/C2 – but not yet a discovery.
So why are particle physicists so conservative when it comes to claiming a discovery?
Last year, Robert Crease explored this issue in his regular column for Physics World, and you can read that column here.
Crease wisely cites past experience as the number-one reason for caution. Indeed he quotes University of Oxford physicist and data-analysis guru Louis Lyons as saying “We have all too often seen interesting effects at the 3σ or 4σ level go away as more data are collected.”
As Crease points out, nearly everyone he spoke to in writing his article “had tales – many well known – of signals that went away, some at 3σ: proton decay, monopoles, the pentaquark, an excess at Fermilab of high-transverse-momentum jets”.
So if the rumours are true, 2012 could be a very exciting year for the LHC as more data are collected and this interesting effect grows. But until the key CERN briefing on 13 December, when more information emerges, it would be wise to “keep calm and carry on”.