13 December: please follow the live blog for up-to-date news
A rumour that reached our comment section suggests that a signal for the Higgs boson has been seen at 125 GeV with 2-3 sigma significance. This would be a great result if confirmed because at this mass the standard model has problems with vacuum stability that are likely to require supersymmetry or something similar to stabilize. If on the other hand the Higgs were at 140 GeV we would be left with a simple but unsatisfying model that could exist without modification up to energies well beyond anything we can explore in man-made experiments. In other words we might never be able to detect anything new. A Higgs that is just 15 GeV lighter is a different story altogether, so theorists will be very happy if that is the answer.
A statement by the CERN DG circulated to staff says that the results that will be released on 13th December will be inconclusive. This means that a 5 sigma signal is not yet available. A Higgs signal at 140 GeV would probably be conclusive, at least with the 10/fb of data combined, but of course the combination has not yet been done. In other words, the inconclusion is consistent with the lighter mass but not conclusively. Another rumour says that the signal is only seen in the diphoton end state for both experiments. This again suggest the lighter mass because anything in the range 130 GeV to 150 GeV would show up strongly in the ZZ to 4lepton channel but 125 GeV wont. Oddly enough the diphoton channels up to 3/fb combined showed no excess at 125 GeV, but events at this mass would be very rare and if there is a signal it will be just a few events on the 10/fb sample.
It has to be said that the best way to foil rumours is to spread false rumours, but the consistency of the rumours we have suggests that they are genuine. The only thing I know that counts against them is that the Tevatron search in the bb mode shows no signal at 125 GeV where they have good reach. This could have been just bad luck. Even so it will be interesting to see the whole plots which might have other potential signals at higher energy. A Higgs at 125 GeV may well be accompanied by other heavier Higgs states that may show only a partial signal, either because they have the possibility to decay into the lighter Higgs or because they have odd CP (rather than the even CP of the standard model Higgs)
With an inconclusive signal the combination of results from ATLAS and CMS is all important. Approximate combinations of the type I have been doing will be good indicators but only the carefully prepared official combination can lead to a definitive result. Last month I looked at best fits to the combined summer data and found the 140 GeV signal to be best. If I do a fit to a two Higgs model I get a second one at 128 GeV. The lighter peak at 119 GeV favoured by Italian bloggers has error bars too big to grab the second place. It is going to be very interesting to repeat this with the 10/fb of data and see if both of these signals survives the fit.
One last thing worth mentioning is that the gfitter calculations have been estimating 125 GeV for the Higgs mass for some time as the best fit. Well done to them. This would mean that if it is confirmed at that mass, no further physics is required at this energy to account for precision tests. On the other hand, gfitter calculations also fit doublet models well to the data so other physics is not ruled out either.
Another piece of good news is that the results meeting on 13th December will be webcast. Unless they enlist the services of a heavy-duty streaming service their normal webcast channels will certainly be overwhelmed by the public interest in this event which has already been reported by the BBC, Telegraph, Guardian, and many others.
Update 3-Dec-2011: Some clarification at NEW is that ATLAS has a 3 sigma excess at 126 GeV while CMS has a smaller excess at 126 GeV, perhaps 2 sigma, both in diphoton channels. These are close enough to combine to give a 3.5 sigma. That would be enough to claim an “observation” but is well short of “discovery”. There will be interest in whether other channels such as ZZ or WW add anything to the result. By the end of 2012 they will have up to four times the data which is enough to multiply the significance by two if the signal holds up. ( I am assuming that the results to be shown on the 13th will use the full 5/fb collected this year. It could be less. )
Update: The latest version of the rumour at NEW gives 3.5 sigma in ATLAS and 2.5 sigma in CMS which amounts to about 4.3 sigma combined for the 10/fb. This is about right for the expected significance at this mass.
Tommaso’s post at QDS is worth reading but we will need to wait until the official announcement for his analysis because he knows too much.
Update 4-Dec-2011: Nature blog has an interesting snippet of news about the rumours including a comment from Bill “Nonsense” Murray who says that ATLAS collaboration approvals will (hopefully) be finalized at a meeting on 7th December, followed by management approvals.
Update 8-Dec-2011: The BBC has run another story including an interview with John Ellis and a quote from Sergio Bertolucci that “I think we may get indications that are not consistent with its non-existence.” As director of research at CERN Bertolucci is likely to be one of a very small number of people who have officially seen both sets of results from CMS and ATLAS.