Once again rumours are circulating that the Higgs Boson has been seen and now they are more stronger than ever. At the EPS conference it was seen that both ATLAS and CMS have an excess of events peaking at around 144 GeV. Fermilab had a signal in the same place but much weaker. At the Lepton-Photon conference starting 22nd August ATLAS and CMS will unveil their combined plot. The question is, will the combined signal at 144 GeV be enough to announce an observation over 3-sigma significance?
Needless to say some early versions of the combined plot have already been leaked but rather than show results that may change I am just going to discuss my own unofficial combinations that are not very different. So here again is my combined plot for CMS, ATLAS and the Tevatron.
This shows a brought excess peaking at 144 GeV where it is well over 3-sigma significance. It extends from 120 GeV to 170 GeV above 2-sigma most of the way but it shows an exclusion above 147 GeV at 95% confidence. The signal is the expected size for a standard model Higgs boson from 110 GeV up to 145 GeV but is excluded by LEP below 115 GeV. What could it be, a Higgs boson, two Higgs bosons or something else?
The width of the Higgs boson is determined by its lifetime and at this mass it should be no more than 10 GeV. However there is a lot of uncertainty in the measured energy in some of the dominant channels. Some useful plots shown at Higgs Hunting 2011 by Paris Sphicas show what a simulated signal looks like in the WW channels and it is clear from these that a Higgs boson at 130 GeV or 140 GeV is perfectly consistent with the broad signal now observed.
There is also a hint of a signal around 120 GeV but it is not strong enough for a claim. I would say that overall this plot is consistent with a single Higgs boson with mass between about 125 GeV and 145 GeV or more than one Higgs boson in the range 115 GeV to 150 GeV. Whatever it is, the significance is enough to claim that a Higgsless model is now unlikely to be right unless some other particle is mimicking the Higgs boson in this plot and it is probably a scalar. Afterall, we can’t really say that the signal is definitely a Higgs boson until we can confirm that it has the right cross-section in some of the individual channels.
What does this say for SUSY and other models? The MSSM requires a Higgs boson below 140 GeV. In detail the signature would be different from the standard model Higgs boson. If there were a Higgs below about 130 GeV the vacuum would be unstable (but perhaps metastable) I think something as light as 120 GeV would be hard to accept as a standalone Higgs boson and would have to be stabilised with something that looks like either a SUSY stop or a Higgsino. On the other hand a 140 GeV Higgs can easily exist on its own and requires no new physics even at much higher energy scales. At this point we cannot rule out either MSSM or a lone Higgs boson.
Earlier I said that the electroweak fits could kill the standard model and that is still the case. At Higgs Hunting 2011 Matthias Schott from the gfitter group told us that a Higgs at 140 GeV has just a p-value of 23% in the fit which includes the Tevatron data. This is far short of what is required to rule it out but it tends to suggest that there may be something more to be found if the gfitter data is good (count the caveats in that sentence.) So just how good is the gfitter data?
The green bar shows the overall preferred fit for the Higgs boson mass giving it a mass of 71 GeV to 122 GeV. But anything below 114 GeV is excluded by LEP. Anything below 122 GeV would certainly favour SUSY which is why this plot has been encouraging for theorists who prefer the BSM models. Indeed it is possible to get a much better fit to the data with just about anything other than the standard model.
How seriously should we take this? To get back some sanity have a look at the effect of the Al measurement. The fit includes two separate measurements of this parameter, one from LEP and one from SLD (SLAC Large Detector). The reason for using the two is that they disagree with each other at about 2-sigma significance. This could just be statistical error in which case we should use the combination of them both, but suppose it is a systematic error in one or other of the experiments, such as a mismodelled background? Removing the SLD measurement would push the preferred Higgs mass up and widen the error bars so that anything up to 160 GeV becomes a reasonable fit. This is just one example of how a measurement could compromise the fit. That being the case I think we should not take the fit too seriously if we have good direct evidence for something different, and now we do.
From reliable sources I am expecting CERN to issue a press release about the status of the search for the Higgs Boson next week in advance of the LP2011 conference. If the official Higgs combination is similar to my version (the leak shows that it is) then they have the right to claim an observation (but not a discovery) of a strong signal consistent with a Higgs boson at 144 GeV (or soewhere else nearby). They cannot excluded other BSM signals including MSSM. I don’t know exactly how they will spin it but they will want the media to take notice.
For more details we will need to await the next analysis. Given present results and the extra data already recorded I am sure we will not have to wait too long.