We have found it – now we have to work out exactly what “it” is. That neatly sums up the thoughts of many physicists at CERN yesterday as they began to absorb the announcement that the Large Hadron Collider (LHC) had discovered a Higgs boson – or at least something like a Higgs. CERN’s director general Rolf-Dieter Heuer was very careful to describe the new particle, which has a mass of about 125 GeV/c2, as a “fundamental scalar boson”. However, even the scalar part of that description – which indicates that the particle has zero spin – has not been completely nailed down.
To learn more about the particle they have found, CERN physicists need more data and more time. Heuer therefore announced yesterday that the LHC will run for an extra three months beyond its scheduled December 2012 maintenance shutdown to allow physicists to do just that. According to CERN’s Bill Murray, the performance of an accelerator and its associated experiments usually improves toward the end of the run, which suggests that physicists can expect many more quality data before the LHC is temporarily switched off in early 2013.
The LHC creates the Higgs in proton–proton collisions, with the giant ATLAS and CMS experiments detecting the particles created when the Higgs decays. This occurs in a number of different ways – or “channels” – and by studying how these decays take place, physicists should get a better picture of exactly what they have discovered.
Most of the data contributing to the new discovery come from the so-called precision measurements, whereby the Higgs decays either into two photons (the diphoton channel) or into two Z bosons (ZZ). Since all of the decay products in both of these channels can be detected, physicists can therefore calculate the mass of the Higgs very precisely.
But there are also a number of different channels in which not all the decay products can be detected. These channels are trickier to deal with because some information about the decay is missing and therefore the mass that calculations based on these channels give is not as precise.
Yesterday, both the ATLAS and CMS teams reported that their precision diphoton and ZZ results were enough to push both experiments over the magic 5σ level that is generally considered a discovery in particle physics. ATLAS chose not to present results from other, less-precise channels, although CMS did – something that brought the statistical significance of its finding down to 4.9σ.
Beyond the Standard Model?
The Standard Model of particle physics describes how the Higgs should decay through various channels; so by comparing these predictions with how the decays actually appear to proceed in the LHC, physicists can tell if what they are dealing with is a Standard Model Higgs. So far the results are consistent with a Standard Model particle, with all channels lining up to the Standard Model to within the error bars.
Intriguingly, however, the number of events in the diphoton channel of both CMS and ATLAS continues to be greater than expected as more data are gathered. This excess could therefore be the result of “new physics” that goes beyond the Standard Model, such as a new charged particle, the existence of a multitude of Higgs, or perhaps the effects of “supersymmetry” – the idea that all particles have “superpartners” with very different spin properties.
Deficit of evidence
Whereas there is a surfeit of diphoton events in the LHC data, it is a different story for the decay of the Higgs into two W bosons – the WW channel. Conventional theories about the Higgs predict that this channel should be seen by the LHC, but to date far fewer events than expected are being reported. Indeed, if this channel does not exist, a major shake-up of particle-physics theory could be on the cards, which could be why many physicists at CERN believe that the WW deficit is not real but simply the result of the fact that the WW channel is tricky to measure.
But if there really is a deficit in the WW channel, it could suggest that the new particle might not be a scalar boson with zero spin. “The WW search is designed to look for a spin-0 [scalar] particle,” admits Bill Murray, who is a member of the ATLAS collaboration. “They have not considered the possibility that it might be spin-2.” Although experiments at the Tevatron at Fermilab seem to rule out spin-2 (and spin-1 is ruled out by the diphoton data), physicists are therefore left with the remote – but exciting – possibility that this new particle is not a scalar boson.
Daniela Bortoletto of Purdue University in the US, who is a member of the CMS team, points out that the channels in which the Higgs decays to a pair of tau leptons or a pair of b-particles also appear to have deficits of events. Given that tau and b-particles are both fermions, Bortoletto says that if this deficit endures as more data are collected, then it might mean that the Higgs interacts differently with fermions and bosons.
Murray, though, says that the deficit could point to a “mixed model” of fermion versus boson coupling to the Higgs, adding that the tau–tau and b–b channels will definitely benefit from the three-month run extension. Indeed, most physicists at CERN who were digesting yesterday’s big announcements believe that by the end of the extended run they should have a much better idea of whether they have a Standard Model Higgs.
But one important measurement that the LHC will not be able to make until it is upgraded to collide protons at 14 TeV – rather than 8 TeV today – is the “self-interaction” of the Higgs boson. That is how two Higgs behave when they encounter each other – something that should only be seen at higher collision energies in which two Higgs could be produced. “An important question is whether you need to measure the self-interaction before you can say you really have a Higgs?,” says Murray. As the LHC’s 14 TeV upgrade is not likely to be completed before the end of 2014 at the earliest, it looks like the debate is set to continue.
Before then, all eyes will surely be on who, if anyone, should win a Nobel prize for the new discovery. Peter Higgs – after whom the boson is named – appeared reluctant at yesterday’s press conference to be drawn on the matter, having always maintained that he was not alone in devising the core ideas that led to the prediction of the Higgs boson in the early 1960s. Speaking recently to Physics World, he said at least five other theorists – include the late Robert Brout, François Englert, Gerald Guralnik, Carl Hagen and Tom Kibble – deserve credit too. But given that the Nobel committee can award the physics prize to no more than three physicists each year, it will have a tricky job on its hands.