Surprise behaviour from the new particle will help test theories that transcend the limits of the standard model of particle physics
Read more: "Higgsteria: Hunting the world's most wanted particle"
Editorial: "Particle discovery is a start, not an end"
A NEWLY glimpsed boson is prompting celebration around the world, but the particle could yet break the model that it is credited with completing. Or so most physicists hope.
Although spotted at last, many properties of the new particle - thought to be the Higgs boson, or at least something similar - have yet to be tested. What's more, the telltale signature it left in the detectors at the Large Hadron Collider (LHC) does not exactly match what is predicted by the standard model of particle physics, the leading explanation for the known particles and the forces that act on them. So it is possible the new particle is something much more exotic, such as a member of a more complete model of the universe that includes the mysterious entities of dark matter and gravity. That would end the standard model's supremacy, but it would also be a cause for even greater celebration than the discovery of the Higgs itself.
"Many of my colleagues and I think that this discovery on Wednesday may mark the beginning of the end of the standard model," says Georg Weiglein of the German Electron Synchotron research centre (DESY) in Hamburg. "Maybe these little deviations from the standard model really build up to a significant deviation. Maybe once we make this more precise with more data we will see that this is not the standard-model Higgs."
Rapturous applause, whistles and cheers filled the auditorium at CERN, near Geneva, Switzerland, as the heads of the twin LHC experiments presented their particle discoveries on 4 July. Joe Incandela of CMS and Fabiola Gianotti of ATLAS both reported seeing excesses of particles that fit the profile of a Higgs, with masses of 125 and 126 gigaelectronvolts (GeV) respectively. (In particle physics energy and mass are interchangeable.)
The Higgs doesn't just complete the standard model, it also has a key role to play in the nature of matter itself, as the fundamental component of the Higgs field. According to the standard model, all particles must pass through this omnipresent entity. Some, like the photon, slip through unhindered - they are massless. Others are slowed down, resulting in mass. "This boson is a very profound thing," says Incandela. "It embodies substance to all these other particles that exist."
Given the rumours, leaks and hype leading up to the announcement - and theknowledge that a discovery was in principle possible given the data collected - the particle discovery was not a complete surprise. Remarkably though, ATLAS and CMS both claimed 5 sigma confidence in the result, equivalent to a 5 in 10 million chance that the readings could have been created by background processes in the detector. That exceeded the best of the anticipated outcomes. "I think we have it," concluded Rolf-Dieter Heuer, director general of CERN.
Discussion quickly moved to what exactly "it" was. The Higgs hasn't been glimpsed directly - but via its decay into a plethora of other particles more easily picked up by the LHC detectors.
The standard model predicts the rate at which a Higgs of a given mass should decay into these particles. But the reported rates for the new particle do not exactly match what is predicted for a mass of about 125 GeV (see diagram). The anomalies could disappear, producing a standard-model Higgs - or they could grow. Most physicists are hoping for the latter.
It is clear that the standard model is inadequate, not least because it can't explain 80 per cent of the matter in our galaxy - dark matter - and makes no mention of gravity (See "Can we split the Higgs boson?"). A non-standard-model Higgs would be a big clue as to which of many proposed extensions to the standard model - if any - is a correct description of reality.
Steven Weinberg, who won a Nobel prize in 1979 for theoretical work on elementary particles, has previously said it would be a "nightmare" if a Higgs boson was discovered that neatly fulfilled its duties as laid out by the standard model and did nothing more. Such a particle wouldn't give us clues about what's next. "It is crucial to keep looking for clues to a more comprehensive theory," he told New Scientist following the announcement.
Luckily, there are several gaps in the data presented last week that might yet turn the nightmare into a dream. Conversations with physicists from ATLAS and CMS at the International Conference on High Energy Physics (ICHEP), whichkicked off in Melbourne, Australia, directly after the CERN announcement, and the flurry of papers appearing on the arxiv preprint server since the announcement, suggest that there are grounds for cautious optimism.
According to the standard model, a Higgs boson of about 125 GeV should decay into tau particles about six per cent of the time, but it seems to be doing it a lot less than that. At last week's seminar, the CMS team reported no excess in tau production beyond what is expected due to background processes. ATLAS, meanwhile, did not release any data specifically on tau production. "I think this is a very intriguing thing, which is perhaps trying to tell us something already," says Albert De Roeck of CMS. "It's sort of a really strange game that's going on there," agrees Paul Jackson of ATLAS. "If that continues to be seen, it is certainly not a Higgs boson."
What else would it be? Only two types of elementary particle are known to exist: fermions, which make up matter and include electrons, quarks and neutrinos; and bosons, which are force carriers and include photons and the W and Z bosons. According to the standard model, the Higgs field is responsible for the mass of all the fermions and bosons. But taus are fermions - and if the Higgs is not decaying into taus, it is probably not giving them mass either. Might the Higgs only give mass to bosons?
De Roeck thinks that might be the case. He points out that when Peter Higgsand others came up with the theory in the 1960s, the Higgs mechanism was designed only to explain the mass of the bosons. It wasn't until later that the mechanism was extended to include all other mass-bearing particles, as a simplification, he says. "So maybe the Higgs is doing what it should be doing."
That's interesting, because then something else is needed to give mass to fermions, a possibility that starts to sound a lot like a hypothetical but mathematically elegant extension to the standard model called supersymmetry. This theory proposes a slew of new particles and superpartners to solve several phenomena that the standard model can't address, including dark matter and a thorny contradiction known as the hierarchy problem. Supersymmetry specifies a minimum of five Higgs bosons, plus several superpartner Higgsinos - some of these other Higgses could give fermions mass if it turns out that the boson spotted at the LHC doesn't.
See graphic: "New particle, new questions"
Many are cautious about extrapolating so much from the tau data at such an early stage. Peter Jenni, a founder and former head of ATLAS, doesn't think it is telling us anything yet. He says bigger statistical deviations have disappeared in the past and he expects this one to as well.
But just about everyone agrees that it's a channel to keep an eye on. "It's not something we're yet making any strong statements about, but it will be something interesting to watch," Incandela said at ICHEP.
Decay data
The low tau rate wasn't the only anomaly in the data. The CMS and ATLAS teams also reported that the new boson seems to decay into a pair of photons too frequently - about one-and-a-half times the rate predicted by the standard model (see diagram). If that trend continues, it could mean that another particle is being produced in the detectors, alongside the Higgs-like boson. That particle might be one of the other Higgs particles predicted by supersymmetry, says De Roeck, or something else.
This "diphoton" excess is extremely important, says Kai Wang of Zhejiang University in Hangzhou, China. "If the current situation stays and the precision improves, I believe it will strongly imply the existence of physics beyond the standard model."
Two days after the announcement, Wang's team posted a paper on arxivshowing that the existence of superpartners of tau particles - called "staus" - could explain the diphoton excess. They show that, via a mechanism first outlined by a group based at Fermilab in Batavia, Illinois, these particles could cause the Higgs to produce more photons (arxiv.org/abs/1207.0990).
It won't be smooth sailing for the LHC experimenters, says Wang. The types of collision that occur at the LHC make creating stau particles difficult. Another kind of collider might be needed (see "Physicists propose factory to spew out Higgs particles").
Another intriguing explanation - which could account for the tau deficit and the photon excess - appeared on arxiv on 5 July. Dan Hooper and Matthew Buckley, both of Fermilab, calculate that if the superpartner of the top quark - the stop - is present when the Higgs is decaying, it will alter the decay to create both anomalies (arxiv.org/abs/1207.1445). By contrast, a stau only explains the excess of photons. "If you only want to introduce one new particle, stops are the only one that gets you everything you need to explain the data," Hooper says.
See graphic: "A boson spotter's guide"
A stau or a stop would be good news for finding dark matter. "In either of these scenarios, you expect at least one superpartner that's even lighter than the stau or stop," says Hooper. "That could be the dark matter."
Exciting though these possibilities are, nearly everyone urges caution. "All of these things, they're like pipe dreams," says Christoph Paus of CMS. "Of course I'd like to see a difference, but if I'm honest, at the moment everything looks like the standard-model Higgs." Even if both anomalies disappear, the Higgs needn't leave us in Weinberg's nightmare.
Although ATLAS and CMS had enough data to see the new boson with certainty, they haven't yet got enough to pin down its properties. They now need to identify the particle's spin - a quantum property a bit like the angle of a particle's rotational axis.
The observed decay into photon pairs, combined with the fact that the particle is definitely a boson, constrain its spin to either 2 or 0. To fulfil its duty of giving other particles mass via the Higgs mechanism, the spin must be 0. Only then can the particle be the fundamental component of the non-directional, or "scalar", Higgs field.
Most physicists think it will be 0, because producing a particle with a spin of 2 in a collider is harder and so less likely. However, even if it is 0 there's yet another way it may still be non-standard, which relates to a property called parity, most easily explained via mirror images. Usually, if a particle has spin 0, its mirror image looks identical. But it is possible for a particle to be spin 0 and not have that property. Among the five Higgses in supersymmetry, one - known as a pseudoscalar - has this property.
"Finding something, and it being a pseudoscalar, is right up the alley of supersymmetry," says De Roeck. Although he adds that, maddeningly, if it isn't a pseudoscalar, that doesn't mean supersymmetry is ruled out.
Jenni, meanwhile, bristles at the notion of a nightmare scenario, and says that finding a standard-model Higgs would be just fine. "To find this last piece of the puzzle was one of the main goals of the LHC. We also know that the standard model does not explain it all, so I don't think this will mean that life for the next 15 years at the LHC will be boring at all."
Until next year, when it will hibernate for an upgrade, the LHC is expected to run smoothly, more than doubling the total amount of data collected. By some estimates, that could allow the tau, diphoton and spin questions to be settled within a year.
That's good news for those of us waiting on tenterhooks to find out if we are living in a supersymmetric universe, or something even more weird and wonderful. But it's a mixed bag for those already tired after the scramble to produce the Higgs result in time for ICHEP. "Now all hell breaks loose," says De Roeck. "I was thinking about taking a vacation. But now..."
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