New particle, new questions

SPECIAL REPORT / beyond the Higgs

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In this section n Exome test reveals root of rare disease, page 10 n There were three waves of “first” Americans, page 11 n Ride an inflatable robot, page 17

SOURCE: CMS

A NEWLY glimpsed boson is prompting exactly “it” was. The Higgs hasn’t been Deviant decay celebration around the world, but the glimpsed directly – but via its decay The standard model predicts the rate at particle could yet break the model that into a plethora of other particles more which the Higgs should decay into five it is credited with completing. Or so easily picked up by the LHC detectors. particle types. Decays for the new boson most physicists hope. The standard model predicts the rate are not matching this exactly Although spotted at last, many at which a Higgs of a given mass should Observed rate divided by expected rate properties of the new particle – thought decay into these particles. But the Normalised expected rate to be the Higgs boson, or at least reported rates for the new particle do Pair of bottom something similar – have yet to be not exactly match what is predicted for quarks (bb) tested. What’s more, the telltale a mass of about 125 GeV (see diagram, Pair of taus (ττ) signature it left in the detectors at the left). The anomalies could disappear, Large Hadron Collider (LHC) does not producing a standard-model Higgs – or Pair of photons (γγ) exactly match what is predicted by the they could grow. Most physicists are standard model of particle physics, the hoping for the latter. Pair of W bosons (WW) leading explanation for the known It is clear that the standard model is Pair of Z bosons (ZZ) particles and the forces that act on inadequate, not least because it can’t them. So it is possible the new particle explain 80 per cent of the matter in our -1 0 1 2 3 Can fall below zero if lower than is something much more exotic, such galaxy – dark matter – and makes no expected background rate as a member of a more complete model mention of gravity (See “Can we split of the universe that includes the the Higgs boson?”, below). A nona very profound thing,” says Incandela. standard-model Higgs would be a big mysterious entities of dark matter and “It embodies substance to all these gravity. That would end the standard clue as to which of many proposed other particles that exist.” model’s supremacy, but it would also extensions to the standard model – if Given the rumours, leaks and hype be a cause for even greater celebration any – is a correct description of reality. leading up to the announcement – and than the discovery of the Higgs itself. Steven Weinberg, who won a Nobel “Many of my colleagues and I think “This discovery the knowledge that a discovery was in prize in 1979 for theoretical work on may mark the principle possible given the data that this discovery on Wednesday may elementary particles, has previously beginning of collected – the particle discovery was mark the beginning of the end of the said it would be a “nightmare” if a the end of the not a complete surprise. Remarkably standard model,” says Georg Weiglein Higgs boson was discovered that neatly standard though, ATLAS and CMS both claimed of the German Electron Synchotron fulfilled its duties as laid out by the model” 5 sigma confidence in the result, research centre (DESY) in Hamburg. standard model and did nothing more. equivalent to a 5 in 10 million chance “Maybe these little deviations from Such a particle wouldn’t give us clues that the readings could have been the standard model really build up to a about what’s next. “It is crucial to keep created by background processes in the looking for clues to a more significant deviation. Maybe once we detector. That exceeded the best of the make this more precise with more comprehensive theory,” he told New anticipated outcomes. “I think we have Scientist following the announcement. data we will see that this is not the it,” concluded Rolf-Dieter Heuer, standard-model Higgs.” Luckily, there are several gaps in the director general of CERN. Rapturous applause, whistles and data presented last week that might yet Discussion quickly moved to what cheers filled the auditorium at CERN, turn the nightmare into a dream. > near Geneva, Switzerland, as the heads of the twin LHC experiments presented Could we split the higgs boson? their particle discoveries on 4 July. Joe The standard model of particle akin to dark matter. composite Higgs is a possibility. Incandela of CMS and Fabiola Gianotti physics assumes the Higgs boson is To see if the boson reported last The idea is not new, and “it’s not of ATLAS both reported seeing excesses an elementary particle. But what if, week at CERN near Geneva, so crazy because we’ve seen it all of particles that fit the profile of a rather like the proton, it is itself Switzerland, could be such a before with the proton”, says Tony Higgs, with masses of 125 and 126 made up of particles? composite, Alex Pomarol from the Gherghetta, a theoretical physicist gigaelectronvolts (GeV) respectively. We know the standard model Autonomous University of at the University of Melbourne. (In particle physics energy and mass is incomplete because it cannot Barcelona in Spain has started to Although the proton was are interchangeable.) explain all the phenomena we compare decay data for the new discovered near the beginning of The Higgs doesn’t just complete the observe (see main story). Tweaking particle with predictions of how a the 20th century, it wasn’t until the standard model, it also has a key role to the model to make the Higgs a composite Higgs would decay inside 1970s that physicists realised, “oh play in the nature of matter itself, as composite of quark-like particles, the Large Hadron Collider. He told wait a minute, the proton is not an the fundamental component of the bound together by a new force, the International Conference on elementary particle, it’s actually Higgs field. According to the standard could solve this problem. It turns High Energy Physics in Melbourne, made up of other constituents”, he model, all particles must pass through out that there is more than one Australia, that the observed decays says. These constituents are now this omnipresent entity. Some, like the way to arrange these new particles are not outside the range predicted known to be quarks and gluons. photon, slip through unhindered – and forces to produce something by composite models – and that a Michael Slezak they are massless. Others are slowed down, resulting in mass. “This boson is 14 July 2012 | NewScientist | 7

Conversations with physicists from ATLAS and CMS at the International Conference on High Energy Physics (ICHEP), which kicked 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

VOLKER SPRINGEL/MAX PLANCK INSTITUTE FOR ASTROPHYSICS/SCIENCE PHOTO LIBRARY

SPECIAL REPORT / beyond the Higgs

If the Higgs is more exotic than the standard model predicts, it could give us clues about entities like dark matter

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?

LET’S BUILD A HIGGS FACTORY – on the cheap No sooner has one mammoth accelerator delivered its first big result, than discussions begin on what should replace it. At the annual get together of Nobel prizewinners in Lindau, Germany, all the talk was of whether the Large Hadron Collider is the right instrument to find out what exactly the LHC has found. The problem is that the LHC collides two beams of protons. These particles are made of a melange of quarks and gluons, so when two hit, the result is a confusing array of shrapnel. Finding something that looks like the Higgs boson has required painstaking reconstructions of what was fleetingly produced in the collisions. Pinning down the new particle’s true properties in this confusing environment and

8 | NewScientist | 14 July 2012

reaching a conclusive identification might simply be impossible. “The question is, will the LHC be able to do it at all? Or do we need something else?” asks Nobel prizewinner Carlo Rubbia who was key to getting the LHC built. Particle physicists’ traditional answer to this problem has been to suggest building a machine to collide electrons and their antiparticles, positrons. Unlike protons, electrons are elementary particles, so don’t disintegrate on impact, making it clearer what is produced when they collide. A proposal to build such a machine, the International Linear Collider, has been on the table for some time. Some hope that the discovery of the boson last week will revitalise these efforts. The problem is that an

electron-positron collider of the right energy would be even bigger than the LHC. With an estimated price tag of $20 billion, few countries are willing to commit. So Rubbia has another solution. Muons are particles similar to electrons, but 200 times more massive. That means a muonantimuon accelerator could reach the necessary energies over a far shorter distance. With the putative Higgs checking in at about 125 gigaelectronvolts, you would need to bash together a muon and an antimuon with each having just over 60 GeV of energy. “That should be doable with a machine 100 times smaller than the LHC,” says Rubbia. The result would be a “Higgs factory” producing the particle and little else.

Richard Webb, Lindau, Germany

De Roeck thinks that might be the case. He points out that when Peter Higgs and 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 massbearing 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. 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.

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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.

A boson-spotter’s guide The new particle is Higgs-like – here’s how we will know if it’s the Higgs we were expecting or some other beast *The boson decays into 2 photons each with spin 1. So symmetry constraints mean that it can’t also have spin 1

NEW BOSON

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 oneand-a-half times the rate predicted by the standard model (see diagram, page 7). 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 arxiv showing 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 “Let’s build a Higgs factory – on the cheap”, left). 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.

What is its spin?*

2

Not Higgs, new mechanism for mass needed

0 Do its mirror images look identical?

NO

YES Does it frequently decay to taus?

NO

YES Does it decay YES more often than predicted to photons

Might be non-standard model Higgs, only giving mass to bosons but not fermions Evidence for 2nd new particle

NO STANDARD MODEL HIGGS Leaves us with no clues to attack other mysteries of the universe

EVIDENCE FOR SUPERSYMMETRY Candidate “theory of everything”

YES

Is that particle predicted by supersymmetry NO EVIDENCE FOR NON-SUPERSYMMETRIC NEW PHYSICS

“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. 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 “Even if both the stau or stop,” says Hooper. “That anomalies could be the dark matter.” disappear, the Exciting though these possibilities Higgs needn’t are, nearly everyone urges caution. leave us in “All of these things, they’re like pipe Weinberg’s dreams,” says Christoph Paus of CMS. nightmare” “Of course I’d like to see a difference, but if I’m honest, at the moment everything looks like the standardmodel 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 nondirectional, 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…” Additional reporting by Lisa Grossman n 14 July 2012 | NewScientist | 9