SO PETER HIGGS didn’t get this year’s Nobel for physics after all. It would have been the Hollywood ending to a story that began half a century ago with a few squiggles in his notebook, and climaxed on 4 July this year with a tear in his eye as physicists armed with a $6 billion particle collider announced they had found the particle that bears his name. Or something very like it anyway.
Higgs wasn’t the only one feeling a little emotional. This was the big one, after all. The Higgs boson completes the grand edifice that is the “standard model” of matter and its fundamental interactions. Job done.
If only things were that simple. As particle physicists gather in Kyoto, Japan, next week for their since July’s announcement, they are still asking whether that particle truly is the pièce de résistance of the standard model. And meanwhile, even more subversive thoughts are doing the rounds: if it is, do we even want it?
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Higgs’s squiggles aimed to solve a rather abstruse problem. Back in the early 1960s, physicists were flushed with their ability to describe electromagnetic fields and forces through the exchange of massless photons. They desperately wanted a similar quantum theory for the weak nuclear force, but rapidly hit a problem: the calculations demanded that the particles that transmit this force, now known as the W and Z bosons, should be massless too. In reality, they weigh in at around 80 and 90 gigaelectronvolts (GeV), almost 100 times meatier than a proton.
The solution hit upon by Higgs and others was a new field that filled space, giving the vacuum a positive energy that in turn could imbue particles with different amounts of mass, according to how much they interacted with it. The quantum particle of this field was the Higgs boson.
As the standard model gradually took shape, it became clear how vital it was to find this particle. The model demanded that in the very early hot universe the electromagnetic and weak nuclear forces were one. It was only when the Higgs field emerged a billionth of a second or less after the big bang that the pair split, in a cataclysmic transition known as electroweak symmetry breaking. The W and Z bosons grew fat and retreated to subatomic confines; the photon, meanwhile, raced away mass-free and the electromagnetic force gained its current infinite range. At the same time, the fundamental particles that make up matter – things such as electrons and quarks, collectively known as fermions – interacted with the Higgs field and acquired their mass too. An ordered universe with a set hierarchy of masses emerged from a madhouse of masslessness.
It’s a nice story, but one that some find a little contrived. “The minimal standard model Higgs is like a fairy tale,” says of CERN near Geneva, Switzerland. “It is a toy model to make the theory match the data, a crutch to allow the standard model to walk a bit further until something better comes along.” His problem is that the standard model is manifestly incomplete. It predicts the outcome of experiments involving normal particles to accuracies of several decimal places, but is frustratingly mute on gravity, dark matter and other components of the cosmos we know or suspect to exist. What we need, say Altarelli and others, is not a standard Higgs at all, but something subtly or radically different – a key to a deeper theory.
Questions of identity
Yet so far, the Higgs boson seems frustratingly plain and simple. The particle born on 4 July was discovered by sifting through the debris of trillions of collisions between protons within the mighty ATLAS and CMS detectors at CERN’s Large Hadron Collider. For a start, it was spotted decaying into W and Z bosons, exactly what you would expect from a particle bestowing them with mass.
Even so, a definitive ID depends on fiddly measurements of the particle’s quantum properties (see “Reflections on spin“). “The task facing us now is ten times harder than making the discovery was,” says of the University of Bristol, UK, a member of the CMS collaboration.
Beyond that, a standard-model Higgs has to decay not just into force-transmitting bosons, but also to matter-making fermions. Here the waters are little muddier. The particle was also seen decaying into two photons, which is indirect proof that it interacts with the heaviest sort of quark, the top quark: according to the theory, the Higgs cannot interact directly with photons because it has no electric charge, so it first splits into a pair of top quarks and antiquarks that in turn radiate photons. Further tentative evidence for fermion interactions comes from the US, where researchers on the now-defunct Tevatron collider at Fermilab in Batavia, Illinois, have seen a hint of the particle decaying into bottom quarks.
But equally, the CMS detector has measured a shortfall of decays into tau leptons, a heavier cousin of the electron. If substantiated, that could begin to conflict with standard model predictions; ATLAS is expected to present its first tau-decay measurements in Kyoto next week. Both ATLAS and CMS see more decays into photons than expected, perhaps signalling the influence of new processes and particles beyond the standard model.
It is too early to draw any firm conclusions. Because we know the new particle’s mass fairly well – it is about 125 GeV, or 223 billionths of a billionth of a microgram – we can pin down the rates at which it should decay into various particles to a precision of about 1 per cent, if it is the standard Higgs. Because of the limited number of decays seen so far, however, the measurement uncertainty on the new particle’s decay rates is more like 20 or even 30 per cent. By the end of the year, ATLAS and CMS will have around two and a half times the data used for the July announcement, but that still won’t reduce the uncertainty enough. Then the LHC will be shut down for up to two years to be refitted to collide protons at higher energies. “We’re probably not going to learn significantly more about the new particle in the immediate future,” says Newbold.
What physicists would like to fill this vacuum is a new collider altogether. The LHC is not exactly ideal anyway: it smashes protons together, and protons are sacks of quarks and other innards that make measurements a messy business. Researchers are lobbying for a cleaner electron-positron collider, possibly in Japan, to close the Higgs file, but that too is a distant prospect.
So we are left with a particle that looks like the standard Higgs, but we can’t quite prove it. And that leaves us facing an elephant in the accelerator tunnel: if it is the standard Higgs, how can it even be there in the first place?
“The elephant in the accelerator tunnel is: if it is the Higgs, how can it even be there in the first place?”
The problem lies in the prediction of quantum theory, confirmed by experiments at CERN’s previous mega-accelerator, the Large Electron Positron collider, that particles spontaneously absorb and emit “virtual” particles by borrowing energy from the vacuum. Because the Higgs boson itself gathers mass from everything it touches, these processes should make its mass balloon from the region of 100 GeV to 1019 GeV. At this point, dubbed the Planck scale, the fundamental forces go berserk and gravity – the comparative weakling of them all – becomes as strong as all the others. The consequence is a high-stress universe filled with black holes and oddly warped space-time.
Conspirators sought
One way to avert this disaster is to set the strength of virtual-particle fluctuations that cause the problem so they all cancel out, reining in the Higgs mass and making a universe more like the one we see. The only way to do that while retaining a semblance of theoretical dignity, says Altarelli, is to invoke a conspiracy brought about by a suitable new symmetry of nature. “But where you have a conspiracy you must have conspirators.”
At the moment, most physicists see those conspirators in the hypothetical superpartners, or “sparticles”, predicted by the theory of supersymmetry. One of these sparticles would partner each standard model particle, with the fluctuations of the partners neatly cancelling each other out. These sparticles must be very heavy: the LHC has joined the ranks of earlier particle smashers in ruling them out below a certain mass, currently around 10 times that of the putative Higgs.
That has already put severe pressure on even the simplest supersymmetric models. But all is not lost, according to of CERN’s theory group. If you don’t find sparticles with low masses, you can twiddle the theory, to an extent, and “dial them up” to appear at higher masses. “We expected that the Higgs would be found and that a supporting cast would be found with it, but not necessarily at the same energy scale,” he says.
Even so, the goalposts cannot be shifted too far: if the sparticles get too heavy, they won’t stabilise the Higgs mass in a convincingly “natural” way. Sparticles are also hotly sought after as candidates to make up the universe’s missing dark matter. Updates will be presented in Kyoto next week, and there is also hope for indirect leads to supersymmetry from measurements of anomalies in the decay rates of other standard-model particles. If nothing stirs there, all eyes are on what happens when the LHC roars back early in 2015 at near double its current collision energy. A revamped LHC should be able to conjure more massive sparticles from thin air, or perhaps even more radical particles such as those associated with extra dimensions of space. These particles amount to another attempt to fill the gap between where the Higgs “should be” – at the Planck scale – and where it actually is.
The weirdest scenario of them all, though, is if there is nothing but tumbleweed between the energies in which the standard model holds firm and those of the Planck scale, where quantum field theories and Einstein’s gravity break down. How then would do we explain the vast discrepancy between the Higgs’s actual mass and that predicted by quantum theory?
Teetering on the brink
One solution is to just accept it: if things were not that way, the masses of all the particles and their interactions strengths would be very different, matter as we know it would not exist, and we would not be here to worry about such questions. Such anthropic reasoning, which uses our existence to exclude certain properties of the universe that might have been possible, is often linked with the concept of a multiverse – the idea that there are innumerable universes out there where all the other possible physics goes on. To many physicists, it is a cop-out. “It looks as if it’s an excuse to give up on deeper explanations of the world, and we don’t want to give up,” says of University College London, who works on the ATLAS experiment.
But a second fact about the new particle gives renewed pause for thought. Not only is its 125 GeV mass vastly less than it should be, it is also about as small as it can possibly be without dragging the universe into another catastrophic transition. If it were just a few GeV lighter, the strength of the Higgs interactions would change in such a way that the lowest energy state of the vacuum would dip below zero. The universe could then at some surprise moment “tunnel” into this bizarre state, again instantly changing the entire configuration of the particles and forces and obliterating structures such as atoms.
As things stand, the universe is seemingly teetering on the cusp of eternal stability and total ruin. “It’s an interesting coincidence that we are right on the border between these two phases,” says CERN theorist , who set about calculating the implications of a 125 GeV Higgs as soon as the first strong hints came out of the LHC in December last year.
“The universe is seemingly teetering on the cusp of eternal stability and total ruin”
He doesn’t know what the answer is. In any case, finding any new particles will change the game once more. “There are many questions in the history of science whose answers have turned out to be environmental rather than fundamental,” says Giudice. “The slightest hint of new physics and my calculation will be forgotten.” So that is what all eyes will really be on in Kyoto. Higgs’s squiggles seem to have become reality – but for a more satisfying twist to the tale, we must hope some other squiggles show similar signs of life soon.
Reflections on spin
For a particle to be confirmed as a Higgs boson, it must pass some strict tests. The first is the value of its quantum mechanical “spin”. Matter particles such as electrons – fermions – have spins of ½. Bosons that transmit forces have whole-number spins: photons, for example, have a spin of 1.
To make physics as we know it work, the Higgs field must look the same everywhere. This is only possible if Higgs itself has no spin at all. Experimental results from CERN’s Large Hadron Collider (LHC) already preclude the particle announced in July from being anything but a spin-0 or spin-2 particle: it decays into pairs of photons, something spin arithmetic forbids for a fermion or a spin-1 boson.
of the LHC’s CMS collaboration thinks it is “highly probable” that the new particle has spin 0 based on the evidence we already have but finer measurements are needed to be sure.
A Higgs should also have even parity, meaning it behaves in exactly the same way when observed in a mirror. Nailing down spin and parity should allow physicists to identify any obvious character flaws, such as the new particle being some other agglomeration of particles we already know about. That should be doable with data the LHC will have collected by the end of the year – but that is just the start of the process (see main story).
