Ever SINCE GALILEO’S famous climb to the top of the Tower of Pisa, where he dropped several items and gained an insight into how gravity works, physicists have used acceleration as a tool to explore the fundamental building blocks of nature. Technological advances have helped physicists make more sophisticated machines that can now accelerate individual particles to almost the speed of light, smash them together and detect in exquisite detail the resulting explosion of tiny particles and jets of energy. Culminating with the Large Hadron Collider (LHC) at the European Organisation for Nuclear Research (CERN), near Geneva, Switzerland, the accelerators have continued to leapfrog one another in size, rate of collisions and the amount of energy used to run it.
Each one is an extravagant and exciting technological feat, pushing the frontiers of technology and resulting in some of the most complex machines in the world. The problem is, this excitement hasn’t been reflected in the physics. There haven’t been any unexpected discoveries since the 1970s. Not one significant surprise. Not because the experimentalists are doing something wrong; rather, the theory is just too good – maddeningly, frustratingly good.
The 1970s saw the finishing touches added to particle physics’ big theory, uninspiringly named the Standard Model. Incorporating all known particles and most forces, the Standard Model has been uncannily successful. It is yet to fail a single experimental test, and has predicted all subsequent particle discoveries, including the bottom quark in 1977, the W and Z bosons in the early 1980s, the top quark in 1995 and the tau neutrino in 2000. The last remaining particle predicted by the Standard Model was the Higgs boson – and its probable discovery was announced to excited crowds in Geneva and Melbourne on 4 July 2012.
NOW, PHYSICISTS ARE stuck. What if the Higgs boson behaves exactly as the Standard Model predicts? The theory would then be complete. The avenues to new physics, to new discoveries, would close. In many ways, the Higgs would be considered a disappointment.
“It is a pity, in a way, because the great advances in physics have come from experiments that gave results we didn’t expect,” Stephen Hawking told the BBC after the Higgs announcement. Hawking had often stated that he thought the Higgs boson wouldn’t be found. In 2008, when the LHC started up, he said, “I think it will be much more exciting if we don’t find the Higgs. That will show something is wrong, and we need to think again. I have a bet of $100 that we won’t find the Higgs.”
Hawking’s sentiment is reflected among many who work directly for the collaborations at CERN: “If it’s just the Standard Model Higgs that we are finding, it’s exciting… but a little boring,” says Albert de Roeck, from the CMS (Compact Muon Spectrometer) collaboration and the University of Antwerp in Belgium.
Despite its success, the Standard Model fails to explain several observations made elsewhere in physics. It has no candidates for dark matter, which makes up most of the universe; it doesn’t explain why the universe predominantly comprises matter and not antimatter; it cannot incorporate the full theory of gravity, as described by Einstein; and it cannot account for a discovery in 2002 that neutrinos must have at least a tiny bit of mass.
These observations have left physicists scratching their heads. Experimental results that point to new physics are needed for physicists to be able to find things they haven’t been able to conjure up using brainpower alone.
And that’s what may be happening. As the physicists accumulate more and more data from the LHC, they are seeing tantalising hints that the Higgs boson may not behave exactly as predicted by the Standard Model. There’s something weird about why its mass was right at the low end of what was possible, and early results show that it exhibits a strange type of decay. What all this means is that the particle announced in July is either the very last piece of the Standard Model, or the first particle of something new and entirely bizarre.
IN THE SMALL HOURS of July 4, exhausted but excited postdoctoral and postgraduate students camped outside the main auditorium at CERN, hoping to make it in for the much-anticipated announcement from the scientists working on the ATLAS and CMS collaborations, which run the two largest detectors on the LHC. The auditorium was too small to hold the thousands of experimental physicists who work there, as well as the journalists and prominent theoretical physicists who had made the journey to CERN to hear the results. One of the key figures in the auditorium was British theoretical physicist Peter Higgs, after whom the particle was named.
It had been 48 years since the Higgs boson was first predicted, the longest time between prediction and discovery for any particle. In the 1960s, Higgs, and two other groups comprising Robert Brout, François Englert, Gerald Guralnik, Carl Hagen and Tom Kibble, first proposed the existence of a field that permeates all space. They thought the field should give particles their mass as they move through it, like some invisible cosmic treacle that slows them down. Without the field, all particles would travel at the speed of light for eternity, never slowing down to make atoms, planets and galaxies, let alone the complex molecules that make up life. Of these six physicists, Higgs was the only one who explicitly stated that there would be a new particle, the result of an excitation or ‘vibration’ of the field. Because of this, the particle and field took on his name.
In a two-hour presentation in July, two collaborations announced an observation of a new particle, in the mass range of 125–126 gigaelectron volts (GeV) – about 126 times the mass of a proton. It was the long-awaited proof that physicists truly understand the origin of mass, and was delivered to thundering applause. The announcement ricocheted around the world, dominating Twitter for hours and headlining more than 5,000 news websites. It was called “science’s great leap forward” by The Economist and likened to the discovery of DNA.
Towards the end, Higgs, who has often evaded the media, was asked to comment. “I would like to add my congratulations to everyone involved in this achievement,” he said. After a pause, he added, “It’s really an incredible thing that it’s happened in my lifetime.”
“WE HAVE TO BE PRECISE in what we mean by ‘Higgs boson’ because the Higgs boson is a general class of particles,” says British physicist Richard Hawkings, who was the physics coordinator of the ATLAS collaboration at the time of the discovery. “The simplest version of this is the Standard Model Higgs boson, which means that there is only one Higgs boson.”
Several other theories, not included in the Standard Model, predict there may be more Higgs bosons. Supersymmetry is one, and it proposes that for every particle, there is a heavier ‘super-partner’ particle. In the simplest version of supersymmetry, there are no fewer than five Higgs bosons: three of them neutral, one positively charged and one negatively charged. It could be that the LHC discovery is one of the Higgs bosons of supersymmetry.
One of the clues to its true identity is in how it decays. The Higgs boson – or, rather, all the Higgs bosons – are very unstable, short-lived particles, which quickly decay into other particles. There are several possible decays, among them: into two Z bosons; two photons; two bottom quarks; two tau leptons; or two W bosons.
“The most important decay is the two photon decay – both ATLAS and CMS have seen more of this decay than the Standard Model predicts. If this trend continues as we add more data this year, this would be not just a discovery of the Higgs boson, but a discovery of some new physics,” says Christopher Hill from Ohio State University in Columbus, and the deputy physics coordinator for the CMS collaboration.
“We see it at a rate that is higher than what is expected by the Standard Model. However, the uncertainties are not small enough that we should be making any statement that this is not consistent with the Standard Model,” says Hawkings. “Of course, there’s a lot of speculation. But for me, I’d want to see more data before I could be convinced.”
They haven’t yet observed the Higgs decay into leptons, or into quarks, which they would expect if it behaved as the Standard Model predicts. But these two types of decay are very difficult to detect. More data is needed before they’ll know if there is truly a signal, and this is “the next big question that we have to answer,” says Hawkings.
THERE IS ANOTHER hint from the Higgs boson that suggests this is not the end of the story: the mass of the boson itself. “What is rather strange is that the mass of the Higgs boson is quite low, at 125GeV. It’s right on the low end of the range. And many, many theoretical physicists believe that this indicates there is actually something else to be found, something that keeps the Higgs mass low,” says Hawkings.
The mass of the Higgs boson has contributions in quantum mechanics from many different particles. “This Higgs boson is not just a particle sitting there by itself, it’s surrounded by a cloud of other particles, coming in and out of existence. For example, its mass is affected by quarks, in particular the top quark, which is very heavy. When you calculate its mass, these corrections come in and they try to push its mass higher.
“Now, if there are some other particles, for example in the theory of supersymmetry, the top quark has a partner called the ‘stop’ or the ‘top squark’ and that will tend to cancel the effect of the top quark pushing the Higgs boson’s mass higher. So, the fact that the Higgs boson’s mass is quite low indicates that there may be other particles lurking just around the corner.”
The theory of supersymmetry is also popular among physicists because it is mathematically elegant; it ties up a lot of loose ends in their equations. And it holds the promise of helping solve some of the outstanding anomalies, such as candidate particles for dark matter.
“Supersymmetry provides a very good candidate for dark matter particles that maybe have been produced a short time after the Big Bang and have survived until now,” says Peter Jenni, former spokesperson for the ATLAS collaboration. The key particle is the lightest supersymmetric particle (LSP) that is stable and has no charge. Normally, it would not be produced directly, but is the end product of the decay chain of much heavier supersymmetry particles.
“The energy densities were only high enough in the universe shortly (0.00000000001 seconds) after the Big Bang for the production of heavy supersymmetry particles. After that, the universe expanded and cooled down. So, enormous numbers of LSPs [could have] survived since then, and as they feel gravitation like all other particles, they also gathered in the visible galaxies,” says Jenni.
If they do indeed exist, heavy supersymmetry particles could be produced inside the LHC. “ATLAS and CMS search intensively for supersymmetry particles at the LHC, which could have enough energy to produce them. One would observe the particles – ordinary jets [of energy] and leptons – into which they decay, plus missing transverse energy because the neutral LSP would escape the detector without leaving any track,” Jenni adds.
MEANWHILE, DATA CONTINUES to pour from the LHC. But from early 2013 until mid 2014, the accelerator will be shut down to undergo maintenance and upgrade work. When it starts up again, it will perform at double the energy, to reach the full design energy of the accelerator.
Results from the last round of data collection, from 2012, will be revealed in March 2013, but it may or may not be the last word on the Higgs.
“We’ll be back in 2015, taking more data,” says Hawkings. “This will allow us to explore more about the Higgs boson, hopefully shed some more light on these other questions, such as whether supersymmetry exists, whether there are other new things out there, it may shed some light on questions about dark matter… and hopefully some new discoveries.”
By then, physicists should know whether their Standard Model theory is complete, or if there is a bright new future for particle accelerators and physicists in working out the details of an entirely new model.