The final hiding place?
The Large Hadron Collider has left little room for the Higgs to hide after a spectacular tour de force that kicked off the summer conference season.
Among particle physicists there is a palpable feeling that, one way or another, we will soon know whether the famously elusive particle exists or not. The answer might well set the course of physics for decades to come.
The summer meetings are always worth watching: these are major dates on the scientific calendar when the world of high-energy physics gathers to announce its latest findings. The work can sound arcane at first pass, but the big discoveries made public at these meetings can go on to earn Nobel prizes.
The season began last week, with scientists heading to France for the Europhysics Conference on High Energy Physics in Grenoble. If anything was clear beforehand, it was that the Large Hadron Collider was performing well beyond expectations.
Even with that in mind, the results from the world’s most powerful particle collider were nothing short of breathtaking. In one fell swoop, the LHC had searched and dismissed a vast range of masses that the Higgs particle - as described by the Standard Model at least - might take. If the particle exists in this form, the likelihood now is that it lurks in a low and narrow range of masses that will be fully explored within months.
The hunt for the Higgs at the LHC plays out almost exclusively in two general-purpose detectors called Atlas and CMS. The search proceeds by smashing protons into one another and looking for hints of the Higgs in the debris of those collisions. The Higgs particle, if real, has only a fleeting existence, and decays immediately into other well-known particles. Which ones depend mostly on the mass of the Higgs particle, which nobody yet knows.
Back in 2000, the LHC’s predecessor at CERN, the Large Electron Positron collider, ruled out a Standard Model Higgs boson below around 115 GeV (gigaelectron volts). Since then the Tevatron has excluded the Higgs particle between 157-173 GeV. Indirect evidence – that is inferring the Higgs mass from precision measurements of the top quark and so on – suggest the Higgs is no heavier than 185GeV.
So what did the LHC detector groups find?
This means that if the Higgs boson is the kind described by the Standard Model – that is, the simplest possible - it probably lies in the region 115 – 149GeV. It will take more data to scour this region for signs of the Higgs.
These results were impressive enough, but there was more to the story last week. Atlas reported a bump in its data – due to an apparent excess of particle decays – in the region 120GeV to 140GeV. That excess might just be due to the Higgs particle, but the strength of the signal was too low to claim official evidence.
The Atlas bump was intriguing in itself, but signals like these can easily be caused by statistical fluctuations in data or poorly modelled backgrounds. What makes the bump slightly more exciting is that the CMS detector saw a similar, though smaller bump in the same region. This is tantalising stuff, but it helps to keep a level head.
There is a chance that both detectors have seen their first glimpse of the Higgs particle, but this is only one possibility. The detectors model their backgrounds – that is the decays from other particles – in similar ways, so if the modelling is flawed, both detectors could see a spurious bump in the same place. In short, this is more interesting than exciting.
I tried to write about this potential glimpse of the Higgs particle in the Guardian, but what appeared was a minor car crash. The headline, “Cern scientists suspect glimpse of Higgs boson God particle”, was only the most cringe-inducing problem, and managed to be simultaneously wrong and embarrassing. Hardly anyone outside a newsroom likes the phrase “God particle”. (More on that here.) More seriously, no-one I spoke with at Cern suspected they had glimpsed the Higgs: there are simply too many other explanations right now. These needless exaggerations grate. There is a chance England will win the next World Cup, but I don't suspect it will happen.
If you have never followed the hunt for the Higgs, now is a good time to start. By the end of August, Atlas and CMS plan to combine their results using data from more collisions. If the low mass excesses get bigger, it will be hard not to start wondering whether the Higgs has finally begun to show itself. We should hear more at the Lepton-Photon conference in Mumbai.
And what then? When I interviewed Steven Weinberg, the Nobel prize-winning physicist at the University of Texas at Austin, during my research for Massive, I asked him what it would mean for physicists to find the Standard Model Higgs boson. He replied that rather than pull physics out of the doldrums, it would plunge physics into the doldrums. The problem is this: Find the single SM Higgs boson and the Standard Model becomes a jigsaw complete. As wonderful as that would be, it gives us no new clues as to what lies beyond the Standard Model. And something must. Much better would be to find more than one Higgs.
One of the most popular theories for taking physics beyond the Standard Model is supersymmetry, which says that each type of known particle has a heavy and as-yet-undiscovered twin. There is a beauty to supersymmetry, but its real appeal is in describing gravity and the other three known forces of nature. It also predicts more than one kind of Higgs particle. But results released from the LHC at Grenoble did nothing to bolster confidence in supersymmetry. They ruled out supersymmetric particles below 1.2Tev (teraelectron volts), putting the squeeze on supersymmetry itself.
What is amazing about all of this is that so much has come so quickly from the Large Hadron Collider. There is more to come. The Large Hadron Collider has begun to wield its might.