2012: The year that the Higgs boson is discovered
By Jim Baggott
The new year is a time for bold and often foolhardy predictions. Certainly, most of us will take the prophesy of impending doom on 21 December, 2012 with a large pinch of salt. This date may represent the end of a 5,125-year cycle in the Mesoamerican Long Count calendar, but it doesn’t necessarily signal the end of all things (not even in Mayan history, contrary to popular belief). I think that when the time comes, we can plan for Christmas 2012 with a reasonably clear conscience.
But, despite the obvious pitfalls, I am prepared to stick my neck out and make a prediction. I predict that this will be the year that the Higgs boson is discovered.
This elusive particle was first ‘invented’ in 1964, by English theoretical physicist Peter Higgs. It is the characteristic particle of the hypothetical ‘Higgs field’ – an energy field thought to pervade the entire universe. The field was introduced into theories of particle physics around the same time by Higgs in Edinburgh, Belgian physicists Robert Brout and François Englert, and Americans Gerald Guralnik and Carl Hagen and British physicist Tom Kibble at Imperial College in London.
The Higgs field is hypothetical but we can be reasonably confident that something like it must exist. It is believed to be responsible for breaking the symmetry of the primordial electro-weak force, moments after the big bang, and forever dividing the weak nuclear force (responsible for beta-radioactivity) from electromagnetism (responsible for much of physics, chemistry and life). As a result of their interactions with the Higgs field, the massless carriers of the weak force – called W and Z particles – gain mass. In essence, the Higgs field retards the particles’ acceleration; it drags on them like molasses. We interpret this resistance to acceleration as mass.
In 1967, American theorist Steven Weinberg used the Higgs mechanism to predict the masses of the W and Z particles. In 1983, these particles were discovered at the CERN particle physics laboratory in Geneva, with almost precisely the masses that had been predicted.
The Higgs field is now believed to be responsible for endowing all elementary particles with mass, and is a central plank of the so-called standard model of particle physics. This is a collection of quantum field theories that describe the strong and weak nuclear forces and electromagnetism, and the elementary particles that make up all the visible matter in the universe. To a certain extent, we could say that we know the Higgs field must exist because particles have mass. However, having an instinct that the Higgs field must exist is not the same as proving its existence. And we prove the existence of the Higgs field by finding the Higgs boson, also known in the popular media as the ‘God particle’.
Although CERN physicists will argue that the search for the Higgs boson is not the only purpose of the Large Hadron Collider (LHC), it is fair to say that – faster-than-light neutrinos aside – this is where most of the current interest resides. The LHC reached a peak proton-proton collision energy of 7 trillion electron volts in March 2010 and throughout 2011 engineers have tweaked the beam luminosity, a measure of the number of collisions the beam can produce, to ever higher values. The machine out-performed nearly everybody’s expectations.
The summer proved to be something of a roller-coaster ride. In July 2011, at the European Physical Society high-energy physics conference in Grenoble, glimpses of a candidate Higgs boson were reported with an energy around 135 GeV (giga electron-volts, or billion electron-volts) by both the detector collaborations, ATLAS and CMS. But by the time of the Fifteenth International Symposium on Lepton-Photon Interactions at High Energies, which began on 22 August at the Tata Institute in Mumbai, the physicists’ confidence had evaporated. It seemed that as more data had been gathered, the significance of the events hinting at a Higgs around 135 GeV had actually declined.
I met with Peter Higgs on a wet Thursday afternoon in Edinburgh, a few days before the Mumbai conference was due to commence. Higgs retired in 1996 but has remained in Edinburgh close to the University department where he first became a lecturer in mathematical physics in 1960. He is now a sprightly 82 years old. We sat in a coffee shop with his colleague and friend Alan Walker, and talked about his experiences and his hopes for the near future.
He had waited 47 years for some kind of vindication for the mechanism that bears his name. We talked about the prospects for the Mumbai conference, and the grounds we had for optimism that something momentous may be about to be reported. ‘It’s difficult for me now to connect with the person I was then [in 1964],’ he explained, ‘But I’m relieved it’s coming to an end. It will be nice after all this time to be proved right.’
Sadly, as events in Mumbai unfolded, Higgs discovered that he would have to wait a little longer.
By the end of 2011 the LHC had delivered data from about 350 trillion proton-proton collisions to both ATLAS and CMS. At a public conference organized at CERN on 13 December both detector collaborations reported excess events corresponding to a candidate Higgs boson with an energy around 124-126 GeV.
When the data from several different possible Higgs decay channels were combined, the ATLAS collaboration observed an excess of events corresponding to 3.6-sigma above the predicted background. This is a statistical measure reflecting the degree of confidence in the experimental data. Three-sigma significance implies a confidence level of 99.7% – in other words, a 0.3% chance that the data are in error. Although such confidence levels sound pretty convincing, to warrant declaration of a ‘discovery’, particle physicists actually demand five-sigma data, or confidence levels of 99.9999%.
Still, 3-6-sigma significance is quite compelling. CMS reported a combined excess of events with slightly lower statistical significance of 2.4-sigma.
In a blog entry posted the same day, Tommaso Dorigo, an Italian physicist working on CMS, declared this to be ‘firm evidence’ for a standard model Higgs boson with an energy around 125 GeV. There followed a short but intense war of words in the blogosphere as American theorist Matt Strassler adopted a more conservative view, arguing that Dorigo’s use of the word ‘firm’ was unwarranted: ‘If he had said “some preliminary evidence” he would have gotten away with it. As it is, it seems to me that he has crossed a line…’
Although the statistical significance of the ATLAS and CMS results are consistent with the use of the term ‘evidence’, the official line from CERN is that we must wait for more data to be certain. Perhaps their experiences at the Mumbai conference last August has taught the physicists to be cautious. CERN Director-General Rolf Heuer closed the 13 December conference with these observations: ‘[The data provide] intriguing hints in several channels in two experiments, but please be prudent. We have not found it yet. We have not excluded it yet. Stay tuned for next year.’
Higgs himself echoed the party line: ‘Ah well, I won’t be going home to open a bottle of whisky and drown my sorrows, but equally I am not going home to crack open a bottle of champagne either!’
The LHC is scheduled to re-start proton physics in April, so the focus of attention will be once again on the big summer conferences.
So, what grounds do I have for confidence in my prediction? After all, there is a very real possibility that the excess events reported in December may decline in significance as more data are collected. But I have been watching events unfold at CERN for nearly two years and my instinct tells me that this is it. I’m not by nature a gambler, but I’d be willing to gamble on this.
As my ATLAS contact recently put it: ‘We really do need data to be sure, but I would bet on this myself. [It] depends how much of a betting man you are.’
Jim Baggott was born in Southampton, England, in 1957. He graduated in chemistry from the University of Manchester in 1978. After completing his doctorate in physical chemistry at the University of Oxford, he worked as a postgraduate research fellow at Oxford and at Stanford University in California. He has been studying and writing about science, philosophy and science history for nearly 20 years, and has won awards for both scientific research and science writing. His published work includes The Quantum Story, Beyond Measure: Modern Physics, Philosophy, and the Meaning of Quantum Theory, and A Beginner’s Guide to Reality. Watch Jim Baggott explain the history of the Large Hadron Collider and CERN here. His previous post for OUPblog can be found here.