REPORTER’S NOTEBOOK: Higgs boson developments
First-hand observations of Higgs research at CERN
This summer I have had the opportunity to work with the MIT physics faculty at CERN, the European Organization for Nuclear Research, near Geneva, Switzerland. CERN is home to the Large Hadron Collider (LHC), and I am here with a group of MIT professors, postdocs, grad students, and undergrads working on the Compact Muon Solenoid (CMS). And yes, this is at the heart of the search for the Higgs boson. I have witnessed most of the biggest behind-the-scenes events over the past month and will share them here.
What is the Higgs boson? It is an important part of the Standard Model, which is a theory in physics that explains three of the only four fundamental forces in nature — strong nuclear force, weak nuclear force, and electromagnetic force — that mediate all interactions of subatomic particles. The Higgs boson is part of a theoretical mechanism proposed by several physicists in 1964 for the electroweak symmetry breaking in the Standard Model, which is the phenomenon where the photon — the mediator of the electromagnetic force — is massless, while the W and Z bosons — the mediators of the weak nuclear force responsible for radioactive decay of subatomic particles — are 80 and 91 times heavier than protons, respectively. The consequence of this prediction is a Higgs field that permeates all space and interacts with some elementary particles, causing them to appear massive, while particles that don’t interact with the field remain massless.
The Higgs boson decays so quickly, however, that the detectors cannot possibly observe it. The decay products can be observed, though — the mass at the center of the event can be reconstructed. Many standard processes produce similar decay products, so there is a great deal of “background,” but if the Higgs boson exists, there should be an excess of events with its mass. When plotting histograms of events per mass, this excess shows up as a bump in data whose statistical significance can be measured. There are several types of decay processes — i.e. channels — that the Higgs boson undergoes useful for this type of study, and different teams work on producing the extensive analyses for each decay. In December of 2011, CERN announced a small signal excess at the mass of 125 giga-electronvolts (GeV), but definitive conclusions couldn’t be drawn until the signal significance reached the level of five sigma (five standard deviations above background).
I arrived at CERN in early June. Following the December announcement, CERN decided to intensify the data collection for the Higgs search, conducting dedicated runs of the LHC between April and June. This step was taken in the hopes that the analyses would produce enough data to present significant results at the International Conference of High Energy Physics (ICHEP) in July. To prevent any bias in designing the complex analysis algorithms, the numerical region of interest for the Higgs mass was kept hidden (“blinded”) in the new data. With the analysis architecture completed, it was time to “unblind” the region of interest, all just a week after I arrived.
On a Friday afternoon in June, I joined hundreds of CMS physicists gathered in a large room at CERN for the unblinding meeting. If the signal “bump” reappeared in the new data, it would almost certainly be due to the proposed explanation, while its absence would strongly suggest that the previous finding was a fluke and possibly exclude the Higgs boson entirely. The information on a few PowerPoint slides would essentially tell us if we were heading toward a confirmation or rejection of a major theory in particle physics. MIT graduate student Mingming Yang G delivered the talk for the analysis of the decay channel of Higgs to two photons. She announced that the 2011 and 2012 data revealed a four sigma signal excess at 125 GeV for this single channel. This elicited a great deal of excitement from the audience. Other analysis channels reported smaller excesses — most notably three sigma from the channel of Higgs to two Z particles to four leptons — but it was clear that the two-photon channel would drive the result, and a combination with other channels would produce an excess close to or exceeding five sigma. Despite the excitement, the results had to stay within CMS.
The next few weeks were a flurry of activity. These results had to be presented and approved by publication committees. There were important meetings almost every day, and the schedule became even tighter when CERN announced that there would be a seminar with an update on the Higgs search just one day before ICHEP. I was impressed how intensely all the physicists scrutinized the results, even discussing details of single plots or histograms for extended periods of time. The analyses then had to be “topped up” with the new data collected since the unblinding, and these results also had to be approved. The combination of the results from each channel was presented, producing a 4.7 sigma excess before the final data was added.
The scale and hype of the announcement seminar were on a different level entirely. I began waiting outside the door of the auditorium at 2 a.m. for the 9 a.m. meeting and there were already fifty people in front of me. By 7:30 a.m., over one thousand people, ranging from college kids to research scientists, were waiting outside, and most did not get a seat. The media was out in full force, and CERN had invited the original theorists of the mechanism, including Peter Higgs, to attend. Finally, in a fashion more dramatic than one would expect from a physics presentation, Joe Incandela, the CMS spokesperson, announced that CMS’ analyses produced an overall significance of 4.9 sigma, or 5.0 if only considering the most sensitive channels. The ATLAS (A Toroidal LHC Apparatus) collaboration at the LHC, which worked completely independently from CMS, also announced near identical results, with a signal also present at 125 GeV.
The excitement following the announcement was extensive and somewhat amusing. I briefly met Peter Higgs, and watched normally laid-back scientists get swarmed by the media. In the end, I was simply grateful to have witnessed some of the biggest events leading up to an important event in the history of physics. Perhaps I am a bit spoiled to only see it at the most exciting time, but this experience has given me more respect for the scientific method as implemented in large inquiries, even when bureaucracy and media leaks get in the way.
The real question that remains is what this all means. It is true that we have not yet observed the Standard Model Higgs boson per se. We do know beyond a reasonable doubt that there is a new state (i.e. particle) at 125 GeV and we do know that it is a boson. It seems consistent with the Higgs exactly as theorized, but it is still possible that there are major differences that could lend support to other alternative theories, like supersymmetry. But I wouldn’t bet against it.