The Higgs Boson: Searching for the God Particle

by Scientific American Editors

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  GATHER ROUND: Dozens of students and physicists gathered at Columbia University's Low Library early Wednesday morning to get the latest news on the Higgs boson.

  Credit: John Matson

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  Both research teams announced their results during a morning seminar at CERN, the European laboratory for particle physics that operates the Large Hadron Collider, or LHC. But the morning start in Geneva meant that U.S. physicists and other curious observers were tuning in to the announcement during the predawn hours. Tuts and his Columbia colleagues decided to host a viewing party at the campus library, with a live video feed from CERN as well as coffee, cookies, soft drinks and chips. About 50 people, many of them students, turned up for the event, which began around 2:30 A.M.

  Unlike some past announcements centered on the Higgs in the past few years, which have produced as much ambiguity and confusion as anything else, this one did not disappoint. ATLAS physicists said that their most recent data reveal the presence of an unknown particle with a mass of about 126.5 GeV, or 126.5 billion electron-volts. An electron-volt is a physicist’s unit of mass or energy; for comparison, the proton has a mass of about 1 GeV. The CMS collaboration found evidence for a new particle with a mass of 125.3 GeV.

  Crucially, both teams' findings appear exceptionally robust. In physics terms, evidence for a new particle requires a “3-sigma” measurement, corresponding to a 1-in-740 chance that a random fluke could explain the observations, and a claim of discovery requires a 5-sigma effect, or a 1-in–3.5 million shot that the observations are due to chance. In December representatives of the two experiments had announced what one called “intriguing, tantalizing hints” of something brewing in the collider data. But those hints fell short of the 3-sigma level. The new ATLAS finding met not just that level of significance but cleared the gold standard 5-sigma threshold, and CMS very nearly did as well, with a 4.9-sigma finding.

  "This is the payoff," Tuts said after the two teams had announced their latest analyses in the Higgs hunt. "This is what you do it for." Peter Higgs himself, who was in Geneva for the seminar along with other eminent physicists who developed the theory, sounded a similar note after the ATLAS and CMS teams had unveiled their conclusions. "For me, it's really an incredible thing that it's happened in my lifetime," Higgs said to the audience at CERN. He was among a half-dozen physicists who in the 1960s proposed what is now known as the Higgs mechanism, hypothesizing the existence of a field permeating all of space, along with an associated particle. The field imparts particles with mass by exerting a sort of drag on them, slowing them down much like a human being slows down when she tries to walk through water instead of air.

  The newfound particle fits the bill for the Higgs boson, but the researchers cautioned that more work is needed to compare the properties of the particle to those predicted for the Higgs. After all, the LHC’s detectors cannot identify the Higgs directly. The LHC accelerates protons to unprecedented energies of four trillion electron-volts (4 TeV) before colliding a clockwise-traveling proton beam with a counterclockwise beam. From the smash-up new particles emerge, some of them existing for just an instant before decaying to other particles.

  In the case of the Higgs, physicists can only infer its existence and its properties from the more mundane particles it decays to produce—say, gamma-ray photons or pairs of electrons. The new particle has the right mass to be the Higgs and broadly decays as predicted, although a few ambiguities remain. Fortunately, more data are right around the corner. "We have only recorded one third of the data expected in 2012," ATLAS spokesperson Fabiola Gianotti of CERN said during her presentation. "This is just the beginning. There is more to come."

  Both Gianotti and CMS spokesperson Joe Incandela of the University of California, Santa Barbara, were greeted by large outbursts of applause when they displayed the slides outlining the results of their Higgs search.

  "There aren’t many discoveries like this," Columbia physicist Brian Cole told the group assembled on campus for the early-morning viewing party. "This trumps, I would say, everything in my physics career.…So I hope you all remember this for the rest of your lives."

  Just before sunrise, four Columbia undergraduates made their way out of the library and back across campus. Only two of them were studying physics, they said—one was focusing on chemistry and another on math. But they all agreed that it had been worth staying up late to see history in the making.

  -Originally published: Scientific American online July 4, 2012

  Tantalizing Hints of Elusive Higgs Particle Announced

  By Davide Castelvecchi

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  TRACKING THE HIGGS: A reconstructed particle collision in the CMS detector of the LHC.

  Credit: CMS/ CERN

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  GENEVA—The two largest collaborations of physicists in history Tuesday presented intriguing but tentative clues to the existence of the Higgs boson, the elementary particle thought to endow ordinary matter with mass.

  Representing the 6,000 physicists who work on two separate detectors at the Large Hadron Collider (LHC), called CMS and ATLAS, two spokespersons said that both experiments seemed to agree, as both their data sets suggested that the Higgs has a mass close to that of about 125 hydrogen atoms. The LHC is an international facility hosted by CERN, the European particle physics laboratory outside Geneva.

  "We are talking of intriguing, tantalizing hints," said CMS spokesperson Guido Tonelli, speaking to a room filled with dozens of journalists and TV crews. "It's not evidence."

  The experiments, in which protons traveling at nearly the speed of light collide head-on, cannot directly detect the Higgs, because the boson would decay within a fraction of a nanosecond into other particles. Instead, physicists must search through the debris of many different types of particle decay to find precise combinations of by-products that the Higgs would produce—and different chains of particle decays may well have the same signatures. A particular combination that appears more often than expected from other, "background" processes may signal the presence of the Higgs. But if it does not appear often enough compared with the expected background, it could just be a statistical fluctuation. Today, neither CMS nor ATLAS could claim to have the "3-sigma" statistical significance needed to claim evidence for a new particle—let alone 5 sigma for the accepted standard to claim a discovery. (A 3-sigma result implies a fraction of a 1 percent chance of a statistical fluke.) Instead, so far each experiment could only claim a statistical significance of around 2 sigma.

  Both the detectors and the LHC accelerator itself, however, have been performing better than expected; so all the ducks are now in a row for settling the question soon, according to the researchers. "The nice thing to know is that by the end of 2012—sooner if we are lucky—we should be able to say the final word," Fabiola Gianotti, the ATLAS spokesperson, said at the press conference.

  "I find it fantastic that we have the first results on the search for the Higgs, but keep in mind that these are preliminary results. And keep in mind that we have small numbers," said CERN Director General Rolf-Dieter Heuer in summarizing presentations that both Tonelli and Gianotti gave during a CERN seminar earlier that day.

  "I think the evidence is very encouraging, though it's still too early to be sure," comments Steven Weinberg, a leading theoretical physicist at the University of Texas at Austin and a winner of the Nobel Prize in Physics.

  A generation of high-energy physicists came of age studying and testing the Standard Model of particle physics, a theory devised in the 1970s that has withstood all experimental challenges. One final piece is missing, though, and it is one without which the whole model could fall. Without the Higgs boson, physicists cannot explain how other particles have mass. The Higgs itself has mass, and going by exclusion, researchers from the LHC and from its predecessor particle colliders were able narrow down the range of its value to between 115 and 140 giga–electron volts, or GeV. (One GeV is roughly the mass of a
hydrogen atom.)

  Together, the LHC detectors have now reduced the allowed range further: Tonelli said that according to CMS data its mass cannot be greater than 127 GeV. That was not for lack of data—in fact, quite the opposite. "We were not able to exclude the range below 127 GeV because of excesses," or more of certain particle by-products than would be expected in the absence of the Higgs, he remarked during his seminar talk—which was an understated way of saying that the CMS experiment had actually seen hints of a Higgs existing and having a mass of 124 GeV or so. ATLAS saw excesses in a similar range of energies, although the graphs did not quite line up—the ATLAS data favor a Higgs around 126 GeV.

  Not everyone is impressed with the new findings. The data are "unconvincing," says Matt Strassler, a theoretical physicist at Rutgers University who was visiting CERN for the occasion. "I was a little disappointed," he adds, that the results did not live up to the expectations and the rumors—some called it a "Higgsteria"—that had circulated in the run-up to the announcement. On the other hand, he grants, no one expected to have a discovery at this stage—the experiments have not yet amassed enough data.

  Vivek Sharma, Higgs search coordinator at the CMS collaboration, agrees that the two experiments have a small discrepancy on what the supposed Higgs mass would be, and that tantalizing hints of new physics from other experiments have often turned out to be statistical anomalies. "People should curb their enthusiasm," he cautions.

  Joe Lykken, a theoretical physicist at Fermi National Accelerator Laboratory in Batavia, Ill., who is a member of the CMS collaboration, is more optimistic about the discrepancy. "Even though we are only seeing hints of the Higgs boson, it is encouraging that the ATLAS and CMS hints seem to be consistent with each other," he says.

  A Higgs with a mass of 125 GeV would fit with a hypothesized extension of the Standard Model called supersymmetry, which posits that every known particle has a heavier, as-yet-undiscovered partner. "The low-mass Higgs is not so bad for supersymmetry, to say it diplomatically," CERN's Heuer said.

  The LHC first fired up in September 2008, but within a week it was crippled by a serious accident that put it out of order for more than a year. "It was a big setback," says Lyn Evans, a CERN accelerator physicist who oversaw the construction and commissioning of the LHC from 1994 until his retirement a year ago. After repairs, however, the machine restarted in 2009 and has delivered more collisions than predicted, enabling the ATLAS and CMS collaborations to amass data five times faster than expected.

  As recently as a year ago, one would not have thought that the LHC would make so much progress in its Higgs search by the end of 2011, observes Dmitri Denisov, spokesperson for the DZero experiment, one of the detectors at Fermilab's recently retired Tevatron collider. "It performed better than anyone expected," Denisov says.

  If the Higgs really exists, it will answer the long-standing question of how matter gets its mass. It will also reveal the nature of the connection between two fundamental forces, the weak nuclear force and the electromagnetic force—a relationship termed the electroweak interaction. The two forces were unified for the first instants of our universe, but now they behave differently. Weinberg says the new results suggest that "it should be possible to reach a definite decision about whether this is the particle associated with the breakdown of the symmetries of the electroweak theory. I'll bet that it is."

  -Originally published: Scientific American online, December 13, 2011

  Heather Gray: A Diary of the Higgs

  By Kelly Oakes

  Heather Gray, a researcher working on the ATLAS experiment at CERN, was at this year’s Lindau meeting. I spoke to her over email before it started to find out about her expectations, and afterwards she told me about her impressions of the meeting and what it was like to watch the announcement from CERN with other young researchers. She also made a video diary of the Higgs exciement at Lindau that you can watch below.

  What were your first impressions of the meeting when you arrived?

  My arrival at the meeting was somewhat chaotic as my bag had been sent on the wrong flight by the airline and then when dashing out for some new clothes I dropped my wallet! However the staff at the Lindau meeting where wonderful so they quickly helped me to sort things out. This did mean that I was a little distracted during the initial stages and the opening ceremony. I enjoyed the formality and tradition clearly inherent in the opening ceremony and obtained a better understanding of the history of the Nobel meeting. Of course, it was exciting to see so many Nobel Laureates in one place at the same time, although this was something I got more accustomed to over the week. Finally, I found the number of young researchers quite surprising – there really were many, many young people all doing fascinating research in many different corners of the globe.

  You mentioned in our last Q&A that you’re working on the Higgs boson. How busy were you in the weeks leading up to the meeting, and how did you feel on Wednesday when the results were announced?

  Oh, whenever we have data, we are always extremely busy working to understand it. I was very busy the week before I left because I was taking a shift as run manager: essentially looking after our experiment’s data-taking for a week, in the last few days that we collected data that was used for this result, so it was definitely a time in which we wanted things to go smoothly. I do work on the Higgs, but I don’t work on the two channels that ATLAS showed this time, so for me the busiest time is yet to come as we collect more data. However, in the last few days before the result went public the entire collaboration was involved in the process of understanding the results, approving it and converging on the message we wanted to present to the outside world.

  At Lindau, the seminar in which the spokespeople of the two experiments presented the new results unfortunately clashed with some of the scheduled talks, so there wasn’t an opportunity to watch it together with the other participants. However, quite a group of us got together and watched in the tent on a few laptops, which I enjoyed a lot. I found the seminar very exciting, even if I already knew what ATLAS was going to show, and I even found myself surprisingly emotional when the Director General said “I think we’ve got it!” We couldn’t stop ourselves from clapping as everyone had broad grins on their faces.

  What’s next for the Higgs in general, and for your own research?

  In this case, the two are really quite well aligned. What we want and need to do next re more careful measurements to determine whether this particle is the Higgs boson or something else. The decay channel that I work on directly is the decay to a pair of bottom quarks and we need data before the channel becomes sensitive. Of course, this doesn’t mean we’re waiting, but rather actively working on the analysis and optimising things so that we can obtain the best results possible.

  In order to understand if it is the Higgs, we need to repeat the current measurements with more data and check nothing changes. Then we need to see if we observe this particle in all predicted decay channels and whether we see it decaying in different ways at the rate at which we expect. Here’s where the channel I work on fits in, as the bb channel is the only decay to quarks that we could expect to see, and if it’s the Higgs, it should really couple to all particles. We’ll be taking data until the end of this year or early next year and we hope that this should give us enough data to obtain at least the first answer to this question.

  You took part in a master class with David Gross – what was that like?

  The master class was one of my favourite parts of the meeting. I couldn’t have asked for better timing to be giving a talk about the Higgs than the day after that seminar! I found the contributions from all participants were of a very high quality such that I found them fascinating. The discussion was excellent with many people attending the class participating. I also enjoyed the discussion I had with David Gross about how sure we experimentalists need to be about a result before making it public.

  What will you take away from the meeting?

  I will ta
ke away some great experiences, great friends, new ideas and most of all the impressions I got of the Nobel Laureates, not just as great scientists, but also human beings.

  -Originally published: Scientific American online, July 11, 2012

  SECTION 5

  Beyond the Higgs

  Beyond Higgs: On Supersymmetry (or Lack Thereof)

  By Glenn Starkman

  With the search for the Higgs boson, the last missing piece of the Standard Model of particle physics, apparently reaching its long-anticipated-and-finally-successful conclusion, anticipation of the next set of discoveries is growing.

  Recently the Stanford campus hosted a smallish gathering celebrating the 60th birthday of Savas Dimopoulos, justly acclaimed by each of the attendees as the (or at least one of the few) most insightful particle physics model builders of the last 30 years. (And my PhD adviser.) Now you’d think that the leading topic of discussion at such an event would be the details of the ongoing Higgs search – has it or hasn’t it been discovered? Does the fact that the two relevant experiments at CERN’s Large Hadron Collider (LHC) – ATLAS and CMS – both have a signal indicative of a new particle with the same mass? And what about the supportive analysis coming from Fermilab’s Tevatron?

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  Credit: CERN

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