The Particle at the End of the Universe: How the Hunt for the Higgs Boson Leads Us to the Edge of a New World

by Sean Carroll

  The bear is caught

  The general feeling among physicists was that if the 2011 hints were signs of something real, the data collected in 2012 would be enough to reach the magical five-sigma threshold necessary to declare a discovery. We knew how many collisions were happening at the LHC, and the feeling worldwide was that we would be able to declare discovery (or crushing disappointment) a year later, in December 2012.

  After its yearly winter shutdown, the LHC resumed collecting data in February. The International Conference on High Energy Physics (ICHEP) in Melbourne was planned for early July, and both experiments anticipated giving updates of their progress at that meeting. Conditions in 2012 were somewhat different from those in 2011, so it wasn’t immediately obvious how quickly progress could be made. They were running at a higher energy—8 TeV rather than 7 TeV—and also at a higher luminosity, so they were getting more events per second. Both of those sound like improvements, which they are, but they are also challenges. Higher energy means slightly different interaction rates, which means slightly different numbers of background events, which means you have to calibrate the new data separately from the old data. Higher luminosity means more collisions, but many of those collisions are happening simultaneously in the detector. This leads to “pileup”—you see a bunch of particle tracks but have to work hard to separate which ones came from which collisions. It’s a nice problem to have; but it’s still a problem you have to solve, and that takes time.

  The ICHEP is a major international event, and a logical venue at which to provide an update on the progress of the Higgs search after the new data had started coming in at higher energies. What people expected to hear was that the machine was doing great, and ideally that the statistical significance of the December hints was growing rather than shrinking. The LHC was scheduled to pause in its data collection in early June for routine maintenance purposes, and that was chosen as a natural point at which to look at the data carefully and see what they had.

  Both experiments were analyzing their data blind. The “box” containing the true data in the region of interest was opened on June 15, leaving about three weeks for the experimentalists to figure out what they had and how to present it in Melbourne.

  Almost immediately the rumors started flying. They were a little bit more vague than they had been in December, which is understandable; the experimenters themselves were scrambling to figure out what it was that they had. In the end, I don’t know of any rumors that got the final result precisely correct. But the general tenor was unmistakable: They were seeing something big.

  What they were seeing, of course, was a new particle—the Higgs, or something near enough. Even a glance at the data was enough to see that. The stakes were immediately raised; a simple update wasn’t going to be an appropriate tack to take when the results were presented to the public. You either have a discovery, or you don’t; and if you do, you don’t bury the lede, you trumpet it to the world.

  As subgroups within the experiments frantically analyzed the data in the various different channels, higher-ups debated how best to deploy the trumpets. On the one hand, both experiments were scheduled to give updates in Melbourne, and it would seem petty to pull out. On the other, there were hundreds of physicists at CERN who weren’t going to fly around the world, and this day belonged to them as much as to anyone. In the end, a compromise was reached: Each experiment would give a seminar on the day the conference opened, but the seminars would be located in Geneva and simulcast in Australia.

  If that weren’t enough to convince people on the outside that important news was coming, word quickly spread that CERN was inviting big names to be present at the seminars. Peter Higgs, now age eighty-three, was at a summer school in Sicily at the time; he was scheduled to fly back to Edinburgh, his travel insurance had run out, and he didn’t have any Swiss francs with him. But he changed his plans after John Ellis, the eminent theorist at CERN and longtime Higgs boson aficionado, left him a phone message: “Tell Peter that if he doesn’t come to CERN on Wednesday, he will very probably regret it.” He came, as did François Englert, Gerald Guralnik, and Carl Hagen, other theorists who had helped pioneer the Higgs mechanism.

  In December 2011, I was back in California and slept right through the seminars, which started at five a.m. Pacific time. But in July 2012, I managed to book a flight to Geneva and was there at CERN for the big day. I and many others were running from building to building at the lab, scrambling to get the proper credentials. At one point I had to sweet-talk my way past a security guard to get back into a building from which I had just exited, and explained that I was kind of short on time. “Why is everybody in a hurry today?” he asked.

  As in December, hundreds of people (mostly younger folks) had camped out overnight to get good seats in the auditorium. Gianotti once again gave the talk reporting results from ATLAS, but Tonelli’s term as CMS spokesperson had run out and the CMS talk was given by his successor, Joe Incandela, from the University of California, Santa Barbara. Incandela and Gianotti had both cut their teeth working together on UA2, one of the detectors at CERN’s previous hadron collider, and they had searched for Higgs bosons in the data from that experiment. Now they were about to see their long-standing quest come to fruition.

  Everyone in the room knew that all this fuss wouldn’t be happening if the signal had gone away. The primary question was, how many sigma? Between rumors and back-of-the-envelope estimations, the prevailing opinion seemed to favor the idea that each experiment would reach four-sigma significance, but not quite five. Combining the two, however, might bump us over the five-sigma threshold. But combining data from two different experiments is much trickier than it sounds, and it didn’t seem feasible that it could have been done over just the past three weeks. There was more than a little worry that we were going to be tantalized once more, but not quite able to claim a discovery.

  We needn’t have worried. Incandela, who spoke first, went through the different channels that had been analyzed by CMS one by one. Two-photon events came first, and they displayed a noticeable peak just where we were hoping, at 125 GeV. The significance was 4.1 sigma—more than in the previous year, but not a discovery. Then came events with four charged leptons, which result from the Higgs decaying into two Z bosons. Another peak, in the same place, this time with 3.2 sigma significance. On his sixty-fourth PowerPoint slide, Incandela revealed what you get when you combine these two channels together: 5.0 sigma. The wait was over. We found it.

  Gianotti, like Incandela, went out of her way to praise the hard work of everyone who helped keep the LHC running, and she emphasized the care the ATLAS collaboration went through to analyze their data. When she turned to the two-photon results, there was once again an evident peak at 125 GeV. This time the significance was 4.5 sigma. The four-lepton results also fell into line: a tiny peak, but discernible, with a significance of 3.4 sigma. Combining them gave an overall significance of exactly 5.0 sigma. At the end of her talk, Gianotti thanked nature for putting the Higgs where the LHC could find it.

  ATLAS found a Higgs mass of 126.5 GeV, while CMS got 125.3 GeV, but the measurements are within the expected uncertainty of each other. CMS analyzed more channels in addition to two photons and four leptons, and as a result their final significance ended up dropping just a tiny amount, to 4.9 sigma. But again, that’s consistent with the overall picture. The agreement between the two experiments was amazing, and crucially important. If the LHC had only one detector looking for the Higgs, the physics community would be much more hesitant to take the results at face value. As it was, hesitancy was thrown to the wind. This was a discovery.

  After the seminars were over, Peter Higgs became emotional. He later explained, “During the talks I was still distancing myself from it all, but when the seminar ended, it was like being at a football match when the home team had won. There was a standing ovation for the people who gave the presentation, cheers and stamping. It was like being knocked over by a wave.” In the pre
ssroom afterward, reporters tried to get more comments from him, but he demurred, saying that the focus on a day like this should be on the experimenters.

  In retrospect, a lot of things went right in the first half of 2012 to enable a Higgs discovery earlier than most people expected. The LHC was going full steam, collecting more events in just a few months of operation than it had in all of 2011. Pileup was a challenge, but the data analysts met it heroically, and the overwhelming fraction of events were successfully reconstructed. The higher energy pushed up the rate at which Higgs bosons were produced. And the teams had honed their analysis routines, managing to squeeze more significance out of their data than before. All these improvements ended up giving particle physicists Christmas in July.

  What is it?

  After the seminars were over, Incandela was reflective. “You often think that, once you’ve discovered something, it’s an end. What I’ve learned in science is that it’s almost always a beginning. There’s almost always something very big, just right there, that is within reach, and you just have to go for it. So you can’t let down your guard!”

  There is no question that CMS and ATLAS have found a new particle. There is very little question that the new particle resembles the Higgs boson; its decay rates into different channels match up roughly with what the Standard Model Higgs is expected to do if its mass is 125 GeV or so. But there’s plenty of reason to wonder whether it really is the simplest Higgs, or something more subtle. There are tiny hints in the data that may indicate that this new particle is not just the minimal Higgs. It’s far too early to tell whether those hints are real; they could easily go away, but we can rest assured that the experiments will be following up on them to figure out what’s really going on.

  Remember that particles don’t appear in the detector with labels. When we say that we’ve found something consistent with a Higgs boson, we’re referring to the fact that the Standard Model makes very specific predictions once the mass of the Higgs is fixed. There are no other free parameters; knowing that one number allows us to say precisely how many decays there will be into each channel. Saying that we see something like the Higgs is saying that we see the right amount of excess events in all the channels where they should be visible, not just in one.

  The figures included in the color insert show the data from ATLAS and CMS in 2011 and early 2012, looking specifically at collisions that created two photons. What we see are the numbers of events in which the two photons total up to a specific energy. Notice how few of these events there actually are. The experiment sees hundreds of millions of interactions per second, of which a couple hundred per second pass through the trigger and are recorded for posterity; but in a year’s worth of data, we get only a thousand or so events at each energy.

  The dashed curve in the figure is the prediction for the background—what you would expect without a Higgs. The solid line is what happens when we include the ordinary Standard Model Higgs, with a mass of 125 GeV. Both curves show a small bump with a couple hundred more events than expected. You can’t say which events are Higgs decays, and which decays are background, but you can ask whether there is a statistically significant excess. There is.

  Closer inspection reveals something funny about these data. One of the reasons we were surprised to find the Higgs so quickly in 2012 is that the experiments actually observed more events than they should have. The significance of the two-photon bump in the ATLAS data is 4.5 sigma, but with the number of collisions analyzed the Standard Model predicts that we should have reached only 2.4 sigma. Likewise, in CMS, the significance was 4.1 sigma, but it was expected to reach only 2.6 sigma.

  In other words, there were more excess events with two photons than we should have seen. Not too many more; the sizes of the bumps are a bit bigger than expected but still within the known uncertainties. But the fact that they are consistent between both experiments (and consistent with ATLAS’s result from 2011 alone) is intriguing. There is no question we will need more data to see whether this discrepancy is real or just a tease.

  The CMS data presented another small but noticeable puzzle. While ATLAS stuck with the robust channels of two photons or four charged leptons, CMS also analyzed three noisier channels: tau-antitau, bottom-antibottom, and two Ws. As might be expected, the bottom-antibottom and WW channels didn’t give statistically significant results (although more data will certainly improve the situation). The tau-antitau analysis, however, was a puzzle: No excess was seen at 125 GeV, even though the Standard Model predicts that it should be. This was not quite a statistically significant discrepancy, but it’s interesting. Indeed, the slight tension with the tau data is what brought the final significance of the full CMS analysis down to 4.9 sigma, even though the two-photon and four-lepton channels alone had achieved five sigma.

  What could be going on? None of these hints is serious enough to be sure that anything at all is going on, so it might not be worth taking the discrepancies too seriously. But as theorists, that’s what we do for a living. Within a day or two after the seminars, theory papers were already appearing online, attempting to sort it all out.

  It’s easy to give one simple example of the kind of thing that people are thinking about. Remember how the Higgs decays into two photons. Because photons are massless, and therefore don’t couple directly to the Higgs, the only way this can happen is via some intermediate virtual particle that is both massive (so it couples to the Higgs) and electrically charged (so that it couples to photons).

  By the rules of Feynman diagrams, we are instructed to calculate the rate for this process by adding up independent contributions from all the different massive charged particles that could appear in the loop inside this diagram. We know what the Standard Model particles are, so that’s not hard to do. But new particles can easily change the answer by contributing to those virtual processes, even if we’ve not yet been able to detect them directly. So the anomalously large number of events might be the first signal of particles beyond the Standard Model, helping the Higgs decay into two photons.

  Details matter, of course; if the new particles you have in mind also change the rates of other measured processes, you might be in trouble. But it’s exciting to think that by studying the Higgs we might be learning not only about that particle itself but also about other particles yet to be found.

  Don’t let down your guard.

  TEN

  SPREADING THE WORD

  In which we draw back the curtain on the process by which results are obtained and discoveries are communicated.

  With all the solemn British rectitude he could summon, correspondent John Oliver was putting tough questions to Walter Wagner, the man who had gone to court to stop the Large Hadron Collider from beginning operations. A serious charge had been leveled: the LHC was a hazard to the very existence of life on earth.

  JO: So, roughly speaking, what are the chances the world is going to be destroyed? Is it one in a million, one in a billion?

  WW: Well, the best we can say right now is about a one-in-two chance.

  JO: Hold on a second. It’s . . . fifty-fifty?

  WW: Yeah, fifty-fifty . . . If you have something that can happen, and something that won’t necessarily happen, it’s going to either happen, or it’s going to not happen, and, so, the best guess is one in two.

  JO: I’m not sure that’s how probability works, Walter.

  As the LHC was starting up in 2008, physicists tried their best to spread the word that this was a machine that would help us find the Higgs boson, perhaps reveal supersymmetry for the first time, and possibly discover exciting and exotic phenomena such as dark matter or extra dimensions. But against this uplifting story of human curiosity triumphant, a countervailing narrative struggled for people’s attention: The LHC was a potentially dangerous experiment that would re-create the Big Bang and potentially destroy the world.

  At the time, the mad-scientists-out-of-control scenario was winning the competition for attention. It’s not that jo
urnalists were willing to ignore the truth and seek out sensationalism for its own sake. (At least, not most of them. In the United Kingdom, the Daily Mail tabloid ran a big headline, ARE WE ALL GOING TO DIE NEXT WEDNESDAY?) Rather, much like the label “God Particle,” the disaster scenarios seemed to be a mandatory part of any news story. Once the idea is posed that just maybe the LHC could kill everyone on earth—even if it was something of a long shot—that’s the question that people wanted to see addressed. Added to the mix was Walter Wagner, a litigious former nuclear safety officer, who brought a quixotic suit against the LHC in Hawaii. After the case was thrown out of court on (fairly evident) jurisdictional grounds, Wagner appealed to federal court. A three-judge panel finally dismissed the case in 2010, with a pithy conclusion:

  Accordingly, the alleged injury, destruction of the earth, is in no way attributable to the U.S. government’s failure to draft an environmental impact statement.

  CERN and other physics organizations took the need to proceed safely very seriously, sponsoring multiple expert reports on the subject, all of which concluded that the risk of disaster was completely negligible. Oliver’s interview, which allowed Wagner to discredit himself with his own words, was one of the very few news reports to take an appropriate angle on the topic. It appeared on Jon Stewart’s The Daily Show, a satirical news program from Comedy Central channel. Only a comedy program was smart enough to treat the LHC disaster worry as the farce that it was.