Late on the evening of June 14, 2012, groups of graduate students and postdoctoral researchers working on the Large Hadron Collider began peering into a just opened data cache. This huge machine at CERN, the European laboratory for particle physics near Geneva, had been producing tremendous amounts of data in the months since it awoke from its winter-long slumber. But the more than 6,000 physicists who work on the LHC's two largest experiments were wary of unintentionally adding biases to their analysis. They had agreed to remain completely unaware of the results—performing what are called “blind” analyses—until mid-June, when all would suddenly be revealed in a frenzy of nocturnal activity.
Many of the young scientists worked through that night to untangle the newly freed threads of evidence. Although the LHC is a giant collider feeding multiple experiments, only the two largest ones—ATLAS and CMS—had been tasked with finding the Higgs boson, the long-sought particle that would complete the Standard Model of particle physics, the theoretical description of the subatomic world. Each massive detector records the subatomic debris spewing relentlessly from proton collisions in its midst; a detailed, independent accounting of these remnants can reveal fleeting new phenomena, including perhaps the elusive Higgs boson. Yet the detectors have to sift through the particle tracks and energy deposits while enduring a steady siege of low-energy background particles that threaten to swamp potentially interesting signals. It is like drinking from a fire hose while trying to ferret out a few tiny grains of gold with your teeth.
Fortunately, the scientists knew what they were looking for. After the LHC's disastrous start—an electrical splice between two magnets warmed and melted just nine days after the LHC came online in 2008, triggering a powerful spark that punctured the surrounding vessel, released tons of helium and ripped scores of costly superconducting magnets from their mounts—the collider had been collecting reams of data during 2011, enough to pick up an early hint of a Higgs signal.
After that run ended in October for its scheduled winter shutdown, Fabiola Gianotti, then spokesperson for ATLAS, and one of us (Tonelli), then spokesperson for CMS, delivered a special seminar to an overflowing audience in the main CERN auditorium. Both detectors independently found suggestive bumps in the data.
What's more, these telltale hints of a Higgs boson corroborated one another. Both ATLAS and CMS reported several dozen events above the expected background in which two photons came blazing out with combined energies of 125 billion electron volts, or 125 GeV. (GeV is the standard unit of mass and energy in particle physics, about equal to a proton mass.) If proton collisions had created short-lived Higgs bosons, they could have decayed into these photons. Each experiment also found a few surplus events in which four charged leptons (electrons or muons) carried off similar total energies. These could also have been the result of a Higgs [see box on page 8]. Such a concurrence of signals was unprecedented. It suggested that something real was beginning to appear in the data.
Yet given the stringent norms of particle physics, none of the signals observed in 2011 were strong enough to allow for claims of a “discovery.” Data peaks and bumps like this had often proved ephemeral, mere random fluctuations. And the successful spring 2012 run, which generated more proton collisions in 11 weeks than had come in during all of 2011, could easily have washed out the nascent data peaks, smothering them in background noise.
Of course, the opposite could occur, too. If the bumps were the result of an actual Higgs boson, not just a cruel statistical artifact, all the new data gave researchers a good chance of being able to claim an official discovery—ending this decades-long search and beginning a whole new era in our understanding of matter and the universe.A Three-Decade Search
Never just another particle, the Higgs boson is the cornerstone of a grand intellectual edifice known as the Standard Model, the interwoven set of theories that constitute modern particle physics. This particle's existence had been suggested in 1964 by Peter W. Higgs of the University of Edinburgh as the result of a subtle mechanism—independently conceived by François Englert and Robert Brout in Brussels plus three theorists in London—that endows elementary particles with mass. The Higgs boson is the physical manifestation of an ethereal fluid (called the Higgs field) that permeates every corner of the cosmos and imbues elementary particles with their distinctive masses. With the discovery of quarks and gluons in the 1970s and the massive, weak-force- bearing W and Z bosons during the early 1980s, most of the elements of the Standard Model had fallen neatly into place.
Although theorists asserted that the Higgs boson—or something like it—must exist, they could not predict what its mass might be. For this and other reasons, researchers had few clues about where to look for it. An early candidate, weighing in at less than nine times the proton mass, turned up in 1984 at a refurbished, low-energy electron-positron collider in Hamburg, Germany. Yet the evidence withered away after further study.
Most theorists agreed that the Higgs mass should be 10 to 100 times higher. If so, discovering it would require a much larger and more energetic particle collider than even the Fermi National Laboratory's Tevatron, a six-kilometer proton-antiproton collider completed in 1983. That same year CERN began building the billion-dollar Large Electron Positron (LEP) collider, boring a 27-kilometer circular tunnel that crossed the French-Swiss border four times near Geneva. Although LEP had other important physics goals, the Higgs boson was high on its target list.
U.S. particle physicists, encouraged by the Reagan administration to “think big,” pushed through grandiose plans for a much larger, multibillion-dollar machine, the Superconducting Super Collider (SSC), in the late 1980s. With a proton-proton collision energy of 40 trillion electron volts (40 TeV, or 40,000 GeV), the SSC was designed to track down the Higgs boson even if it were to come in at a mass near 1,000 GeV.
But after the SSC's projected price tag nearly doubled to $10 billion, Congress voted to kill it in 1993. Dismayed, U.S. Higgs hunters thereafter turned back to Fermilab and CERN to pursue this research. Discoveries and precision measurements made at LEP and the Tevatron soon implied that the Higgs boson should be no more than 200 GeV, which put it potentially within reach of these colliders. In over a decade of searching, however, physicists found no lasting evidence for Higgs-like data bumps.
During the final LEP runs in the summer of 2000, physicists decided to push the collision energy beyond what the machine was designed to handle. That is when hints of a Higgs boson began appearing. In September two of the four LEP experiments reported evidence for a handful of events with a Z boson plus another mystery particle that decayed into two bottom quarks—a particle that looked a lot like a 115-GeV Higgs boson. CERN's then director Luciano Maiani granted the machine a six-week stay of execution that autumn, but during that period researchers could unearth only one more candidate event. It was not sufficient. After a heated debate, Maiani decided to shut LEP down and begin its planned conversion into the LHC, a machine designed to find the Higgs boson.Closing In on Discovery
The LHC is the most spectacular collection of advanced technology ever assembled. Built inside the original LEP tunnel by hundreds of accelerator physicists and engineers led by project manager Lyndon Evans, it uses little left from that collider. Its principal components include more than 1,200 superconducting dipole magnets—shiny, 15-meter-long cylinders worth nearly $1 million each. Probably the most sophisticated components ever mass-produced, by firms in France, Germany and Italy, they harbor twin beam tubes that are flanked by niobium-titanium magnet coils bathed in liquid helium at 1.9 kelvins, or −271 degrees Celsius. Inside, twin proton beams circulate in both directions at energies up to 7 TeV and velocities approaching light speed.
The beams resemble those of a pulsed laser rather than a flashlight. Each consists of almost 1,400 “bunches,” containing up to 150 billion protons apiece—about the number of stars in the Milky Way. Under normal operations, 10 to 30 proton collisions occur during each bunch crossing. That corresponds, however, to around half a billion collisions per second.
Proton collisions are far messier than electron-positron collisions. Theorist Richard Feynman of the California Institute of Technology once compared the process to smashing garbage cans into garbage cans, which means that lots of junk comes out. Protons are composite objects made of quarks and gluons; in the most interesting events, two gluons collide at energies above 100 GeV—and occasionally up to 1 TeV. Physicists, aided by sophisticated detectors, custom-built electronics and state-of-the art computers, try to sift the few events corresponding to interesting physics from the billions of dull, uninteresting ones.
The ATLAS and CMS experiments cannot observe a Higgs boson directly—it would decay into other particles far too quickly. They look for evidence that it was created inside. Depending on the Higgs boson's mass, it could decay into lighter particles in a variety of ways. In 2011 attention began to focus on its rare decays into two photons and four charged leptons [see box below] because these signals would stand out starkly against the tremendous backgrounds that could easily swamp a Higgs signal.
The year's delay caused by the 2008 magnet disaster gave Fermilab physicists one last shot at making a Higgs discovery. Just before the scheduled Tevatron shutdown in September 2011, the CDF and D-Zero experiments at the collider reported small excesses of events in which bottom quark pairs appeared at combined energies from 125 to 155 GeV. But as in the LEP closure, the researchers could not convince the lab director to grant them a reprieve, and the Tevatron was soon shut down. (In March 2012 these physicists reported a more detailed analysis that showed a bulge centered at 125 GeV, reinforcing the CERN results.)Crossing the Line
By May 2012 the LHC was producing data 15 times faster than the Tevatron had ever achieved, thanks to efforts of physicists and operators led by accelerator director Stephen Myers. This run was a culmination of two decades of work by thousands of ATLAS and CMS physicists who built and now operate the detectors, designed and now manage a computer system that distributes data around the world, created novel hardware and computer software to identify the most interesting collisions, and wrote the algorithms that dig out the most pertinent events from the great morass of data being recorded. They all worked feverishly, anticipating a discovery. So when the researchers opened their data sets in mid-June, they had torrents of events to sift through. After graduate students and postdocs worked through the night, they anxiously prepared to reveal what had turned up.
It was a hot afternoon on June 15, 2012, when CMS physicists began gathering in Room 222 of the CERN filtration plant to hear the young physicists' reports. Soon the room was crowded with hundreds of collaboration members—out of about 3,000 in all—many of them standing or sitting on the floor. Few had slept much the night before. Tension and excitement gripped the room.
The first speaker discussed one possible Higgs decay route, or “channel,” into pairs of W bosons. A small excess of events appeared in the low-mass region of most interest, but the faint signal generated no great excitement. Then presentations on the rare four-lepton and two-photon decays came one after the other. Now it indeed looked like a Higgs boson was showing up at long last. The signals from the 2012 data were occurring again in the same vicinity—near 125 GeV—that had so tantalized researchers six months earlier. Scientists realized almost immediately that if they were to combine the new data with the 2011 results, chances were good that CMS could claim a Higgs discovery. The crowd cheered at the end of the two key presentations.
Similar revelations occurred in the ATLAS experiment. Spontaneous celebrations broke out in several groups when they first glimpsed the new data. Yet it took more than a week of long workdays and sleepless nights before ATLAS physicists were certain that they had enough good events to conclude that the chances that their results were due to random fluctuations were less than one in three million—corresponding to the stringent “five sigma” standard that particle physicists require before claiming a discovery. At the thrilling moment of recognition, one ATLAS group of about a dozen physicists, meeting in Building 32 on the afternoon of June 25, 2012, erupted in loud clapping and cries of joy, which echoed down the hallway.
By that time word of a discovery had leaked out. Worldwide interest began growing so intense that secrecy was placed at a premium. There were to be no further leaks before the official word was presented, particularly because the exact content of documents under preparation could change. ATLAS members were not supposed to talk about the recent results with CMS physicists, nor vice versa. Individual physicists, however, could not resist discussing the news many had awaited so long. Hushed conversations in the CERN cafeteria and corridors suggested that something big was building up. Pressure to go public swelled.
CERN director Rolf-Dieter Heuer got an early glance at the findings in a June 22, 2012, meeting with Gianotti and Joseph Incandela of the University of California, Santa Barbara, Tonelli's successor as CMS spokesperson. Heuer decided that the evidence was strong enough to make public. He immediately informed the CERN Council (its governing body) to keep them abreast of the fast-moving developments. Heuer then decided to hold a joint seminar at CERN on July 4, 2012, timed to coincide with the opening of the 36th International Conference on High Energy Physics in Melbourne, Australia, followed by a CERN press conference.
The night before the seminar, hundreds of physicists dozed fitfully in the hallways outside the locked main auditorium, desperately hoping to get one of the unreserved seats remaining inside. Myers, Evans and four prior CERN directors who had been heavily involved with the LHC since its conception were seated in the front row. Having just flown to Geneva, Peter Higgs walked in to warm, sustained applause and sat down next to Englert.
Incandela and then Gianotti showed blizzards of slides about the new data and results, mostly covering the 2012 measurements. As in December, graphs of two-photon data revealed striking peaks jutting out at 125 to 126 GeV. And this time around, the experiments had more than a dozen extra events in which a heavy particle had exploded into four charged leptons at 125 GeV. Subtle peaks had begun to form in that channel, too.
That clinched it. Combining this result with the two-photon one, CMS and ATLAS independently concluded that the chances that the apparition was a fluke, due to random fluctuations, were less than one in three million. It had to be real. When the camera panned to Higgs, he could be seen pulling out a handkerchief to wipe his eyes.
“I think we have it,” exulted Heuer, wrapping up the seminar to sustained applause. “We have a discovery,” he went on, guardedly using the word at last. “We have observed a new particle consistent with a Higgs boson.”A New Era in Physics?
Few physicists doubt that a heavy new particle has turned up at CERN, but there is still debate about its exact nature. CERN officials initially spoke cautiously on this question, calling it a “Higgs-like” boson, but in March announced that it is indeed a Higgs—though not necessarily the only one. Although physicists have not yet proved beyond a doubt that the new particle has the required property of zero “spin,” preliminary data strongly favor that value. The ATLAS experiment continues to observe more two-photon events than expected, while CMS is reporting results consistent with Standard Model expectations based on a similar amount of data. Could something be amiss here?
Since July 2012 attention has become focused on whether the new particle is indeed “the” Higgs boson as predicted by the Standard Model. That question can be resolved by determining how the new boson decays into other particles. Results released by ATLAS and CMS since July 2012 show that the Higgs signal has greatly improved, while difficult-to-measure decays into bottom quarks and tau leptons are beginning to appear at about the expected frequency. Meanwhile Fermilab physicists published evidence from the Tevatron for decays of the new particle into bottom quarks. Analyses of the combined LHC and Tevatron data by CERN theorists John Ellis and Tevong You indicated that the new particle, as they put it, “does indeed walk and quack very much like a Higgs boson.”
The new particle's connection with a pair of high-energy photons has stimulated intrigue. Because the Higgs field imbues elementary particles with mass, it should interact more strongly with heavier particles. Photons have no mass, so the Higgs boson produces them via a mechanism involving other, massive particles. Additional heavy particles (which are required by supersymmetry and other theories) could enhance the process—as may be happening, based on early data. If the tendency holds up, it will strongly suggest physics beyond that described by the Standard Model.
The Higgs discovery marks the end of a long era in particle physics and the beginning of an exciting new phase. After decades in the doldrums, the discipline is energized once again by the heady intercourse of theory and experiment. Questions abound that may find answers from further research on this fascinating particle or its potential partners. Does it play a role in the inflation mechanism considered the force driving the big bang origins of the universe? Does it interact with dark matter particles thought to inhabit the cosmos? And what higher-energy mechanism or process, if any, shields the fragile vacuum from instabilities that may threaten the existence of the universe as we know it?
Although we celebrate the triumph of the Standard Model, such a lightweight Higgs boson should be extremely sensitive to physics lying beyond it. The particle opens up a fabulous new laboratory for further experimentation. Are its properties exactly as predicted? The apparent discrepancies in the early data could be random fluctuations that disappear in months to come. Or perhaps they are offering subtle hints of intriguing new physics.