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

  Like most major particle physics labs, the story of CERN has been one of bigger and better machines reaching ever-higher energies. In 1957, there was the Synchrocyclotron, which accelerated protons to an energy of 0.6 GeV, and in 1959, saw the inauguration of the Proton Synchrotron, which reached energies of 28 GeV. It still operates today, providing beams that are accelerated further by other machines, including the LHC.

  A major step forward came in 1971 with the Intersecting Storage Rings (ISR), which attained 62 GeV in total energy. The ISR was a proton collider as well as an accelerator. Previous machines had accelerated protons and aimed them at stationary blocks of matter, which are relatively easy targets to hit; the ISR collided beams moving one way with beams moving in the opposite direction. This technique presents a much greater technological challenge but also makes much higher energies accessible; not only does each beam carry energy, but every bit of the energy is now available to make new particles. (In fixed-target experiments, much of the energy goes into providing a push to the target.) Prospects for building a particle collider were studied in the 1950s by Gerard K. O’Neill, an American physicist, who later became more well-known for proposing and advocating human habitats in outer space, and small electron-positron colliders were constructed in Frascati, Italy, in the 1960s by Austrian physicist Bruno Touschek.

  The ISR was about one and three-quarters of a mile in circumference. Big, but bigger was yet to come. The Super Proton Synchrotron (SPS), more than four miles in circumference, opened in 1976, and reached energies of 300 GeV. Just a few years later, in a bold move, CERN reengineered the SPS from its original task of accelerating protons to a new configuration in which it collided protons with antiprotons. As you might expect, antiprotons are hard to collect and work with. They’re not lying around like protons are; you have to make them in lower-energy collisions to start, and then work hard to gather them without their bumping into an ambient proton and annihilating in a flash of light. But if you pull off this trick, you can take advantage of the fact that protons and antiprotons have opposite charges to curve them around in opposite directions but in the same magnetic field. (The LHC collides protons with protons, and therefore has to use two separate beam pipes for the two directions.) Italian physicist Carlo Rubbia used the upgraded SPS in 1983 to discover the W and Z bosons of the weak nuclear force, picking up the Nobel Prize in 1984.

  The SPS is still around, and still hard at work. Thanks to upgrades, it now accelerates protons up to 450 GeV. These are handed off to the LHC, which pushes them to even higher energies. Particle physicists are great believers in recycling.

  CERN inaugurated its next great machine in 1989—the Large Electron-Positron collider (LEP). This required yet another new tunnel, this time seventeen miles around and 330 feet underground, stretching across the Swiss-French border. If those numbers sound familiar, they should—the tunnel that was originally built for LEP is the same one in which the LHC now sits. After a successful run, LEP was shut down in 2000, and the machinery was removed to make way for the LHC.

  The Large Electron-Positron collider

  Protons are hadrons—strongly interacting particles. When you smash two of them together (or a proton and an antiproton), the results are a little unpredictable. What really happens is that one of the quarks or gluons inside the hadron smacks into one of the quarks or gluons in the other, but you don’t know the precise energy of either particle to start. A machine that collides electrons and positrons is a very different beast: built for precision, not brute power. When an electron and positron collide, as in LEP, you know exactly what is going on; the results are better suited for delicate measurements of particle properties than for discovering new particles in the first place. If you’re playing “Where’s Waldo?” particle physics at a hadron collider is like letting your gaze wander over the entire picture looking for a jaunty striped cap; searching at an electron-positron collider is like placing a fine grid over the drawing and painstakingly examining the faces one by one.

  LEP was so precise that it was even able to discover the moon. Or, at least, the tides it causes. Each day, the moon’s gravitational field tugs at the earth as it rotates underneath. At CERN, this tiny stress caused the total length of the LEP tunnel to stretch and contract by about a millimeter (one-twenty-fifth of an inch) every day. Not such a big deal in a seventeen-mile-long beam pipe, but enough to cause a tiny fluctuation in the energy of the electrons and positrons—one that was easily detectable by the high-precision instruments. After some initial puzzlement at the daily energy variations, the CERN physicists quickly figured out what was going on. (At heart, this way of detecting the moon isn’t that different from how astrophysicists detect dark matter in the universe, through its gravitational influence.) LEP was also able to detect the passage of high-speed TGV trains entering Geneva, whose leaking electrical currents were able to measurably disturb the precisely tuned machine.

  But the LEP physicists weren’t there to detect the moon or trains; they wanted to discover the Higgs boson. And for a while, they thought they had.

  After a very successful run making precision measurements of properties of the Standard Model (but not discovering any new particles), LEP was scheduled to be turned off in September 2000 and dismantled to make way for the LHC. Knowing that their machine had only a few months of operation left, the technicians went for broke, using every trick they could think of to boost it to 209 GeV, a higher energy than its design specifications had ever contemplated. If it broke, it was a lame duck accelerator anyway.

  As the beams attained these new energies, a team at an experiment named ALEPH led by Sau Lan Wu of the University of Wisconsin–Madison noticed a handful of events that stood out above the rest. Just a few tantalizing hints, but exactly what we might expect if there was a Higgs boson lurking at a mass of 115 GeV, right at the edge of what LEP was capable of seeing. Wu has a number of important results to her name, including sharing the European Physical Society Prize for a 1979 experiment that helped establish the existence of gluons. She was hot on the trail of the Higgs, and wouldn’t let this opportunity slip by carelessly.

  Ordinarily, a few suggestive events in a particle detector aren’t much reason to get excited, even if they look exactly like the Holy Grail that you and your colleagues have been chasing for years. Particle physics is about statistics: For almost anything you can see in a detector, there is more than one way to make it happen, and the whole trick is comparing the rate you should expect against the rate you might get with a new particle. So if a few events are teasing you, just collect more data. The signal will either grow stronger or fade away.

  The problem is, you can’t collect more data if the lab is going to turn off your accelerator. Wu and other physicists petitioned Luciano Maiani, an Italian physicist who was director general of CERN at the time, to extend the LEP run in order to collect more data. Everyone appreciated the possible significance of the potential discovery, and the enormous regret they would feel if they shut down the machine just before finding the Higgs. You don’t often get to see an elementary particle for the first time, especially one this central to our understanding of physics. As physicist Patrick Janot put it at the time, “We are writing a line in the history of mankind.” And they also knew they had competition: The Tevatron accelerator at Fermilab, outside Chicago, was also taking aim at the Higgs boson, and might be able to find it at 115 GeV before the LHC could come up to speed. Particle physics relies on international collaboration, but a competitive fire burns inside every scientist.

  Maiani, appreciating what was at stake, chose a compromise: LEP would still be shut down, but only after one additional month of running, through October 2000. The Higgs hunters grumbled a little bit but set about collecting more data in search of events that matched what the Higgs should produce. And they found them; just a few, but scattered over the four different experiments running at LEP, not just at the ALEPH detector where Wu’s team was working. But they also collected many more �
��background” events that didn’t look like the Higgs at all.

  When the run finally came to an end, the total statistical significance of the apparent Higgs events had actually decreased; the signal was being swamped by the background. LEP could have kept running, but that would have meant a serious delay in the schedule for building the LHC, which would have meant both increased costs and more time before the bigger machine would finally come online. As tempting as it was to make one last grab for the brass ring, it was time for LEP to retire and for other accelerators to take up the chase.

  SLAC, Brookhaven, Fermilab

  While CERN has successfully combined the efforts of many European countries (and more recently, the world) to create a leading physics lab, other facilities have also been responsible for major advances in our understanding of particles and forces. Three labs in the United States, in particular, have helped put together the Standard Model: SLAC at Stanford University in California, Brookhaven on Long Island, and Fermilab outside Chicago.

  SLAC originally stood for “Stanford Linear Accelerator Center,” but in 2008, the Department of Energy officially changed it to “SLAC Linear Accelerator Center,” perhaps because someone in a position of power is fond of infinite recursion. (More plausibly, because Stanford University didn’t want the Department of Energy to trademark an acronym containing their name.) Founded in 1962, SLAC holds a unique place in particle physics by hosting a high-energy linear accelerator—a straight line rather than a circular ring. The building containing the accelerator is two miles long, the longest building in the United States and third-longest in the world. (The top two are the Great Wall of China and the Ranikot Fort, a nineteenth-century military fortification in Pakistan.) Originally, the accelerator used electrons and slammed them into fixed targets. Starting in the 1980s, it was upgraded to collide electrons with positrons, and eventually a ring was added, using the linear accelerator as a first stage.

  SLAC played a key role in the discovery of several particles, including the charm quark and the tau lepton, but undoubtedly its greatest contribution was showing that the very idea of quarks was on the right track. In 1990, the Nobel Prize was awarded to Jerome Friedman and Henry Kendall of MIT and Richard Taylor of SLAC, who in the 1970s used SLAC’s electron beam to closely examine the inner structure of protons. The SLAC-MIT team showed that low-energy electrons went right through the protons without much deflection, while high-energy electrons (which you might have expected to go through even more easily) were more likely to careen off at odd angles. Particles with high energies correspond to field vibrations with short wavelengths, and are therefore sensitive to resolve what’s going on at very short distances. What the physicists were seeing were very small particles living inside the protons—what we now know as quarks.

  Brookhaven National Laboratory was founded in 1947, and has contributed to seven different Nobel Prizes: five in physics and two in chemistry. The muon neutrino, for which Lederman, Schwartz, and Steinberger shared the Nobel, was discovered at Brookhaven. Currently, its main contribution to particle physics comes from the Relativistic Heavy Ion Collider (RHIC), a 2.4-mile-long ring that smashes heavy nuclei together to create the kind of quark-gluon plasma that existed shortly after the Big Bang. Officials from the Guinness Book of World Records have certified that RHIC is responsible for the highest artificially produced temperature of all time: more than 7 million degrees Fahrenheit, or 250,000 times the temperature at the center of the sun. The goal of physics at RHIC is not so much to search for new particles, as to figure out how quarks and gluons behave in these extreme circumstances.

  The other major high-energy physics complex is the Fermi National Accelerator Laboratory, or Fermilab for short. Specializing in giant rings that accelerated protons and antiprotons to high energies, Fermilab has been a direct competitor to CERN for much of its existence. It was founded in 1967 under the guidance of Robert Wilson, a polymath scientist and innovative administrator who was renowned among physicists for his creativity and apparent ability to achieve the impossible. Not only did he bring in the new laboratory ahead of schedule and under budget, he designed the main building and personally created many of the sculptures that bring the site to life. When Wilson, who had briefly studied sculpture at the Accademia Belle Arti in Rome, proposed building a thirty-two-foot-tall metal obelisk for the lab, he was told that union regulations required that all welding be done by union members. His response was natural (for him): He joined the welders’ union, apprenticed himself to master welder James Forester of the Fermilab Machine Shop, and dutifully followed the appointed course of instruction. The obelisk, constructed by Wilson over lunchtimes and weekends, was installed in 1978 in a reflecting pool outside the main hall.

  The pride of Fermilab was the Tevatron, a massive machine that collided protons and antiprotons together at energies of 2,000 GeV. (Remember that “TeV” stands for one “Tera Electron Volt,” which is 1 trillion electron volts, or 1,000 GeV.) Completed in 1983, the Tevatron was the highest-energy accelerator in the world until the LHC took the crown in 2009. Its crowning achievement was the discovery of the unusually massive top quark, finally pinning it down in 1995. Gordon Watts of the University of Washington, who was a graduate student working at Fermilab at the time, remembers the moment when the signal climbed above the important “three sigma” threshold (explained in Chapter Nine) for claiming evidence for a new particle:

  We were in one of the big top meetings reviewing all the analyses that were about to go out for one of the conferences. Every analysis was seeing a small excess, but it was so small that it wasn’t really meaningful. In fact, they had been doing this for quite some time and we were all used to it—so we basically ignored it. It was the end of one of the normal marathon meetings, the room was packed, I was sitting on the floor in the back, in fact. It was hot, and the room air was . . . umm . . . stuffy (to put it nicely). I think we were about to hear the last talk when one of the people that had gotten there early enough to snag a chair raised his hand . . . “Uh . . . hold it a moment . . . if I do the simplest thing here and add up all the backgrounds and the signals I get over three sigma.” There was a silence in the room while everyone went scrambling back through the talks to figure out if that was actually correct. Either the spokesperson or the top convener spoke next . . . it was a four-letter word. I think everyone felt the chill go down their spine.

  The long sought-after Higgs remained beyond the Tevatron’s grasp. With lower energy and luminosity than the LHC, the American machine was always a long shot to win that race. But after LEP turned off, and before the LHC came to life, Fermilab had a window where they could possibly have claimed the first solid evidence of the mysterious particle. In the end, physicists at the Tevatron were able to exclude certain mass ranges for the Higgs, but they couldn’t claim any hard indication for its existence.

  Facing significant pressure from a gloomy budget situation, as well as the much higher energies of the newly operational LHC, the Tevatron was shut off for good on September 30, 2011, ending the career of the last major high-energy particle collider on U.S. soil. (The Relativistic Heavy Ion Collider at Brookhaven does important work in nuclear physics but doesn’t compete in the search for new particles, reaching energies of less than 10 GeV per nucleon.) Whether it will ever have a successor is currently unknown.

  The Super Collider

  There was supposed to be a successor to the Tevatron, of course: the Superconducting Super Collider, which was endorsed by President Ronald Reagan in 1987 and originally scheduled to begin operation in 1996. The SSC was a grandly ambitious scheme, featuring a brand-new ring fifty-four miles in circumference, colliding protons with 40 TeV of total energy, twenty times higher than the Tevatron. In retrospect, it may have been too ambitious. Support for the project was very high in the early days, when a site for the laboratory had not yet been chosen: Nearly every state’s Congressional delegation could imagine they would bring home the massive project for their constituents,
and forty-three of the fifty states treated the competition seriously enough to undertake geological and economic surveys. The eventual winner was a site near the sleepy town of Waxahachie, Texas, about thirty miles south of Dallas.

  Once the SSC site was selected, enthusiasm for the project immediately dampened among forty-nine of the fifty state delegations in Congress. It was a time of great pressure to bring the federal budget deficit under control, and the SSC cost, high to begin with, had nearly tripled to $12 billion. An additional factor was competition—in the minds of government officials, if not in the minds of scientists—from the International Space Station. The ISS budget was more than $50 billion from NASA, or more than $100 billion if flights of the space shuttle were included in the cost. It did not escape notice that much of the money for this giant project would also end up in Texas, with the Johnson Space Center serving as mission control.

  I asked JoAnne Hewett, now a theorist at SLAC, about how she came to accept her current job. She could pinpoint the day precisely: October 21, 1993, the day Congress voted to kill the SSC for good. Hewett had offers from the SSC lab, as well as from SLAC, and was eager to be part of the exciting atmosphere at the new machine under construction. She spent that autumn morning watching Congress on C-SPAN, observing helplessly as the vote went the wrong way. She spent that afternoon in mourning, and then called the director of SLAC to accept their offer. Her career there has been very successful, building new models of particle physics and inventing clever ways to test them against the data—but one can’t help but feel wistful about the prospect of actually having such data in hand, earlier and from higher-energy collisions.