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 same thing happens with the Higgs field. In the very early universe, the temperature is unbelievably high, and the Higgs field is being jostled constantly. As a result, its value at any one point keeps hopping around and averages out to zero. In the early universe, symmetry is restored. W and Z bosons are massless, as are the fermions of the Standard Model. The moment at which the Higgs went from being zero on average to some nonzero value is known as the “electroweak phase transition.” It’s something like liquid water freezing to become ice, but nobody was around to see it happen.

  We’re talking about very early times in the history of the universe here: about one trillionth of a second after the Big Bang. If you re-created the conditions from the early universe in your living room, the Higgs would evolve from zero to its usual nonzero value so quickly that you’d never notice it had been zero. But physicists can use equations to predict a long sequence of events that happened in that first trillionth of a second. At the moment we don’t have any direct experimental data to test those ideas, but we’re working on making predictions that will someday confront the observations.

  Messy but effective

  This story might sound a bit far-fetched, what with nonzero fields in empty space, nature discriminating between left and right, and bosons putting on weight by chowing down on other bosons. It’s a picture that was only put together gradually, over the course of many years, and against a tide of skeptical voices chiming in along the way. But . . . it fits the data.

  When this theory of the weak interactions was finally put together by Steven Weinberg and Abdus Salam in independent papers from the late 1960s, it was pretty thoroughly ignored. Too much artifice, too many fields doing too many weird things. At the time, people had deduced that something like the W bosons must exist in order to carry the weak force. But Weinberg and Salam predicted a new particle, the neutral Z boson, for which there wasn’t any evidence. Then in 1973, an experiment at CERN with the whimsical name of Gargamelle found evidence for the interaction carried by what we now call the Z. (The particle itself wasn’t discovered until ten years later, also at CERN.) Since then, experiment after experiment has piled on data that continues to support the basic picture of a weak-interaction symmetry broken by a Higgs field.

  As of 2012, we seem to have finally put our fingers on the Higgs itself. But that’s not the end of the story, it’s only the beginning. There’s no question that the Higgs theory fits the data, but in many ways it seems more than a bit contrived. Other than the Higgs, every particle we’ve ever found is either a fermionic “matter particle” or a boson derived from the connection field associated with a symmetry. The Higgs seems different; what makes it so special? Why just those symmetries, broken in just that way? Is it possible there’s a deeper theory that would work even better? Now that we’re confronting data rather than just inventing models, there is good reason to hope that we will be inspired to come up with a better theory than brainpower alone has yet given us.

  NINE

  BRINGING DOWN THE HOUSE

  In which we figure out how to find the Higgs boson, and how we know we’ve found it.

  After years of waiting, the discovery of the Higgs boson came faster than anyone had expected.

  In one sense, anticipation had been building for more than four decades, since the Higgs mechanism became the accepted model of the weak interactions. But once the LHC started running, excitement grew in earnest in December 2011.

  Early that month, CERN had put up a fairly innocuous notice, advertising seminars on December 13 entitled “Update on the search for the Higgs boson by the ATLAS and CMS experiments at CERN.” Updates happen all the time, so by itself that wasn’t anything to get excited about. But with two experiments, each representing a group of more than three thousand physicists, word quickly spread that these wouldn’t be any old seminars. As early as December 1, the British Telegraph featured a story by science correspondent Nick Collins, headlined, SEARCH FOR GOD PARTICLE IS NEARLY OVER, AS CERN PREPARES TO ANNOUNCE FINDINGS. The article itself wasn’t nearly as breathless as the headline, but the implications were clear. On the physics blog viXra log, pseudonymous commenter “Alex” pithily noted, “Today[’s] rumour is: Higgs at 125 Gev around 2-3 sigma,” leading other commenters to gleefully start speculating about the theoretical implications.

  “Alex” could have been anyone, of course, from a mischievous teenager in Mumbai who enjoys tweaking particle physicists to Peter Higgs himself. But multiple blogs and online articles seemed to be pointing in the same direction: This wasn’t any old update, this was going to be important news about the Higgs . . . maybe even the long-awaited discovery announcement.

  CMS and ATLAS, the two large LHC experimental collaborations, are each miniature republics, in which the citizens elect leaders to represent them. The topmost office is simply called the “spokesperson.” To ensure that the collaboration speaks with a unified voice, the preparation and communication of new results is tightly controlled—not only official publications, but even talks by individual collaboration members must be carefully vetted. Talks this important are given by the spokespersons themselves. In December 2011, both spokespersons hailed from Italy: Fabiola Gianotti, a CERN staff member, was the leader of ATLAS, while Guido Tonelli from the University of Pisa headed up CMS.

  Gianotti is a major player in experimental particle physics, voted by the Guardian as one of the top one hundred women scientists in the world. She came to the field relatively late, as a college student, after concentrating in Latin, Greek, history, and philosophy in high school, and pursuing piano seriously at a conservatory. It was a professor’s explanation of the photoelectric effect—Einstein’s suggestion that light always comes in discrete quantized packets—that ignited her interest in physics. Now she was leading one of the largest scientific endeavors of all time, on the verge of discovering a major piece of nature’s puzzle. Asked to explain the importance of this quest, Gianotti didn’t hesitate to use poetic language: “Fundamental knowledge is a little bit like art. It’s something very much related to the spirit, the soul, the brain of men and women, as clever beings.”

  Both speakers had exciting news to report, but they did so in the most cautious manner possible. There were hints. ATLAS, in particular, saw some evidence that looked compatible with a Higgs around 125 GeV. In particle physics, “evidence” for unusual things comes and goes quite frequently, but this wasn’t any old unusual thing; it was the kind of signal expected from decaying Higgs bosons, after we had ruled out almost every other place it could be. When you’ve lost your keys and have searched for them almost everywhere, you shouldn’t be surprised when they turn up in the last place you look. To make the case stronger, CMS also saw a wisp of a signal at just about the same mass. Again, nothing to write home about on its own, but in the context of ATLAS’s result it was more than enough to get the room buzzing.

  Gianotti did her best to bring the enthusiasm under control: “It’s too early to tell if this excess is due to a fluctuation of the background, or if it is due to something more interesting.” Later she expressed the same sentiment in more colloquial fashion by quoting an Italian saying, “Don’t sell the skin until you have caught the bear.”

  This particular skin was sold and space in the living room was set aside for a nice new rug long before the bear was actually caught. Statistically, the December results might not have been anything to write home about, but they fit perfectly into what physicists expected to see if there was a Higgs at 125 GeV. It seemed just a matter of time before more LHC data would settle the case. It ended up taking less time than we had any right to expect.

  What goes in

  Let’s take a step back and think about what it takes to discover the Higgs boson, or even find tantalizing evidence for its existence. To dramatically oversimplify things, we can boil it down to a three-step process:

  Make Higgs bosons.

  Detect the particles that they decay into.

  Convince you
rself that the particles really came from the Higgs, and not something else.

  We can examine each step in turn.

  We know the basic idea of making Higgs bosons: Accelerate protons to high energy in the LHC, smash them together inside one of the detectors, and hope that a Higgs is produced. There are more details, of course. We can hope to produce the Higgs when we reach very high energies, because E = mc2 tells us that we have a chance of creating high-mass particles. But thinking that there’s a chance is different from knowing that it will happen. What are the precise processes by which we can expect to make a Higgs boson?

  Your first thought is, “Well, protons smash together, the Higgs comes out.” But a little more thought reminds you that protons are made of quarks and gluons, not to mention virtual antiquarks. So it must be that some combination of quarks and gluons smash together to form a Higgs. Then you remember that in Chapter Seven we talked about conservation laws—quantities like electric charge, quark number, or lepton number remain unchanged in any known particle interaction. So we simply can’t have, for example, two up quarks smash together and form a Higgs. The Higgs has zero electric charge, while each up quark has charge +2/3, so the numbers don’t add up. Adding insult to injury, two up quarks have a total quark number of 2, while the Higgs has a quark number of zero, so that doesn’t add up either. If you had a quark and an antiquark come together, you’d have a chance.

  What about the gluons? The short answer is, “Yes, two gluons can combine to make a Higgs,” but there’s a long answer that is a bit more complicated. Remember that the whole point of the Higgs field (or one of the points, anyway) is to give mass to other particles. The more the Higgs interacts with something, the more mass it ends up having. The converse is also true: The Higgs interacts very readily with heavy particles, only reluctantly with light particles, and it doesn’t interact directly at all with massless particles like photons and gluons. But through the magic of quantum field theory, it can interact indirectly. Gluons don’t interact directly with the Higgs, but they do interact with quarks, and quarks interact with the Higgs; so two gluons can collide to produce a Higgs by going through quarks as an intermediate step.

  Particle physicists have developed a very detailed and rigorously tested formalism for understanding how particles interact with one another. Richard Feynman, the colorful Nobel Prize–winning physicist, invented an extraordinarily helpful method for keeping track of these comings and goings: Feynman diagrams. These pictures are little cartoons of particles interacting and evolving over time into other particles. Force-carrying bosons are drawn as wavy lines, fermions are solid lines, and the Higgs is a dashed line. By starting with a fixed set of fundamental interactions, and mixing and matching the corresponding diagrams, we can figure out all the different ways particles can be produced or converted into other particles.

  For example, two gluons can come along, represented by wavy lines. These vibrations in the gluon field set up vibrations in the quark fields, which can be thought of as a quark-antiquark pair. Because it’s one quark and one antiquark in each case, the total charge and quark number is zero, matching that of the initial gluon. These quarks are virtual particles, playing a crucial intermediary role, but doomed to disappear before they ever show up in a particle detector. One matched quark-antiquark pair meets and they cancel each other out; the other meets and gives rise to a Higgs boson. Every kind of quark contributes to this process, but top quarks contribute the most, since (as the heaviest flavor of quark) they couple to the Higgs most strongly. All of this could be precisely described using a couple of lines of intimidating mathematical machinery; alternatively, it is elegantly captured in a single friendly diagram.

  Feynman diagrams provide a fun, evocative way of keeping track of what kinds of things can happen when particles come together to interact. Physicists, however, use them for the very down-to-earth task of calculating the quantum probability of the depicted interaction taking place. Every diagram corresponds to a number, which can be computed by following a series of straightforward rules. These rules can be confusing at first glance; for example, a particle going backward in time counts as an antiparticle, and vice versa. When two particles join to make a third (or one decays into two), the total energy and all other conserved quantities must balance. But the virtual particles—the ones that move around in the interior of the diagram but aren’t present in the initial collection or the final products—don’t have to have the same mass that a real particle would have. The right way to think about the diagram above is that two vibrations in the gluon field come together and set up a vibration in the quark field, which ultimately produces a vibration in the Higgs field. What we actually see are two gluon particles joining to create a Higgs boson.

  A Feynman diagram representing two gluons fusing together to create a Higgs boson, via the intermediate step of virtual quarks.

  The first person to realize that “gluon fusion” was a promising way to create Higgs bosons was Frank Wilczek, the American theorist who had helped pioneer our understanding of the strong interactions—work he did in 1973 as a graduate student, and for which he eventually shared the Nobel Prize. In 1977, he was on the faculty at Princeton, but he took time to visit Fermilab over the summer. Even the world’s great thinkers must take care of the mundane challenges of everyday life, and on this occasion Wilczek had spent a long day attending to his wife, Betsy Devine, and their infant daughter, Amity, both of whom were struggling with illness. After his wife and daughter fell asleep for the day, Wilczek took a walk around the Fermilab grounds to think about physics. Even at that time it was becoming clear that the basic outline of the Standard Model was “pretty much a done deal,” as he put it, but that the properties of the Higgs boson were relatively unexplored. His thesis work had given him a great fondness for gluons and their interactions, and while walking he realized that gluons were a great way of making Higgs bosons (and that Higgs bosons could in turn decay into gluons). Here we are thirty-five years later, and this process is the most important single way that the Higgs is produced at the LHC. On the same walk, Wilczek also came up with the idea for the “axion,” a hypothetical low-mass cousin of the Higgs that is now a promising candidate for constituting the dark matter in the universe. A testament to the importance of long, peaceful walks to the progress of physics.

  In Appendix Three, we discuss the various ways that particles can interact in the Standard Model, and the Feynman diagrams corresponding to each possibility. Not carefully enough to get anyone a PhD in physics, but hopefully enough to give you the general idea. One thing should be clear: It’s a bit of a mess. It’s easy to say, “We smash protons together and wait for a Higgs to come out,” but it’s a lot of work to sit down and do the calculations carefully. When all is said and done, a number of different processes contribute to creating Higgs bosons at the LHC: the fusion of two gluons as we just discussed; the analogous fusion of a W+ with a W-, or two Z bosons, or a quark and an antiquark; and the production of a W or Z that spits off a Higgs before going on its way. Details depend on the mass of the Higgs, as well as the energy of the original collisions. Calculating the relevant processes provides full employment for theoretical physicists.

  What comes out

  So you’ve made a Higgs boson! Congratulations. Now comes the tricky part: How are you ever going to know?

  Heavy particles tend to decay, and the Higgs is very heavy indeed. The lifetime of the Higgs is estimated to be somewhat less than a zeptosecond (10-21 seconds), which means it gets to travel less than a billionth of an inch between when it’s produced and when it decays. Even with the very advanced detectors inside ATLAS and CMS, there’s no way we’re seeing that. Instead, we see what the Higgs decays into. We will also see things that other non-Higgs particles decay into, many of which look just like what the Higgs decays into. The trick is to pick out the tiny signal from the huge amount of background noise.

  The first step is to figure out exactly what your Higgs is going to
decay into, and how often. In general, the Higgs likes to couple to heavy particles, so we might expect it to decay frequently to top and bottom quarks, W and Z bosons, and the tau lepton; not so much to lighter particles like up and down quarks or electrons. And that’s basically right, although there are subtleties (as you knew there would be).

  For one thing, the Higgs can’t decay into something that’s heavier than it is. It can temporarily convert into heavy virtual particles that themselves quickly decay away, but processes like that become very rare if the virtual particles are much heavier than the original Higgs. If the Higgs were 400 GeV, it could readily decay into a top quark and an antitop, which come in at 172 GeV each. But for a more realistic Higgs mass like 125 GeV, top quarks are unavailable, and bottom quarks are the favored decay mode. That’s one reason why heavier versions of the Higgs (up to 600 GeV) would have actually been easier to find, even though it takes more energy to create them—the rate of decay into heavy particles is much higher.

  The figure shows a pie chart giving the approximate ratio of different decay modes for a Higgs boson with a mass of 125 GeV, according to the Standard Model. The Higgs will decay into a bottom quark and an antibottom most of the time, but there are a number of other important possibilities. Although this value for the Higgs mass makes it hard to detect, once we do there’s a tremendous amount of interesting physics to be studied—we can measure each decay mode separately and compare it with the predictions. Any deviation would be a sign of physics beyond the Standard Model, such as additional particles or unusual interactions. We’ve even seen hints that such deviations might actually exist.