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

  But absent such a construction (and it’s not as if those suggestions are very good), it’s hard to do justice to the history by choosing a naming convention. Higgs himself refers to “the boson that has been named after me,” and sometimes talks about the “ABEGHHK’tH mechanism”—that’s Anderson, Brout, Englert, Guralnik, Hagen, Higgs, Kibble, and ’t Hooft, for those of you scoring at home. Joe Lykken at Fermilab switched out ’t Hooft in favor of Nambu to come up with “HEHKBANG,” which is at least a pronounceable acronym, but no more attractive. “That would be foolish,” as he himself admits.

  Ultimately one has to admit that the name of a particle is just a label. It’s not supposed to be, and shouldn’t be taken as, a comprehensive and fair history of the development of an idea. We can call it the “Higgs boson” without pretending that Higgs is the only one who deserves credit. (Given the funding pressures in modern particle physics, I suspect that the naming rights would be happily sold for about $10 billion. “The McDonald’s boson,” anyone?)

  The verdict of history

  As we have recounted the story, Nambu and Goldstone helped establish our understanding of spontaneous symmetry breaking, but they concentrated on the case of global symmetries. Anderson pointed out that gauge symmetries are different, and in particular that they didn’t leave any remnant massless particles, but he didn’t construct an explicitly relativistic model. That was done independently by Englert and Brout, by Higgs, and by Guralnik, Hagen, and Kibble. All three took slightly different routes but achieved essentially the close up same answers, and all three deserve a hefty measure of credit. As does ’t Hooft, who showed that the idea made mathematical sense.

  By tradition, the Nobel Prize in sciences is given to individuals rather than groups, and no more than three individuals in any one year. There’s no question that the candidates are jockeying for position, at least discreetly. ’t Hooft and Veltman have already won a Nobel for their work on renormalizing electroweak theory. Anderson won a Nobel for something completely different, but realistically that does hurt his chances for a second prize (even if he does have a good case for being there first). Robert Brout passed away in 2011, and Nobels are not given posthumously.

  In 2004, the Wolf Prize in Physics—sometimes described as the second-most prestigious award after the Nobel—was given to Englert, Brout, and Higgs, but not to Guralnik, Hagen, and Kibble. At a 2010 “Higgs Hunting” meeting in France, the advertising poster made direct mention of “Brout, Englert, and Higgs,” leaving out GHK entirely. This caused a certain amount of push-back, with supporters of the Anglo-American team threatening to boycott the conference. Organizer Gregorio Bernardi was taken aback by the criticism, saying, “People took this very seriously, which we didn’t expect.” That seems at least somewhat disingenuous; if you care enough about assigning credit that you attach the names of Englert and Brout to a boson that has universally been known as the “Higgs,” you can’t be surprised when Guralnik, Hagen, and Kibble (or their partisans) are upset. Part of the sting was taken away when the American Physical Society awarded its 2010 Sakurai Prize in theoretical physics to Hagen, Englert, Guralnik, Higgs, Brout, and Kibble—in that order, which seems to have been chosen specifically to make it impossible for anyone to complain. (Anderson might have reasonably complained.)

  As Anderson ruefully notes, “If you want the history right in detail, you better write it yourself.” Over the past several years Guralnik, Higgs, Kibble, and Brout and Englert have all written reminiscences of their work in 1964, attempting to put their own contributions in perspective. And, this being the modern age, a controversy flared up on Wikipedia, the online encyclopedia that can be edited by anyone. In August 2009, a user known only as “Mary at CERN” put up a new entry entitled “1964 PRL Symmetry Breaking Papers.” There were already separate entries on “Spontaneous Symmetry Breaking,” “Higgs Mechanism,” and so forth; this new article aimed squarely at the question of how credit should be attributed. While discussing all the papers, it was clear who the new entry was meant to support: “A case can be made that, while first to publish by a couple months, Higgs and Brout-Englert solved half of the problem—massifying the gauge particle. Guralnik-Hagen-Kibble, while published a couple months later, had a more complete solution—massifying the gauge particle and also showing how the numbing influence from Goldstone’s theorem is avoided.” But what one person can write on Wikipedia, another can edit; the current revision is a bit more even-handed.

  I have no particular preference concerning who, if anyone, should win the Nobel Prize for inventing the idea of the Higgs boson, nor do I have a prediction. The prizes are good for science, as they help draw attention to interesting work that might not otherwise be publicized. But they’re not what science is about; the reward for helping to discover the mechanism in the first place is enormously larger than any prize the Nobel committee can bestow.

  The real disappointment is that it seems difficult to imagine any experimentalist claiming a Nobel for actually discovering the boson. It’s a simple problem of numbers: Too many people contributed to the experiments in too many ways for any one or two or three to be picked out as responsible. One achievement that is unquestionably Nobel-worthy is the successful construction of the LHC itself, so Lyn Evans would be a sensible candidate. It’s probably past time for the Nobel foundation to think about relaxing the tradition that collaborations cannot win any of the prizes in science. Whoever gets that rule change implemented might deserve the Nobel Peace Prize.

  TWELVE

  BEYOND THIS HORIZON

  In which we consider what lies beyond the Higgs boson: worlds of new forces, symmetries, and dimensions?

  From the age of ten, Vera Rubin was fascinated by the stars. Her interest never waned, and when she applied to college it was natural that she would seek to study astronomy. But this was in the 1940s, and women were not exactly welcome in science. At one point she spoke to a Swarthmore College admissions officer, who asked whether she had any other interests. She admitted that she enjoyed painting. The admissions officer seized on that, asking, “Have you ever considered a career in which you paint pictures of astronomical objects?” She ended up attending Vassar College instead, but the question made an impression. She later recalled, “That became a tag line in my family: for many years, whenever anything went wrong for anyone, we said, ‘Have you ever considered a career in which you paint pictures of astronomical objects?’”

  Rubin persevered, proceeding to graduate studies at Cornell and Georgetown University. The road wasn’t easy; when she wrote to Princeton asking for a graduate school catalogue, they refused to send her one, noting that the astronomy department didn’t accept female graduate students. (That policy eventually ended in 1975.)

  One secret to success as a scientist is to look where others don’t. As larger telescopes were becoming available, many astronomers turned their gaze to the centers of distant galaxies, in regions rich with stars and activity. Rubin chose to concentrate on their outer fringes, studying the dynamics of the thinly spread stars and gas orbiting slowly on the edges. This technique provides a way to measure the total mass of a galaxy: The more matter inside, the higher the gravitational field on the outer stars will be, and the faster they will have to orbit.

  Rubin and her collaborator Kent Ford found something astonishing. We expect that stars should move more and more slowly as we move away from the center of the galaxy, just as more distant planets in the solar system orbit more slowly around the sun. The gravitational field is lower, so there is less force to resist, requiring less velocity to maintain an orbit. But Rubin and Ford found something very different: Stars move at equal speeds as we examine larger and larger distances from the dense central region of a galaxy. The implication is straightforward, although hard to accept: There is much more matter in a galaxy than we observe, and much of it is distributed far from the center, unlike the visible stars.

  What Rubin and Ford had stumbled upon was a surprising phenomenon
that today sits at the center of modern cosmology: dark matter.

  They weren’t the first; as far back as the 1930s, Swiss-American astronomer Fritz Zwicky had demonstrated that there was much more matter in the Coma cluster of galaxies than we can observe in our telescopes, and Dutch astronomer Jan Oort showed that our local galactic neighborhood had more matter than was immediately evident. For a long time, however, there was hope that the matter was simply “missing”; it was just ordinary stuff, but in a form that wasn’t easy to see. As we learned more and more about galaxies and clusters and the universe as a whole, we were able to precisely measure two numbers separately: the total amount of matter in the universe and the total amount of “ordinary matter,” where ordinary matter includes atoms, dust, stars, planets, and every kind of particle known in the Standard Model.

  The two numbers don’t match. The total amount of ordinary matter in the universe is only about one-fifth of the total amount of matter. The vast majority is dark matter, and the dark matter can’t be any of the particles in the Standard Model.

  The Higgs boson is the final piece of the Standard Model puzzle, but the Standard Model is certainly not the end of the road. Dark matter is just one indication that there is a lot more physics out there remaining to be understood. One exciting prospect is that the Higgs can serve as a bridge between what we know and what we hope to learn. By studying carefully the properties of the Higgs, we hope to shed light on the dark worlds beyond our own.

  The early universe

  Let’s think about dark matter a bit more carefully, as it provides some of the strongest evidence we have for physics beyond the Standard Model, and a great example of how the Higgs could be involved in understanding this new physics better. A crucial feature of dark matter is that it can’t be ordinary matter (atoms and so forth) in some “dark” form, like brown dwarfs or planets or interstellar dust. That’s because we have very good measurements of the total amount of ordinary matter, from processes in the early universe.

  To understand dark matter, we need to think about where it came from. Imagine you have an experimental apparatus that is basically a super-oven: a sealed box with some stuff inside, and a dial you can adjust to make the temperature as high or low as you like. An ordinary oven might go as high as 500 degrees Fahrenheit, which in particle physics units is about 0.04 electron volts. At that temperature, molecules can rearrange themselves (which we call “cooking”), but atoms maintain their integrity. Once we get up to a few electron volts or higher, electrons are stripped away from their nuclei. When we hit millions of electron volts (MeV), the nuclei themselves are stripped apart, leaving us with free protons and neutrons.

  Another thing also happens at high temperatures: The collisions between particles are so energetic that you can create new particle-antiparticle pairs, just like in a particle collider. If the temperature is higher than the total mass of a particle plus its antiparticle, we expect such pairs to be copiously produced. So at sufficiently high temperatures, it almost doesn’t matter what was in the box to begin with; what we get is a hot plasma filled with all the particles that have masses lower than the temperature inside. (Remember that mass and temperature can both be measured in GeV.) If the temperature is 500 GeV, our box will be buzzing with Higgs bosons, all the quarks and leptons, W and Z bosons, and so forth—not to mention possible new particles that haven’t yet been discovered here on earth. If we were to gradually lower the temperature inside of that box, these new particles would gradually disappear as they bumped into their antiparticles and annihilated, leaving us with only the particles we started with.

  The early universe is much like the plasma inside our ultrahot oven, with one crucial extra ingredient: Space is expanding at an incredible rate. The expansion of space has two important effects. First, the temperature cools off, so it’s as if the temperature dial on our oven starts very high but is quickly turned down. Second, the density of matter decreases rapidly as particles move away from one another in the expanding space. That latter feature is a crucial difference between the early universe and an oven. Because the density is decreasing, particles that were produced in the original plasma might not have a chance to annihilate away; it might simply be too hard for one of them to find a corresponding antiparticle.

  As a result, we get a relic abundance of these particles from the primordial plasma. And we can calculate precisely what that abundance should be, if we know the masses of the particles and the rate at which they interact. If the particles are unstable, like the Higgs boson is, the relic abundance is pretty irrelevant, as the particles just decay away. But if they’re stable, we’re stuck with them. It’s easy to imagine that a leftover stable particle from the early universe constitutes the dark matter today.

  In the Standard Model, we can play this game with the atomic nuclei. One crucial difference is that we start with more matter than antimatter, so the matter can never completely annihilate away. Start at a fairly high temperature, say around 1 GeV. The plasma will consist of protons, neutrons, electrons, photons, and neutrinos; all the heavier particles will have decayed. That temperature is sufficiently hot that protons and neutrons cannot form nuclei without being instantly ripped apart. But as the universe expands and cools, nuclei begin to form a few seconds after the Big Bang. Just a couple of minutes later, the density is so low that nuclei stop running into one another, and those reactions cease. We are left with a certain combination of protons and light elements: deuterium (heavy hydrogen, one proton and one neutron), helium, and lithium. This process is known as “Big Bang nucleosynthesis.”

  We can make precise calculations of the relative abundance of those elements, with just one input parameter: the initial abundance of protons and neutrons. And then we can compare the primordial element abundances with what we see in the real universe. The answer matches precisely, but only for one specific density of protons and neutrons. That happy result is reassuring, since it indicates that the way we think about the early universe is basically on the right track. Since protons and neutrons make up the overwhelming fraction of mass in ordinary matter, we know quite well how much ordinary matter there is in the universe, no matter what form it might take today. And it’s not nearly enough to make up all the matter there is.

  WIMPs

  One promising strategy for dark matter is to play that same game as we did with nucleosynthesis, but starting at a much higher temperature and adding a new particle into the mix—a particle that will be the dark matter. We know that dark matter is dark, so the new particle should be electrically neutral. (Charged particles are precisely those that interact with electromagnetism, and therefore tend to give off light.) And we know it’s still around, so it should be stable, or at least have a lifetime longer than the age of the universe. We even know something a bit more detailed: Dark matter doesn’t interact very strongly with itself. If it did, it would settle into the middle of galaxies, rather than forming big puffy halos as the data seem to indicate. So the dark matter doesn’t feel the strong nuclear force, either. Of the known forces of nature, the dark matter certainly feels gravity, and it may or may not feel the weak nuclear force.

  Let’s imagine a particular kind of new particle: a “Weakly Interacting Massive Particle,” or WIMP. (Cosmologists are nothing if not cheeky when it comes to inventing new names.) By “weakly interacting” we don’t just mean “doesn’t interact very much”; we mean that it feels the weak interactions of particle physics. For simplicity, we assume the WIMP has a mass compatible with other particles involved in the weak interactions, like the W and Z bosons or the Higgs. Around 100 GeV, let’s say, or at least between 10 and 1,000 GeV. Other details of the way such a particle interacts are relevant for high-precision calculations, but just these basic properties are enough to perform back-of-the-envelope estimates.

  Then we compare the predicted abundance of such a WIMP with the actual abundance of dark matter. What we find—amazingly—is that they match beautifully. There’s some wiggle room, ha
ving to do with what other particles might exist and how exactly the WIMPs annihilate, but the rough agreement is striking. Stable particles with weak-scale interactions generally have the right relic abundance to account for the dark matter, without even trying too hard.

  This interesting coincidence is known as the “WIMP miracle” and has given many particle physicists hope that the secret to dark matter lies in new particles with similar masses and interactions to the W/Z/Higgs bosons. All those particles decay quickly, of course, so the WIMP must have some good reason to be stable, but that’s not hard to invent. There are many other plausible theories of dark matter—including a particle called the “axion,” invented by Steven Weinberg and Frank Wilczek, which is like a very lightweight cousin of the Higgs—but WIMP models are by far the most popular.

  The possibility that the dark matter is a WIMP opens up some very exciting experimental possibilities, precisely because the Higgs will interact with it. Indeed, in many (arguably most, but it’s hard to count) viable models of WIMP dark matter, the strongest coupling between the dark matter and ordinary matter will be through exchanging a Higgs boson. The Higgs could be the link between our world and most of the matter in the universe.

  The Higgs portal

  This feature—interacting via Higgs exchange—turns out to be common in many theories of physics beyond the Standard Model. You have a whole bunch of new particles in what’s known as a “hidden sector,” and they don’t interact very noticeably with the particles we’ve already studied. The Higgs is a little more sociable than the known fermions and gauge bosons, which means that it’s more likely to interact with the new particles. That’s the sense in which our discovery of the Higgs is both the completion of one grand project—constructing the Standard Model—but also the beginning of the next—finding hidden worlds beyond that model. Wilczek and his collaborator Brian Patt have dubbed this possibility the “Higgs portal” between the Standard Model and hidden sectors of matter.