The Particle at the End of the Universe: How the Hunt for the Higgs Boson Leads Us to the Edge of a New World (34 page)

The separate papers by Weinberg and Salam had all the impact, as Kurt Vonnegut once said in a different context, of a pancake twelve feet in diameter dropped from a height of two inches. In academia, and science in particular, the most concrete way of quantifying the influence of a piece of research is to count how many times the paper has been cited by other papers. Between 1967 and 1971, Weinberg’s paper was cited just a handful of times. The two authors did not even pursue their own ideas to any great extent in the years immediately thereafter. Since 1971, however, Weinberg’s paper has been cited more than 7,500 times—an average of more than once every two days for four decades.

What happened in 1971? Some surprising experimental result? No, a surprising theoretical result: Gerard ’t Hooft, a young graduate student in the Netherlands, working under Martinus “Tini” Veltman, proved that theories with spontaneously broken gauge symmetries are renormalizable, even though the gauge bosons are massive. In other words, ’t Hooft showed that the electroweak theory made mathematical sense. This had been conjectured by both Weinberg and Salam, but many people in the field had remained skeptical, which partly accounts for the obscurity of these ideas up to that point. In Sidney Coleman’s words, ’t Hooft “revealed Weinberg and Salam’s frog to be an enchanted prince.” Gerard ’t Hooft has since gone on to earn a reputation as one of the most creative and brilliant minds in physics. He and Veltman shared the Nobel Prize in 1999 for their work on the electroweak theory and spontaneous symmetry breaking.

The surprising experimental results weren’t long in coming, however. The main novel prediction of the Glashow, Salam-Ward, and Weinberg models was the existence of a heavy neutral boson, the Z. The effects of the W bosons were well-known: They change the identity of a fermion when they are emitted (for example, changing a down quark to an up during neutron decay). If the Z existed, it would imply a version of the weak interactions in which particles kept their identities; for example a neutrino could scatter off an atomic nucleus. Events of precisely this kind were observed at CERN’s Gargamelle detector in 1973, setting the stage for Glashow, Salam, and Weinberg to share the Nobel Prize in 1979. (Ward was left out, but only three people can share the prize in any one year.) The W and Z bosons themselves, as opposed to their indirect effects, weren’t discovered until Carlo Rubbia found them a few years afterward.

All that remained was to discover the Higgs boson.

The name game

Physicists are human beings. They are typically motivated by what Richard Feynman called “the pleasure of finding things out,” but once they find out something interesting they appreciate getting credit for their work. Throughout this book, following nearly universal practice within the physics community, I’ve been referring to the “Higgs mechanism” for given mass to gauge bosons via spontaneous symmetry breaking, as well as the “Higgs boson” for the scalar particle that this model predicts. It’s clear, however, that while Higgs’s contributions were important, he was hardly alone. Why is that the name, and what should be the name?

Nobody is precisely sure where the name “Higgs boson” originally came from; it certainly wasn’t from Higgs himself. Particle physics lore points the finger at Benjamin Lee, a talented Korean-American physicist who died in a tragic car accident in 1977. Lee had learned about spontaneous breakdown of gauge symmetries from talking with Higgs, and the story goes that he gave an influential talk at a conference at Fermilab in 1972, where he repeatedly referred to the “Higgs meson.” That was in the immediate aftermath of ’t Hooft’s revolutionary result, when everyone was scrambling to learn about these ideas. Precisely because physicists are human beings, they tend to lazily stick with the first words they hear attached to the subject, so a widely heard talk can spread a piece of nomenclature far and wide.

Another theory goes back to Weinberg’s 1967 paper. When the original papers came out in 1964, not too many physicists were thinking about spontaneous symmetry breaking in gauge theories; after ’t Hooft’s breakthrough in 1971, many rushed to catch up, and Weinberg’s paper was a good starting point. In his discussion of the Higgs mechanism, he references three papers by Higgs, as well as the paper by Englert and Brout and the one by Hagen, Guralnik, and Kibble. However, Higgs comes first in his reference list, due to a mix-up between
Physical Review Letters
(where Higgs’s second paper appeared) and
Physics Letters
(where the paper by Englert and Brout appeared). From such minor lapses are long-lasting consequences forged.

Perhaps most important, “Higgs boson” sounds like a good name for a particle. It was Higgs’s papers that first drew close attention to the boson particle rather than the “mechanism” from which it arose, but that’s not quite enough to explain the naming convention. We might ask, however, what is the alternative? There may have been a chance, in the early days, to come up with a label that wasn’t derived from the name of a person. The “radial boson,” perhaps, or the “relicon,” since the boson is the only surviving relic of the symmetry-breaking process. The “electroweak boson” would work, although it runs the danger of being confused with the W and Z bosons, so the “electroweak scalar boson” might be most accurate.

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?

F
rom 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.