Authors: Sean Carroll
The Particle at the End of the Universe: How the Hunt for the Higgs Boson Leads Us to the Edge of a New World (23 page)
electronelectron neutrino
muon
tau
The particles that we’ve joined up in pairs here seem very different to us at first glance; they have different masses and charges. That’s because the Higgs field lurking in the background breaks the symmetry between them. If it weren’t for the masquerade put on by the Higgs, the particles in each pair would be completely indistinguishable, just like we think of red/green/blue quarks as three different versions of the same thing.
The Higgs field itself rotates under the symmetry of the weak interactions; that’s why, when it gets a nonzero value in empty space, it picks out a direction and breaks the symmetry, just like the air picks out a velocity that we can measure things with respect to when we’re traveling in our train. Back in our pendulum example, the lowest-energy state of the regular pendulum was perfectly symmetric, pointing straight down. The upside-down pendulum, like the Higgs field, breaks the symmetry by falling either left or right.
If you were hopelessly lost in a forest in the middle of the night, all directions would seem the same to you. You could rotate how you were standing, and your situation would be just as dire. But if you had a compass, and you knew you wanted to walk north, the direction picked out by that compass would break the symmetry; now there’s a right direction to walk, and there are wrong directions. Likewise, with no Higgs field the electron and the electron neutrino (say) would be identical particles. You could rotate them into each other, and the resulting combinations would remain indistinguishable. The Higgs field, like the compass, picks out a direction. There is now one particular combination of fields that interacts most strongly with the Higgs field, which we call the “electron,” and one that doesn’t, which we call the “electron neutrino.” It’s only with respect to the Higgs field filling space that such a distinction makes sense.
If it wasn’t for the symmetry breaking, there would actually be
four
Higgs bosons, rather than just one; two pairs of particles that transform into each other via the weak interaction symmetry. But when the Higgs field fills space, three of those particles get “eaten” by the three gauge bosons of the weak interactions, which thereby go from being massless force-carriers to being the massive W and Z bosons. Yes, physicists really do talk that way: The weak-interaction bosons gain mass by consuming the extra Higgs bosons. You are what you eat.
Back to the Bang
The analogy between the Higgs field and the upside-down pendulum is actually a pretty good one. Like the Higgs, the underlying laws of physics for the pendulum are perfectly symmetric; they don’t favor either left or right. But there are only two stable configurations for the pendulum to be in: pointing left or pointing right. If we tried to balance it carefully so that it was pointing in a symmetric configuration pointing directly upward, any tiny bump would send it falling left or right.
The Higgs field is the same way. It
could
be set to zero in empty space, but that’s an unstable configuration. For the pendulum, if it’s lying peacefully to the left or right, we would have to exert some energy to lift it so that it pointed directly upward. The same is true for the Higgs field. To move it from its nonzero value at every point in space back to zero would require a superhuman amount of energy—much more than the total energy in the observable universe today.
But the universe used to be a much denser place, with a lot more energy packed into a much smaller volume. At times near the Big Bang, 13.7 billion years ago, matter and radiation were squeezed much closer together, and the temperature was enormously higher. In terms of the pendulum analogy, think of that upside-down pendulum sitting on a table rather than being bolted to the floor. “High temperature” means a lot of random motions of particles; in terms of the analogy, it’s like someone takes a hold of the table and starts shaking it. If the shaking is sufficiently energetic, we might imagine that the pendulum is pushed so hard that it flips over from left to right (or vice versa). If the shaking is
really
energetic, the pendulum will vibrate like crazy, flipping quickly back and forth. On average, it will spend as much time on the left as on the right. In other words, at high temperatures, the upside-down pendulum becomes symmetric again.
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.
A
fter 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.