Higgs Discovery: The Power of Empty Space

Lisa Randall

• Publisher: Ecco
• Year: 2012
• ISBN:
  9780062245311
• Topic: Scienze fisiche
• Subtopic: Fisica atomica e molecolare

On July 4, 2012, physicists at the Large Hadron Collider in Geneva madehistory when they discovered an entirely new type of subatomic particle that many scientists believe is the Higgs boson. For forty years, physicists searched for this capstone to the Standard Model of particle physics—the theory that describes both the most elementary components that are known in matter and the forces through which they interact. This particle points to the Higgs field, which provides the key to understanding why elementary particles have mass. In Higgs Discovery, Lisa Randall explains the science behind this monumental discovery, its exhilarating implications, and the power of empty space.

HIGGS DISCOVERY

On July 4, 2012, along with many other people around the globe who were glued to their computers, I learned that a new particle had been discovered at the Large Hadron Collider (LHC) near Geneva. In what is now a well-publicized but nonetheless stunning turn of events, spokespeople from CMS and ATLAS, the two major LHC experiments, announced that a particle related to the Higgs mechanism, whereby elementary particles acquire their masses, had been found. I was flabbergasted. This was actually a discovery, not a mere hint or partial evidence. Enough data had been collected to meet the rigorous standards that particle physics experiments maintain for claiming a new particle’s existence. The accumulation and analysis of sufficient evidence was all the more impressive because the date of the announcement had been fixed in advance to coincide with a major international physics conference occurring in Australia that same week. And what was more exciting still was that the particle looks a lot like a particle called the Higgs boson.

A Higgs boson is not just a new particle, but a new type of particle. The thrill in this particular discovery was that it was not simply a confirmation of definite expectations. Unlike many particle discoveries in my physics lifetime, for which we pretty much knew in advance what had to exist, no physicist could guarantee that a Higgs boson would be found in the energy range that the experiments currently cover—or even found at all. Most thought something like a Higgs boson should be present in nature, but we didn’t know with certainty that its properties would permit experiments to find it this year. In fact, some physicists, Stephen Hawking among them, lost bets when it was found.

This discovery confirms that the Standard Model of particle physics is consistent. The Standard Model describes the most elementary components that are known in matter, such as quarks, leptons (like the electron), and the three nongravitational forces through which they interact—electromagnetism, the weak nuclear force, and the strong nuclear force. Most Standard Model particles have nonzero masses, which we know through many measurements. The Standard Model including those masses gives completely consistent predictions for all known particle phenomena at the level of precision of a fraction of a percent.

But the origin of those particle masses was not yet known. If particles had mass from the get-go, the theory would have been inconsistent and made nonsensical predictions such as probabilities of energetic particles interacting that were greater than one. Some new ingredient was required to allow for those masses. That new ingredient is the Higgs mechanism, and the particle that was found is very likely the Higgs boson that signals the mechanism’s existence and tells how it is implemented. With improved statistics, which is to say with more information after the experiments run longer, we will learn more about what underlies the Higgs mechanism and hence the Standard Model.

Though a discovery was indeed announced, it was in fact made with some of the caution I had come to expect from particle physics announcements. Because the measurements had identified barely enough Higgs boson events to claim a discovery, they certainly didn’t yet have enough data to measure all the newly discovered particle’s properties and interactions accurately enough to assure that it is a single Higgs boson with precisely the properties such a particle is expected to have. A deviation from expectations could turn out to be even more interesting than something in perfect accord with predictions. It would be conclusive evidence for a new underlying physical theory beyond the simple model that implements the Higgs mechanism that current searches are based on. This is the sort of thing that keeps theorists like me on our toes as we try to find matter’s underlying elements and their interactions. Precise measurements are ultimately what tell us how to move forward in our hypotheses. The Higgs boson is a very special particle indeed and we ultimately want to know as much as we can about it.

Whatever has been found—the Higgs boson, the particular implementation of the Higgs mechanism that seems simplest or something more elaborate—it is almost certainly something very new. The interest from the public and press has been very gratifying, indicating a thirst for knowledge and scientific advances that humanity to a large extent shares. After all, this discovery is part of the story of the universe’s evolution as its initial symmetry was broken, particles acquired masses, atoms were formed, structure, and then us. News stories featured members of the public who were fascinated but weren’t necessarily quite sure by what. Perhaps the ultimate recognition of the pervasiveness of Higgs boson awareness was the appearance of jokes and spoof news stories indicating the interest—but also some of the bewilderment.

So I’m writing this to respond to many of the questions I’ve been asked—to share what the discovery means and to explain a bit about where it takes us. Some of what I’ll say is already in chapters from my previous books, Warped Passages and Knocking on Heaven’s Door, two of which are appended. Those books didn’t isolate the Higgs boson for extra special attention; rather they covered many topics, including information about the collider, the larger physics story for which this is the capstone, and the nature of science itself. They give the larger context of which this discovery is one—albeit a very important—part. But at least for the time being, the Higgs boson deserves its moment in the limelight. So in addition to those older chapters about the Higgs particle, this book offers a few new (and old) thoughts. It’s an unbelievably exciting moment in physics and I’d like to share some of what occurred and what it means.

THE CHALLENGE OF DISCOVERY

I guess I was better off on July 4, 2012, than during the last Higgs report in December 2011. On the earlier date, I woke up before five in the morning to do an interview and to listen to talks from CERN, as I was in California and the time zone was not very congenial. At the time of the recent announcement, on the other hand, I found myself on a Greek island where I was taking an all too rare vacation. Although I had poor Internet connectivity and was isolated from my colleagues, at least I was only one time zone away when Joe Incandela, the spokesperson for CMS, first took the stage. Because my somewhat rustic apartment had no Internet, I first learned of the Higgs discovery while sitting in a balcony café—which happily for me opened at 10 A.M., the time of the talks.

In fact, I hadn’t imagined when making my holiday plan that this would happen. I had known the Higgs evidence would increase, but I hadn’t known that the engineers would have done such a heroic job in increasing the collision rate, and the experimenters an equally impressive improvement to their analysis methods, that would allow the speakers on July 4 to say with certainty (by physicists’ standards) that a particle had been found. One other factor that contributed to the Higgs boson discovery was the decision to run at slightly higher energy—8 TeV rather than the 7 TeV of the previous year—which by itself increased Higgs production by about 30 percent. I was very grateful to the Internet for keeping everyone connected and to Twitter for providing an outlet for my excitement (and for sharing information once people caught on to what was happening and the connection diminished in quality).

Maybe to compensate for that disconnect, I spoke a few days later on a radio program on WNYC. In the pre-show discussion that one typically has before such a program, we reviewed the types of topics that might arise. Most were ones I was prepared for. But I was a little flustered when I was told I would be asked to compete with Dennis Overbye’s delightful description of the Higgs boson discovery: “Like Omar Sharif materializing out of the shimmering desert as a man on a camel in ‘Lawrence of Arabia,’ the elusive boson has been coming slowly into view since last winter . . .” (New York Times, July 4, 2012).

I certainly wasn’t going to think of something as magical in the half hour I had before the interview—especially as I love that movie. To further complicate the situation, I was on a rock-climbing cliff in Kalymnos, where I had to belay my partner up a climb that we had already set up (I didn’t tell the producers that, since they would rightfully have worried about the connection).

So in anticipating my interview, I thought about the question. My partner suggested I say it was the Messiah whom physicists had been awaiting for fifty years, which I thought rather funny but not really helpful. I wanted to create an analogy that reproduced the physics better than as an actor or a deity representative.

What I came up with is not perfect but captures the lead-up to the actual discovery. I said it happened in the way you might find your friend in a crowded stadium full of shouting individuals, where everyone—including your friend—is making noise, but your friend, despite his distinctive voice, is a small peep in the noisy crowd. You would be hard-pressed at first to find him amid the huge din in the background. You might occasionally think you heard his voice, but then it would be drowned out or difficult to distinguish from that of others in the hordes of people.

But imagine now that you knew roughly where to look. You knew what section your friend was in and who he would be hanging out with. So you focus your attention in a particular region. When you hear the sound of his voice, you then begin to be increasingly confident that you have located him correctly. You might not know for sure right away but you might then begin to focus on an even more specific region of the stadium. Eventually you reach the right location where the voice is unmistakable and you know your friend has been found.

The Higgs discovery really did work like that. Higgs events are rare among the far more numerous ordinary particles that are produced. A Higgs boson is connected to elementary particle masses. That means that physicists can predict how it should interact (assuming of course that it exists), so you know its “voice.” Alas, the interactions with the ingredients of a proton are in fact rather weak. Quarks and gluons experience the strong nuclear force, which is much more powerful than their interactions with a Higgs boson, so Higgs bosons will be produced only a small fraction of the time and get lost in the “crowd.”

Out of the billions of particle collisions that occur every second, only rarely does a Higgs boson get produced. More often than not, collisions result in boring Standard Model collisions that we know to exist. Those collisions give us more detailed information about quarks and the strong nuclear force. But they muddy the waters for experimenters looking for a clear Higgs boson signal.

The only way to find the particle is to have a pretty good idea where to look so that experimenters can distinguish signal from the “din” of background. Where doesn’t refer to a physical location like your friend’s section in a stadium. Instead it refers to where in the data you expect Higgs evidence to lie—meaning which types of collider events are expected if a Higgs boson exists.

So as with your friend in the stadium, the Higgs boson was initially lost in the background data. Experimenters subsequently looked through trillions of events so that they could begin to see evidence for a bit of a deviation that could signal something special. This was something like the December announcement, where evidence for a particle was presented (at only 3 sigma level, 3 standard deviations, which is a statistical term meaning the probability is less than one in a few hundred that the result isn’t a signal). For those interested in the precise criteria, the level 3 sigma is thought to be too small for discovery, mostly based on experience. Sometimes experimenters don’t yet understand all the details of their system or prediction, fluctuations do happen, and everyone knows that if they wait, the answer will sort itself out. That is why experiments require a signal more than 5 sigma, which means that the odds are less than one in 1.7 million that it is simply background noise arising from known familiar particles.

But as time went by and more data accumulated, physicists zoned in on the region that looked a little different and sorted through at least twice as much data. With enough data and enough understanding of the properties expected for a Higgs boson, a clear signal emerged. That signal was what was revealed on July 4 and is almost certainly connected to the Higgs mechanism by which particles acquire their masses.

THE HIGGS MECHANISM, THE HIGGS FIELD, AND THE HIGGS BOSON

In order to fully appreciate this discovery and what there remains to explore, it helps to know a bit of particle physics, including some deep and subtle underlying concepts. Of critical importance is the distinction between the Higgs mechanism, the Higgs field that is involved in the mechanism, and the Higgs boson particle—which is what an experiment can actually find. Even without experimental proof, physicists were fairly confident about the mechanism, since it was the only consistent way to give elementary particles their masses, as the attached chapters explain in further detail than I do here.

But despite the theoretical consistency of the idea and the failure of any other idea to explain masses, physicists all wanted experimental proof. The experimental results from the LHC have now rather firmly established the relevance of the Higgs mechanism and the Higgs field on which it relies. They have also established the existence of a new particle related to the mechanism—but the Higgs boson is part of a very particular implementation, which only further data will definitively confirm or rule out. That is why my title, Higgs Discovery, is deliberately ambiguous.

The Higgs mechanism is responsible for elementary particle masses, such as the mass of the electron. Mass is what provides resistance when a force is applied. If particles have no mass, they travel at the speed of light. A particle’s mass tells us how it responds to forces and how it travels through space.*

Without its mass, the electron wouldn’t bind into an atom, and just about everything else you take for granted about the world wouldn’t work either. Such an elementary particle mass, that of the electron in this case, relies on the existence of what particle physicists call a field—a quantity that exists throughout space but doesn’t necessarily involve any actual particles. Admittedly, the concept of a field is a bit esoteric and confusing, especially as the word field outside of physics conjures images of cows grazing, which became clearer to me when I read the word champs in French physics textbooks.

But really we enc...