The Particle at the End of the Universe: How the Hunt for the Higgs Boson Leads Us to the Edge of a New World (24 page)

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:

1. Make Higgs bosons.
   
2. Detect the particles that they decay into.
   
3. Convince yourself 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 = mc
²
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.

Probability of a Higgs boson with mass 125 GeV decaying into different particles. Numbers don’t add up to exactly 100 percent due to rounding.

We’re nowhere near done yet, however. Hearken back to our discussion of particle detectors in Chapter Six, where we saw how different layers of the experimental onion helped us pinpoint different outgoing particles: electrons, photons, muons, and hadrons. Then look back at this pie chart. More than 99 percent of the time, the Higgs decays into something that we don’t observe directly in our detector. Rather, the Higgs decays into something that then decays (or transforms) into something else, and
that
is what we end up detecting. This makes life harder, or more interesting, depending on your perspective.