The Particle at the End of the Universe: How the Hunt for the Higgs Boson Leads Us to the Edge of a New World (14 page)

The answer: very carefully. There are tricks, of course, honed over the course of an expert paleontologist’s career; subtle gradations of color and texture that elude the notice of the uninitiated. Bring a group of amateurs to a dinosaur fossil site, and far and away the most common question you will hear is “Is this a bone?” But there is a right answer, and the experts can (almost) always provide it.

While the experience of digging up dinosaurs is worlds away from the everyday life of a theoretical physicist, the similarities with
experimental
particle physics are evident. We speak informally of “seeing a Higgs boson” at the Large Hadron Collider, but the reality isn’t that simple. We never see Higgs bosons, nor do we ever expect to, any more than we expect to see dinosaurs walking down the street. The Higgs is very short-lived—one will survive for one ten-billionth of a trillionth of a second, far too short to be captured directly, even by the technological marvels that are the LHC experiments. (A bottom quark, with a lifetime of one-trillionth of a second, is just barely at the verge of being discernible; the Higgs lifetime is one ten-billionth of that.)

What we expect to find is
evidence
for the Higgs boson, in the form of other particles that are created when it decays. Fossils, if you will.

The last chapter talked about the LHC accelerator itself, which zips hundreds of billions of protons on circular paths around a tunnel underneath the suburbs of Geneva. In this chapter we address the experiments—the massive detectors located at particular installations around the ring, where protons are brought into collision in a rapid-fire series of interactions. In the data from some individual event, we might possibly find two sprays of strongly interacting particles, as well as a high-energy muon-antimuon pair. So did all that come from the decay of a Higgs, or from something else? The task of identifying these fossils correctly is a combination of science, technology, and black magic that lies at the heart of the hunt for the Higgs.

Identifying particles

Particle physics is a detective story. Arriving at the scene of a crime, most detectives aren’t lucky enough to be greeted by clear videotape footage of the perpetrator committing the act or unimpeachable eyewitness testimony or a signed confession. More likely, there are a few haphazard clues—partial fingerprint here, tiny DNA sample there. The tricky part of the job is piecing together those clues to put together a unique story of the crime.

Likewise, when experimental particle physicists are analyzing the results from a collider, they don’t expect to see a little sign attached to a particle saying, “I’m the Higgs boson!” The Higgs will decay quickly into other particles, so we have to have a good idea of what we expect those particles to be—that’s a job for the theorists. Then we collide protons together and watch what comes out. Most of the volume inside a particle detector is filled with material in which particles leave telltale tracks as they pass through, the particle-physics analogue of someone’s muddy footprints at the crime scene. Of course, not all footprints are muddy: Particles like neutrinos, which don’t interact through either electromagnetism or the strong force, don’t leave much of a trail at all, and we have to be more clever.

Sadly, the tracks we do see don’t come with labels reading, “I’m a muon, and I’m moving at 0.958 the speed of light!” either. We have to deduce what particles emerged from the collision, and what that means for the processes that made it happen. We need to know whether this muon was produced by the decay of the Higgs, the decay of a Z boson, or a number of other suspects. And the particles themselves aren’t going to confess.

The good news is that the total number of particles in the Standard Model is relatively manageable, so we don’t have too many suspects to consider. We’re more like the sheriff of Mayberry than a detective in Manhattan. We have six quarks, six leptons, and a handful of bosons: photons, gluons, Ws, Zs, and the Higgs itself. (Gravitons are essentially never produced, just because gravity is so weak.) If we’re able to determine the mass, charge, and whether it feels the strong interaction, we can basically identify it uniquely. So that’s the task of the experimentalist: Keep track as precisely as possible of the particles emerging from a collision, and determine their masses, charges, and interactions. That lets us reproduce the underlying process that caused all the excitement.

It’s pretty easy to judge whether a particle feels the strong interactions, for the happy reason that those interactions are really strong. Quarks and gluons leave completely different signatures in a detector from the ones leptons and photons do. They are quickly confined into different kinds of hadrons—either combinations of three quarks, known as “baryons,” or pairs of one quark and one antiquark, known as “mesons.” These hadrons readily bump into atomic nuclei, making them easy to pick out. In fact, when you produce a single high-energy quark or gluon, the strong interactions usually cause it to fragment into a whole spray of hadrons, known as a “jet.” That makes it very easy to see that you’ve made a quark or gluon, but a little trickier to measure its precise properties.

Likewise, it’s pretty easy to figure out the electric charge on a particle, thanks to the magic of magnetic fields. Just as the LHC tunnel is filled with powerful magnets that nudge protons around the circular beam pipe, the LHC detectors are suffused with magnetic fields that push different particles in different directions, helping us to identify what they are. If a moving particle is deflected in one direction, it has a positive charge; if it’s deflected the other way, it has a negative charge. Moving in a straight line means it’s neutral.

Experiments around the ring

When Carl Anderson discovered the positron back in the 1930s, his cloud chamber was about five feet across and weighed two tons. The experiments at the LHC are a bit larger. The two biggest experiments, the general-purpose behemoths that will be looking for the Higgs, are called ATLAS (A Toroidal LHC ApparatuS) and CMS (Compact Muon Solenoid). They are located on opposite sides of the ring, with ATLAS sitting near the main CERN site and CMS over the border in France. The word “compact” is relative, of course; CMS is nearly 70 feet long, and weighs about 13,800 tons. ATLAS is larger in size but also more lightweight, coming in at 140 feet long and 7,700 tons. That’s the kind of scale you need to dig down to where we hope the Higgs is lurking.

The LHC also features five other experiments: two medium-size ones, ALICE and LHCb, and three small ones, TOTEM, LHCf, and MoEDAL. LHCb specializes in studying the decays of bottom quarks, which are useful for doing precision measurements. ALICE (A Large Ion Collider Experiment) is constructed to study the collisions of heavy nuclei rather than protons, re-creating the quark-gluon plasma that filled the universe shortly after the Big Bang. That’s why it’s the Large “Hadron” Collider, rather than the Large “Proton” Collider—one month a year, the LHC accelerates and collides lead ions instead of protons. TOTEM (TOTal Elastic and diffractive cross-section Measurement), located near CMS, studies the inner structure of protons and will perform precise measurements of the probability they will interact with one another. LHCf (“f” for “forward”) uses splashes from collisions to study the conditions under which cosmic rays propagate through the atmosphere. It’s located near ATLAS, and is much smaller than the other experiments: two detectors, each less than three feet across. MoEDAL (Monopole and Exotics Detector At the LHC) carries out specialized searches for very unusual particles.

It’s the two big experiments, ATLAS and CMS, that have been leading the hunt for the Higgs boson. Unlike the smaller experiments, which are designed with quite specific purposes in mind, these two detectors are made simply to watch protons smash together and do the best possible job at determining what comes out of the collisions. They approached the design challenges somewhat differently, but their capabilities end up being comparable. Needless to say, having two experiments is infinitely preferable to having only one—any dramatic and surprising discovery made by one of the detectors won’t be taken seriously until the other one verifies the finding.

It’s hard to get a feeling for the immensity of these machines without visiting them in person, which I was able to do while they were still under construction. A person is so small compared with CMS or ATLAS that you usually don’t notice them in photographs until someone points them out. Standing next to either detector, you are struck not only by their size but by their complexity. Every piece counts—and, given the international nature of the collaborations, it’s quite likely that two neighboring pieces were constructed in laboratories on opposite ends of the globe.

While CMS might not be “compact” in the sense of “small,” it is certainly compact in the sense of tightly packed together. It was stuck with the less-desirable location, a good car drive from the CERN buildings, because geological analysis revealed that the nearer location was the only one that could handle ATLAS’s greater size. CMS is an extremely dense collection of metal, crystal, and wire. The main magnets, the most powerful of their kind ever built, were constrained to be no more than twenty-three feet across, for very prosaic reasons: Anything larger would have been unable to fit on a truck that could make the trip through the streets of Cessy, the tiny French town where the experiment is located. (The
Wikipedia
page for Cessy, clearly written by physicists working at CMS, advises getting lunch at a certain local pizzeria but warns that “the service can be quite leisurely, so don’t go if you are in a hurry.”) Financial constraints, as well as logistical ones, played a crucial role in design and construction; the brass in the giant cylindrical end caps on either side of the detector was salvaged from Russian artillery shells. A crucial part of the detector is a set of 78,000 lead tungstate crystals, grown in Russia and China over a period of ten years, taking about two days to artificially grow each crystal.

It is ATLAS, however, that is more likely to be depicted in popular photos of the LHC. The reason is simple: It looks like an alien spaceship. The distinguishing features of the detector are the eight giant toroidal magnets that give the experiment its name. You might not recognize an ATLAS magnet as a “torus,” which is the shape of a doughnut, whereas the magnets are tubes that are vaguely rectangular with rounded corners. But physicists learn from topologists, mathematicians who care about general features and not specific shapes, so to them a “torus” is any cylinder that loops back on itself. The ATLAS toroids create a gigantic region of high magnetic field, useful for tracking high-energy muons created in the inner regions of the detector. When the magnets are turned on, the total amount of energy stored in them is more than 1 billion joules—the equivalent of about five hundred pounds of TNT. Fortunately, there’s no way for that energy to be released in an explosion. (Energy isn’t dangerous unless there’s a way to release it. The rest energy in an apple is equivalent to about a million tons of TNT, but it’s not really dangerous unless you bring it in contact with an anti-apple.)

The tremendous physical size of ATLAS and CMS is matched by the size of the collaborations that built and run them. The two groups of people are roughly similar: more than three thousand scientists each, representing more than 170 institutions from thirty-eight different countries. The whole group never gets together in the same place at the same time, but an endless stream of emails and videoconferences keeps the different subgroups in constant contact.

If there are two big collaborations carrying out very similar experiments to look for essentially the same phenomena, does that mean they are in competition with each other? Do you really need to ask? There are extremely high-stakes and serious—although mostly respectful—competition between the two experiments, as both race to make new discoveries. And with teams that large, there is a great deal of competition
within
each experiment, as different physicists jockey for positions of power, as well as debate the relative merits of different ways of analyzing the data.

But the system works. It might lead to some scientists with frayed nerves and a shortage of sleep, but the friendly rivalry between and within the experimental groups leads to topflight science. Everyone wants to be first, but nobody wants to be wrong, and if you’re sloppy, someone else will quickly figure it out. The well-matched capability of the CMS and ATLAS teams is one of the strongest reasons we will have to trust any results they both agree on—including the discovery of the Higgs boson.

Colliding protons

The task of these mammoth experiments is to figure out what happens when two protons collide at enormously high energies. A proton is not an infinitesimally small particle nor an undifferentiated blob of proton-stuff; it’s made of many strongly interacting constituents. We often say, “A proton is made of three quarks,” but that’s a bit sloppy. The two up quarks and one down quark that make a proton a proton are called the “valence quarks.” In addition to those valence quarks, quantum mechanics predicts that there are a large number of “virtual particles” constantly popping in and out of existence: gluons as well as quark-antiquark pairs. It’s the energy contained in these virtual particles that explains why protons are so much heavier than the valence quarks that give them their identity. It’s hard to give a precise count of how many virtual particles there are, as the number depends on how closely we look. (That’s quantum mechanics for you.) But the number of valence quarks remains fixed. If you add up the total number of up quarks inside a proton at any one moment, it would always be exactly two more than the number of antiup quarks; likewise, the number of down quarks is always one more than the number of antidowns.

Basically a proton is a floppy bag of quarks, antiquarks, and gluons, moving around the LHC beam pipe near the speed of light. Richard Feynman dubbed all these constituent particles “partons.” According to relativity, objects moving near light speed are contracted along their direction of motion. So two protons colliding inside the detector resemble pancake-shaped collections of partons, flying at each other face-on. When one proton interacts with another one, it’s actually just one of the partons in one proton that interacts with a parton inside the other proton. As a result, it’s hard to know exactly how much energy is involved in a collision, because we don’t know which of the partons did the interacting.