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

by Sean Carroll

  Quarks and hadrons

  Meanwhile the hadrons were not exactly sitting still. The mid-century advent of particle accelerators led to a boom in the number of so-called elementary particles that physicists had discovered. There were pions, kaons, eta mesons, rho mesons, hyperons, and more. Willis Lamb, during his own Nobel lecture in 1955, cracked, “The finder of a new elementary particle used to be rewarded by a Nobel Prize, but such a discovery now ought to be punished by a ten-thousand-dollar fine.”

  All of these new particles were hadrons—unlike the leptons, they interacted strongly with neutrons and protons. Increasingly physicists began to suspect that the newcomers weren’t really “elementary” at all, but rather reflected some deeper underlying structure.

  The code was finally cracked in 1964 by Murray Gell-Mann and George Zweig, who independently proposed that hadrons were made of smaller particles called “quarks.” Just like leptons, they come in six different flavors: up, down, charm, strange, top, and bottom. The up/charm/top quarks all have electric charge +2/3, while the down/strange/bottom quarks have charge -1/3; these are sometimes grouped as “up-type” and “down-type” quarks, respectively.

  Unlike leptons, each flavor of quark really represents a triplet of particles, rather than just one. The three kinds of each quark are fancifully labeled after colors: red, green, or blue. The names are fun, not realistic; you can’t actually see quarks, and if you could they wouldn’t actually have those colors.

  The quarks of the Standard Model, arranged into three generations. Each type of quark comes in three colors. Larger circles indicate more massive particles, although the sizes are not to scale.

  Quarks are “confined,” which means that they exist only in combinations inside hadrons, never isolated by themselves. When they combine, it is always into “colorless” combinations. Protons and neutrons each have three quarks inside: A proton is two ups and a down, while a neutron is two downs and an up. One of those quarks will be red, one will be green, and one will be blue; together they make white, which counts as colorless by the terms of this analogy. Later we will see that there are also “virtual” quark-antiquark pairs popping in and out of existence inside the nucleons, but they come in color-anticolor combinations, leaving the overall whiteness unaffected.

  It’s impossible to look at the figures portraying the leptons and quarks without noticing some patterns. In both cases we have six types of particles. And these six are precisely arranged into three pairs, with the two particles in each pair differing by one unit of electric charge. Might there be some deeper explanation for this structure? At least in part, the answer is yes. The two particles in each pair, such as an electron and its neutrino, would be identical if it wasn’t for the meddlesome influence of the Higgs field filling empty space. That’s a reflection of the role of the Higgs as a breaker of symmetries, which we’ll examine more carefully later in the book.

  The force that doesn’t fit

  The fermions of the Standard Model are what give the matter all around us its size and shape. But it’s the forces and their associated boson particles that allow those fermions to interact with one another. Fermions can push or pull on one another by tossing bosons back and forth, or they can lose energy or decay into other fermions by spitting out some kind of boson. Without the bosons, the fermions would simply move along straight lines for all eternity, unaffected by anything else in the universe. And the reason why the universe is so bloody complex and interesting is that these forces are all different, and push and pull in complementary ways.

  Physicists often say that there are four forces of nature—they don’t include the Higgs, and not just because it took a long time to discover it. The Higgs is different from the other bosons. The others are what are called “gauge bosons”—as we’ll discuss in Chapter Eight, they are deeply related to underlying symmetries of nature. The graviton is a bit different from the others. Every elementary particle has a certain intrinsic “spin,” and the photon, gluons, and W/Z bosons all have a spin equal to one, while the graviton has a spin of two. (See Appendix One for some details.) We don’t yet know how to reconcile gravity with the demands of quantum mechanics, but it’s still fair to call it a “gauge boson.”

  The Higgs, on the other hand, is completely different. It’s what we call a “scalar” boson, which means it has zero spin. Unlike the gauge bosons, the Higgs is not forced on us by a symmetry or any other deep principle of nature. A world without the Higgs would look very different, but it would be perfectly consistent as a physical theory. As important as it is, the Higgs is somewhat of a blemish on the beautiful mathematical structure of the Standard Model. Nevertheless, it is a boson, and therefore it can be exchanged back and forth by other particles, giving rise to a force of nature.

  The bosons of the Standard Model. (In this book we include gravitons, although not everyone does.) All the bosons are electrically neutral except for the Ws, and all are massless except for the Ws, Z, and the Higgs.

  The Higgs boson is a vibration in the Higgs field, and the Higgs field is what gives mass to all of the massive elementary particles. So the Higgs boson interacts with all of the massive particles in our zoo—the quarks, the charged leptons, and the W and Z bosons. (Neutrino masses aren’t completely understood as yet, so let’s pretend that they don’t interact with the Higgs, although the jury is still out.) And the more massive a particle is, the more strongly it couples to the Higgs. Really it’s the other way around: The more strongly a particle couples to the Higgs, the more mass it picks up by moving through the ambient Higgs field that pervades empty space.

  This feature of the Higgs—it interacts more strongly with more massive particles—is absolutely crucial when it comes to studying the beast at the LHC. The Higgs is a heavy particle itself, and even when we produce it we aren’t able to see it directly; it will very rapidly decay into other particles. We expect there will be a certain rate of decay into (for example) W bosons, a different rate of decay into bottom quarks, a different rate of decay into tau mesons, and so forth. And it’s not random—we know exactly how the Higgs is supposed to interact with other particles (because we know how much mass they each have), so we can calculate quite precisely the expected frequency of different kinds of decays.

  What we really want is to be wrong. It’s a great triumph to discover the Higgs, but things get really exciting when we are surprised by something new. Searching for invisible particles that are hard to produce and decay quickly into other particles is a challenging task. It’s a matter of patience, precision, and careful statistical analysis. The good news is that the laws of physics (or any one hypothetical version of them) are unforgiving; the predictions for what we should see are unambiguous and inalterable. If the Higgs turns out to be different from what we expect, it will be a clear sign that the Standard Model has finally failed us, and the door to new phenomena has been opened at last.

  FOUR

  THE ACCELERATOR STORY

  In which we trace the colorful history of the unlikely pastime of smashing together particles at ever-higher energies.

  When I was about ten years old, I discovered the science section in our local library in Lower Bucks County, Pennsylvania. I was immediately hooked. My favorite books were in astronomy and physics—the 520s and 530s, according to the venerable Dewey Decimal System. One of the books I pored over most intently was a modest volume entitled High Energy Physics, by Hal Hellman. I was doing my reading in the late 1970s, but the book had been written in 1968, before the Standard Model was put together—back when quarks were exotic and somewhat scary-sounding theoretical speculations. But hadrons had been discovered in abundance, and High Energy Physics was full of evocative photographs of particle tracks, each representing a fleeting glimpse of nature’s secrets.

  Many of these photographs had been taken at the mighty Bevatron, one of the leading particle accelerators of the 1950s and ’60s. The Bevatron was located in Berkeley, California, but that’s not where the name ca
me from; it was derived from “billions of electron volts,” the energy the accelerator was able to reach. (As we’ll explain below, an electron volt is a weird unit of energy much beloved by particle physicists.) One billion corresponds to the prefix “giga–,” so a billion electron volts is one GeV, but back in those days Americans would often use “BeV,” and besides, “Gevatron” just doesn’t sound right.

  The Bevatron contributed to two Nobel Prizes: in 1959 to Emilio Segrè and Owen Chamberlain, for the discovery of the antiproton, and in 1968 to Luis Alvarez, for the discovery of too many particles to count—all those pesky hadrons. Sometime later, Alvarez and his son Walter were the ones who first demonstrated that an asteroid impact was the likely cause of the extinction of dinosaurs, by discovering an anomalously high concentration of iridium in geological strata that formed around that time.

  The idea behind particle accelerators is simple: Take some particles, accelerate them to very high velocities, and slam them into some other particles, watching carefully to see what comes out. The procedure has been compared to smashing together two fine Swiss watches and trying to figure out what they are made of by watching the pieces fly apart. Unfortunately, this analogy has it backward. When we smash particles together, we’re not looking for what they are made of; we’re trying to create brand-new particles that weren’t there before we did the smashing. It’s like smashing together two Timex watches and hoping that the pieces assemble themselves into a Rolex.

  To attain these velocities, accelerators use a basic principle: Charged particles (such as electrons and protons) can be pushed around by electric and magnetic fields. In practice, we use electric fields to accelerate particles to ever-higher speeds, and magnetic fields to keep them moving in the right direction, such as around the circular tubes of the Bevatron or the LHC. By delicately tuning these fields to push and nudge particles in just the right way, physicists can reproduce conditions that would otherwise never be seen here on earth. (Cosmic rays from outer space can be even more energetic, but they are also rare and hard to observe.)

  The influence of a magnetic field on moving particles. If the magnetic field is pointing upward, it pushes positively charged particles in a counterclockwise direction, negatively charged particles in a clockwise direction, and neutral particles not at all. Likewise, stationary particles just remain at rest.

  The technological challenge is clear: Accelerate particles to as high an energy as we can, smash them together, and look to see what new particles are created. None of these steps is easy. The LHC represents the culmination of decades of work learning how to build bigger and better accelerators.

  E = mc2

  When the Bevatron created antiprotons, it wasn’t because there were antiprotons hidden in the protons and atomic nuclei they were working with. Rather, the collisions brought new particles into existence. In the language of quantum field theory, the waves representing the original particles set up new vibrations in the antiproton field, which we detect as particles.

  In order for that to happen, the crucial ingredient is that we have enough energy. The insight that makes particle physics possible is Einstein’s famous equation, E = mc2, which tells us that mass is actually a form of energy. In particular, the mass of an object is the minimum energy that object can have; when something is just sitting perfectly still, minding its own business, the amount of energy it possesses is equal to its mass times the speed of light squared. The speed of light is a big number, 186,000 miles per second, but its role here is just to convert units of measurement from mass to energy. Particle physicists like to use units where speed is measured in light-years per year; in that case c is equal to one, and mass and energy become truly interchangeable, E = m.

  What about when an object is moving? Sometimes discussions of relativity like to talk as if the mass increases when a particle approaches the speed of light, but that’s a little misleading. It’s better to think of the mass of an object as fixed once and for all, while the energy increases as it goes faster and faster. The mass is the energy that the thing would have if it was not moving, which by definition doesn’t change even if it happens to be moving. Indeed, energy grows without limit as you get closer and closer to the speed of light. That’s one way of understanding why the speed of light is an absolute limit to how fast things can go—it would take an infinite amount of energy for a massive body to move that fast. (Massless particles, in contrast, always move at exactly the speed of light.) When a particle accelerator pushes protons to higher and higher energies, they are coming closer and closer to the speed of light, never quite getting there.

  Through the magic of this simple equation, particle physicists can make heavy particles out of lighter ones. In a collision, the total energy is conserved but not the total mass. Mass is just one form of energy, and energy can be converted from one form to another, as long as its total amount remains constant. When two protons come together at a large velocity, they can convert into heavier particles if their total energy is large enough. We can even collide perfectly massless particles to create massive ones; two photons can smack together to make an electron-positron pair, or two massless gluons can come together to make a Higgs boson, if their combined energy is larger than the Higgs mass. The Higgs boson is more than a hundred times heavier than a proton, which is one of the reasons it’s so hard to create.

  Particle physicists enjoy using units of measurement that make no sense to the outside world, as it lends an aura of exclusivity to the endeavor. Also, it would be a pain to use one set of units for mass and another for other forms of energy, since they are constantly being converted back and forth to one or the other. Instead, whenever we’re faced with an amount of mass, we simply multiply it by the speed of light squared to instantly convert it into an energy. That way we can measure everything in terms of energy, which is much more convenient.

  Scale of energies. Particle physicists measure temperature, mass, and energy on the same scale, using electron volts as a basic unit. Common expressions include milli-eV (1/1000 eV), keV (1000 eV), MeV (1 million eV), GeV (1 billion eV), and TeV (1 trillion eV). Some values are approximate.

  The energy unit favored by particle physicists is the electron volt, or “eV” for short. One eV is the amount of energy it would take to move an electron across one volt of electrical potential. In other words, it takes nine electron volts’ worth of energy to move an electron from the positive to the negative terminals of a nine-volt battery. It’s not that physicists spend a lot of time pushing electrons through batteries, but it’s a convenient unit that has become standard in the field.

  One electron volt is a tiny bit of energy. The energy of a single photon of visible light is about a couple of electron volts, while the kinetic energy of a flying mosquito is a trillion eV. (It takes many atoms to make a mosquito, so that’s very little energy per particle.) The amount of energy you can release by burning a gallon of gasoline is more than 1027 eV, while the amount of nutritional energy in a Big Mac (700 calories) is about 1025 eV. So a single eV is a small amount of energy indeed.

  Since mass is a form of energy, we also measure the masses of elementary particles in electron volts. The mass of a proton or neutron is almost a billion electron volts, while the mass of an electron is half a million eV. The Higgs boson that the LHC discovered is at 125 billion eV. Because one eV is so small, we often use the more convenient unit of GeV, for giga– (1 billion) electron volts. You’ll also see keV for kilo– (1,000) electron volts, MeV for mega– (1 million) electron volts, and TeV for tera– (1 trillion) electron volts. In 2012, the LHC collided protons with a total energy of 8 TeV, and the eventual goal is 14 TeV. That’s more than enough energy to make Higgs bosons and other exotic particles; the trick is to detect them once they’re produced.

  We can even measure temperature using the same units, because temperature is just an average energy of the molecules in a substance. From this perspective, room temperature is only two-hundredths of an electron volt, while the
temperature at the center of the sun is about 1 keV. When the temperature rises above the mass of a certain particle, that means that collisions have enough energy to create that particle. Even the center of the sun, which is pretty hot, isn’t nearly high enough to produce electrons (0.5 MeV), much less protons or neutrons (about 1 GeV each). Back near the Big Bang, however, the temperature was so high that it was no problem.

  The easiest way for nature to hide a particle from us is to make it so heavy that we can’t easily produce it in the lab. That’s why the history of particle accelerators has been one of reaching for higher and higher energies, and why accelerators get names like Bevatron and Tevatron. Reaching unprecedented energies is literally like visiting a place nobody has ever seen.

  Energizing Europe

  The official name of CERN, the Geneva laboratory where the LHC is located, is the European Organization for Nuclear Research, or in French Organisation Européenne pour la Recherche Nucléaire. You’ll notice that the acronym doesn’t work in either language. That’s because the current “Organization” is a direct descendant of the European Council for Nuclear Research, Conseil Européen pour la Recherche Nucléaire, and everyone agreed that the old abbreviation could stick even after the name was officially changed. Nobody insisted on switching to “OERN.”

  The council was established in 1954 by a group of twelve countries that sought to reenergize physics in postwar Europe. Since that time, CERN has been at the forefront of research in particle and nuclear physics, and has served as an intellectual center for European science, as well as an important component of Geneva’s identity. In the second-largest city in Switzerland, a world center of finance, diplomacy, and watchmaking, one out of sixteen passengers passing through Geneva airport is somehow associated with CERN. When you land there, chances are there’s a physicist or two on your airplane.