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

  In particular, the smaller the mass of the particle, the more space it takes up. Atoms are made out of just three types of fermions—up quarks, down quarks, and electrons—held together by forces. The nucleus, made of protons and neutrons, which in turn are made of up and down quarks, is relatively heavy, and exists in a relatively tiny region of space. The electrons, meanwhile, are much lighter (about 1/2,000th the mass of a proton or neutron) and take up much more space. It’s really the electrons in atoms that give matter its solidity.

  Bosons don’t take up any space at all. Two bosons, or two trillion bosons, can easily sit at exactly the same location, right on top of one another. That’s why bosons are force-carrying particles; they can combine to make a macroscopic force field, like the gravitational field that holds us to the earth or the magnetic field that deflects a compass needle.

  Physicists tend to use the words “force,” “interaction,” and “coupling” in practically interchangeable ways. That reflects one of the deep truths uncovered by twentieth-century physics: Forces can be thought of as resulting from the exchange of particles. (As we’ll see, that’s equivalent to saying “as resulting from vibrations in fields.”) When the moon feels the gravitational pull of the earth, we can think of gravitons passing back and forth between the two bodies. When an electron is trapped by an atomic nucleus, it’s because photons are exchanged between them. But these forces are also responsible for other particle processes like annihilation and decay, not just pushing and pulling. When a radioactive nucleus decays, we can attribute that event to the strong or weak nuclear force at work, depending on what kind of decay occurs. Forces in particle physics are responsible for a wide variety of goings-on.

  Aside from the Higgs, we know four kinds of forces, each with its own associated boson particles. There’s gravity, associated with a particle called the “graviton.” Admittedly, we haven’t actually observed individual gravitons, so the graviton is often not included in discussions of the Standard Model, although we detect the force of gravity every day when we don’t all float into space. But given that gravity is a force, the basic rules of quantum mechanics and relativity essentially guarantee that there are associated particles, so we use the word “graviton” to refer to those particles we haven’t yet seen on an individual basis. The way that gravity acts as a force on other particles is pretty simple: Every particle attracts every other particle (although very weakly).

  Then there is electromagnetism—in the 1800s, physicists figured out that the phenomena of “electricity” and “magnetism” were two different versions of the same underlying force. The particles associated with electromagnetism are called “photons,” which we see directly all the time. Particles that do interact via electromagnetism are “charged,” while those that don’t are “neutral.” And just to keep you on your toes, electrical charges can be positive or negative, with like charges pushing each other apart and opposite charges attracting. The ability of like charges to repel each other is absolutely crucial to how the universe works. If electromagnetism were universally attractive, every particle would simply attract every other particle, and all the matter in the universe would do its best to collapse into one giant black hole. Fortunately we have electromagnetic repulsion as well as attraction, which keeps life interesting.

  Nuclear forces

  Then we have the two “nuclear” forces, so called because (unlike gravity and electromagnetism) they only extend over a very short distance, comparable in size to the nucleus of an atom or less. There is the strong nuclear force, which holds quarks together inside protons and neutrons; its particles are charmingly named “gluons.” The strong nuclear force is (unsurprisingly) very strong, and interacts with quarks but not with electrons. Gluons are massless, just like photons and gravitons. When a force is carried by massless particles, we expect its influence to stretch over a very long range, but the strong force is actually very short ranged.

  In 1973, David Gross, David Politzer, and Frank Wilczek showed that the strong force has an amazing property: The attraction between two quarks actually grows in strength as the quarks are moved apart. As a consequence, pulling two quarks apart requires more and more energy, so much so that you eventually just create more quarks. It’s like pulling on a strip of rubber, with each end representing a quark. You can pull the two ends, but you never get one end all by itself. Instead you create two new ends when the rubber snaps. As a result, you will never see an individual quark alone in the wild; they (and the gluons) are confined inside heavier particles. These composite particles made of quarks and gluons are known as “hadrons,” from which the LHC gets its middle name. Gross, Politzer, and Wilczek shared the Nobel Prize in 2004 for this discovery.

  Then there is the weak nuclear force, which lives up to its name. Although it doesn’t play much of a role in our immediate environment here on earth, the weak force is nevertheless important to the existence of life: It helps the sun shine. Solar energy arises from conversion of protons into helium, which requires turning some of those protons into neutrons, which proceeds by the weak interaction. But down here on earth, unless you’re a particle or nuclear physicist, you don’t see too much of the weak force in action.

  Three different kinds of bosons carry the weak force. There is the Z boson, which is electrically neutral, and there are two different W bosons, one with a positive electric charge and one with a negative electric charge, dubbed W+ and W- for short. The W and Z bosons are quite massive by elementary-particle standards (about as heavy as an atom of zirconium, if that’s any help), which means that they are hard to produce and decay away fairly quickly, all of which contributes to why the weak interactions are so weak.

  In casual speech we use the word “force” to refer to all kinds of things. The force of friction when something is sliding, the force of impact when you smash into a wall, the force of air resistance as a feather falls to the ground. You will have noticed that none of these forces made our list of the four forces of nature, nor do any of them have bosons associated with them. That’s the difference between elementary-particle physics and colloquial usage. All of the macroscopic “forces” that we experience as part of our daily routine, from the acceleration when we depress a car’s gas pedal to the tug on a leash when a dog suddenly sees a squirrel and takes off, ultimately arise as complicated side effects of the fundamental forces. In fact, with the notable exception of gravity (which is pretty straightforward, pulling everything down), all of those everyday phenomena are just manifestations of electromagnetism and its interactions with atoms. This is the triumph of modern science: to boil the marvelous variety of the world around us down to just a few simple ingredients.

  Fields pervade the universe

  Of these four forces, one has long stood out as weird: the weak force. Notice that gravity has gravitons, electromagnetism has photons, and the strong force has gluons; one kind of boson for each force. The weak force comes with three different bosons, the neutral Z and the two charged Ws. And these bosons are responsible for strange behaviors, as well. By emitting a W boson one kind of fermion can change into another kind: a down quark can spit out a W- and change into an up quark. Neutrons, which are made of two downs and an up, decay when they’re by themselves outside a nucleus—one of their down quarks emits a W-, and the neutron converts into a proton, which has two ups and a down. None of the other forces change the identity of the particles they interact with.

  The weak interactions, basically, are a mess. And the reason is simple: the Higgs.

  The Higgs is fundamentally different from all the other bosons. The others, as we’ll see in Chapter Eight, all arise because of some symmetry of nature connecting what happens at different points in space. Once you believe in these symmetries, the bosons are practically inevitable. But the Higgs isn’t like that at all. There is no deep principle that requires its existence, but it exists anyway.

  After the LHC announced the Higgs discovery on July 4, hundreds of attempts were made at expla
ining what it was supposed to mean. The biggest reason why this task is such a challenge is that it’s not really the Higgs boson itself that is all that interesting; what matters is the Higgs field from which the boson arises. It’s a fact of physics that all the different particles really arise out of fields—that’s quantum field theory, the underlying framework for everything that particle physicists do. But quantum field theory isn’t something we teach kids in high school. It’s not even something we often discuss in popular physics books; we talk about particles and quantum mechanics and relativity, but we rarely dig into the wonders of quantum field theory underlying it all. When it comes to the Higgs boson, however, it’s no longer adequate to skirt around the ultimate field-ness of it all.

  When we talk about a “field,” we are talking about “something that has some value at every point in space.” The temperature of the earth’s atmosphere is a field; at every point on the earth’s surface (or at any elevation above the surface) the air has a certain temperature. The density and humidity of the atmosphere are likewise fields. But these aren’t fundamental fields—they are just properties of the air itself. The electromagnetic field or the gravitational field are, in contrast, believed to be fundamental. They’re not made of anything else—they are what the world is made of. According to quantum field theory, absolutely everything is made of a field or a combination of fields. What we call “particles” are tiny vibrations in these fields.

  This is where the “quantum” part of quantum field theory comes in. There’s a lot to say about quantum mechanics, perhaps the most mysterious idea ever to be contemplated by human beings, but all we need is one simple (but hard to accept) fact: How the world appears when we look at it is very different from how it really is.

  The physicist John Wheeler once proposed a challenge: How can you best explain quantum mechanics in five words or fewer? In the modern world, it’s easy to get suggestions for any short-answer question: Simply ask Twitter, the microblogging service that limits posts to 140 characters. When I posed the question about quantum mechanics, the best answer was given by Aatish Bhatia (@aatishb): “Don’t look: waves. Look: particles.” That’s quantum mechanics in a nutshell.

  Every particle we talk about in the Standard Model is, deep down, a vibrating wave in a particular field. The photons that carry electromagnetism are vibrations in the electromagnetic field that stretches through space. Gravitons are vibrations in the gravitational field, gluons are vibrations in the gluon field, and so on. Even the fermions—the matter particles—are vibrations in an underlying field. There is an electron field, an up quark field, and a field for every other kind of particle. Just like sound waves propagate through the air, vibrations propagate through quantum fields, and we observe them as particles.

  Just a bit ago we mentioned that particles with a small mass take up more space than ones with a larger mass. That’s because the particles aren’t really little balls with a uniform density; they’re quantum waves. Every wave has a wavelength, which gives us a rough idea of its size. The wavelength also fixes its energy: It requires more energy to have a short wavelength, since the wave needs to change more quickly from one point to another. And mass, as Einstein taught us long ago, is just a form of energy. So lower masses mean less energy mean longer wavelengths mean larger sizes; higher masses mean more energy mean shorter wavelengths mean smaller sizes. It all makes sense once you unpack it.

  Stuck away from zero

  Fields have a value at every point in space, and when space is completely empty those values are typically zero. By “empty” we mean “as empty as can be,” or, more specifically, “with as little energy as it is possible to have.” According to that definition, fields like the gravitational field or the electromagnetic field sit quietly at zero when space is truly empty. When they’re at some other value, they carry energy, and therefore space isn’t empty. All fields have tiny vibrations because of the intrinsic fuzziness of quantum mechanics, but those are vibrations around some average value, which is typically zero.

  The Higgs is different. It’s a field, just like the others, and it can be zero or some other value. But it doesn’t want to be zero; it wants to sit at some constant number everywhere in the universe. The Higgs field has less energy when it’s nonzero than when it’s zero.

  As a result, empty space is full of the Higgs field. Not a complicated set of vibrations that would represent a collection of individual Higgs bosons; just a constant field, sitting quietly in the background. It’s that ever-present field at every point in the universe that makes the weak interactions what they are and gives masses to elementary fermions. The Higgs boson—the particle discovered at the LHC—is a vibration in that field around its average value.

  Because the Higgs particle is a boson, it gives rise to a force of nature. Two massive particles can pass by each other and interact by exchanging Higgs bosons, just like two charged particles can interact by exchanging photons. But this Higgs force is not what gives particles mass, and it’s generally not what all the fuss is about. What gives particles mass is this Higgs field sitting quietly in the background, providing a medium through which other particles move, affecting their properties along the way.

  One major difference between the Higgs field and other fields is that the resting value of the Higgs is away from zero. All fields undergo tiny vibrations due to the intrinsic uncertainties of quantum mechanics. A larger vibration appears to us as a particle, in this case the Higgs boson.

  As we travel through space, we’re surrounded by the Higgs field and moving within it. Like the proverbial fish in water, we don’t usually notice it, but that field is what brings all the weirdness to the Standard Model.

  Executive summary

  There is a great deal of profound and challenging physics associated with the idea of the Higgs boson. But for right now let’s just give the overall summary of how the Higgs field works and why it’s important. Without further ado:

  The world is made of fields—substances spread through all of space that we notice through their vibrations, which appear to us as particles. The electric field and the gravitational field might seem familiar, but according to quantum field theory even particles like electrons and quarks are really vibrations in certain kinds of fields.

  The Higgs boson is a vibration in the Higgs field, just as a photon of light is a vibration in the electromagnetic field.

  The four famous forces of nature arise from symmetries—changes we can make to a situation without changing anything important about what happens. (Yes, it makes no immediate sense that “a change that doesn’t make a difference” leads directly to “a force of nature” . . . but that was one of the startling insights of twentieth-century physics.)

  Symmetries are sometimes hidden and therefore invisible to us. Physicists often say that hidden symmetries are “broken,” but they’re still there in the underlying laws of physics—they’re simply disguised in the immediately observable world.

  The weak nuclear force, in particular, is based on a certain kind of symmetry. If that symmetry were unbroken, it would be impossible for elementary particles to have mass. They would all zip around at the speed of light.

  But most elementary particles do have mass, and they don’t zip around at the speed of light. Therefore, the symmetry of the weak interactions must be broken.

  When space is completely empty, most fields are turned off, set to zero. If a field is not zero in empty space, it can break a symmetry. In the case of the weak interactions, that’s the job of the Higgs field. Without it, the universe would be an utterly different place.

  Got all that? It’s a bit much to swallow, admittedly. It will make more sense when we complete our journey through the rest of the chapters. Trust me.

  The rest of the book will be a back-and-forth journey through the ideas behind the Higgs mechanism and the experimental quest to discover the boson. We’ll start with a quick overview of how the particles and forces of the Standard Model fit togethe
r, then explore the astonishing ways in which physicists use technology and gumption to discover new particles. After that it’s back to theory, as we think about fields and symmetries and how the Higgs can hide symmetries from our view. Finally we can show how the Higgs was discovered, how the news was spread, who will get the credit, and what it means for the future.

  It should be clear why Leon Lederman thought that the God Particle was an appropriate name for the Higgs boson. That boson is the hidden piece of equipment that explains the magic trick the universe is pulling on us, giving particles different masses and thereby making particle physics interesting. Without the Higgs, the intricate variety of the Standard Model would collapse to a featureless collection of pretty much identical particles, and all of the fermions would be essentially massless. There would be no atoms, no chemistry, no life as we know it. The Higgs boson, in a very real sense, is what brings the universe to life. If there were one particle that deserved such a lofty title, there’s no question it would be the Higgs.

  THREE

  ATOMS AND PARTICLES

  In which we tear apart matter to reveal its ultimate constituents, the quarks and the leptons.

  In the early 1800s, German physician Samuel Hahnemann founded the practice of homeopathy. Dismayed by the ineffectiveness of the medicine of his time, Hahnemann developed a new approach based on the principle of “like cures like”—a disease can be treated by precisely the same substance that causes it in the first place, as long as that substance is properly manipulated. The way to manipulate it is known as “potentization,” which consists of diluting the substance repeatedly in water, shaking vigorously each time. A typical method of dilution might mix one part of substance and ninety-nine parts water. You prepare a homeopathic remedy by diluting, shaking, diluting again, shaking again, as many as two hundred times.