The Particle at the End of the Universe: How the Hunt for the Higgs Boson Leads Us to the Edge of a New World (35 page)

The two numbers don’t match. The total amount of ordinary matter in the universe is only about one-fifth of the total amount of matter. The vast majority is dark matter, and the dark matter can’t be any of the particles in the Standard Model.

The Higgs boson is the final piece of the Standard Model puzzle, but the Standard Model is certainly not the end of the road. Dark matter is just one indication that there is a lot more physics out there remaining to be understood. One exciting prospect is that the Higgs can serve as a bridge between what we know and what we hope to learn. By studying carefully the properties of the Higgs, we hope to shed light on the dark worlds beyond our own.

The early universe

Let’s think about dark matter a bit more carefully, as it provides some of the strongest evidence we have for physics beyond the Standard Model, and a great example of how the Higgs could be involved in understanding this new physics better. A crucial feature of dark matter is that it can’t be ordinary matter (atoms and so forth) in some “dark” form, like brown dwarfs or planets or interstellar dust. That’s because we have very good measurements of the total amount of ordinary matter, from processes in the early universe.

To understand dark matter, we need to think about where it came from. Imagine you have an experimental apparatus that is basically a super-oven: a sealed box with some stuff inside, and a dial you can adjust to make the temperature as high or low as you like. An ordinary oven might go as high as 500 degrees Fahrenheit, which in particle physics units is about 0.04 electron volts. At that temperature, molecules can rearrange themselves (which we call “cooking”), but atoms maintain their integrity. Once we get up to a few electron volts or higher, electrons are stripped away from their nuclei. When we hit millions of electron volts (MeV), the nuclei themselves are stripped apart, leaving us with free protons and neutrons.

Another thing also happens at high temperatures: The collisions between particles are so energetic that you can create new particle-antiparticle pairs, just like in a particle collider. If the temperature is higher than the total mass of a particle plus its antiparticle, we expect such pairs to be copiously produced. So at sufficiently high temperatures, it almost doesn’t matter what was in the box to begin with; what we get is a hot plasma filled with all the particles that have masses lower than the temperature inside. (Remember that mass and temperature can both be measured in GeV.) If the temperature is 500 GeV, our box will be buzzing with Higgs bosons, all the quarks and leptons, W and Z bosons, and so forth—not to mention possible new particles that haven’t yet been discovered here on earth. If we were to gradually lower the temperature inside of that box, these new particles would gradually disappear as they bumped into their antiparticles and annihilated, leaving us with only the particles we started with.

The early universe is much like the plasma inside our ultrahot oven, with one crucial extra ingredient: Space is expanding at an incredible rate. The expansion of space has two important effects. First, the temperature cools off, so it’s as if the temperature dial on our oven starts very high but is quickly turned down. Second, the density of matter decreases rapidly as particles move away from one another in the expanding space. That latter feature is a crucial difference between the early universe and an oven. Because the density is decreasing, particles that were produced in the original plasma might not have a chance to annihilate away; it might simply be too hard for one of them to find a corresponding antiparticle.

As a result, we get a relic abundance of these particles from the primordial plasma. And we can calculate precisely what that abundance should be, if we know the masses of the particles and the rate at which they interact. If the particles are unstable, like the Higgs boson is, the relic abundance is pretty irrelevant, as the particles just decay away. But if they’re stable, we’re stuck with them. It’s easy to imagine that a leftover stable particle from the early universe constitutes the dark matter today.

In the Standard Model, we can play this game with the atomic nuclei. One crucial difference is that we start with more matter than antimatter, so the matter can never completely annihilate away. Start at a fairly high temperature, say around 1 GeV. The plasma will consist of protons, neutrons, electrons, photons, and neutrinos; all the heavier particles will have decayed. That temperature is sufficiently hot that protons and neutrons cannot form nuclei without being instantly ripped apart. But as the universe expands and cools, nuclei begin to form a few seconds after the Big Bang. Just a couple of minutes later, the density is so low that nuclei stop running into one another, and those reactions cease. We are left with a certain combination of protons and light elements: deuterium (heavy hydrogen, one proton and one neutron), helium, and lithium. This process is known as “Big Bang nucleosynthesis.”

We can make precise calculations of the relative abundance of those elements, with just one input parameter: the initial abundance of protons and neutrons. And then we can compare the primordial element abundances with what we see in the real universe. The answer matches precisely, but only for one specific density of protons and neutrons. That happy result is reassuring, since it indicates that the way we think about the early universe is basically on the right track. Since protons and neutrons make up the overwhelming fraction of mass in ordinary matter, we know quite well how much ordinary matter there is in the universe, no matter what form it might take today. And it’s not nearly enough to make up all the matter there is.

WIMPs

One promising strategy for dark matter is to play that same game as we did with nucleosynthesis, but starting at a much higher temperature and adding a new particle into the mix—a particle that will be the dark matter. We know that dark matter is dark, so the new particle should be electrically neutral. (Charged particles are precisely those that interact with electromagnetism, and therefore tend to give off light.) And we know it’s still around, so it should be stable, or at least have a lifetime longer than the age of the universe. We even know something a bit more detailed: Dark matter doesn’t interact very strongly with itself. If it did, it would settle into the middle of galaxies, rather than forming big puffy halos as the data seem to indicate. So the dark matter doesn’t feel the strong nuclear force, either. Of the known forces of nature, the dark matter certainly feels gravity, and it may or may not feel the weak nuclear force.

Let’s imagine a particular kind of new particle: a “Weakly Interacting Massive Particle,” or WIMP. (Cosmologists are nothing if not cheeky when it comes to inventing new names.) By “weakly interacting” we don’t just mean “doesn’t interact very much”; we mean that it feels the weak interactions of particle physics. For simplicity, we assume the WIMP has a mass compatible with other particles involved in the weak interactions, like the W and Z bosons or the Higgs. Around 100 GeV, let’s say, or at least between 10 and 1,000 GeV. Other details of the way such a particle interacts are relevant for high-precision calculations, but just these basic properties are enough to perform back-of-the-envelope estimates.

Then we compare the predicted abundance of such a WIMP with the actual abundance of dark matter. What we find—amazingly—is that they match beautifully. There’s some wiggle room, having to do with what other particles might exist and how exactly the WIMPs annihilate, but the rough agreement is striking. Stable particles with weak-scale interactions generally have the right relic abundance to account for the dark matter, without even trying too hard.

This interesting coincidence is known as the “WIMP miracle” and has given many particle physicists hope that the secret to dark matter lies in new particles with similar masses and interactions to the W/Z/Higgs bosons. All those particles decay quickly, of course, so the WIMP must have some good reason to be stable, but that’s not hard to invent. There are many other plausible theories of dark matter—including a particle called the “axion,” invented by Steven Weinberg and Frank Wilczek, which is like a very lightweight cousin of the Higgs—but WIMP models are by far the most popular.

The possibility that the dark matter is a WIMP opens up some very exciting experimental possibilities, precisely because the Higgs will interact with it. Indeed, in many (arguably most, but it’s hard to count) viable models of WIMP dark matter, the strongest coupling between the dark matter and ordinary matter will be through exchanging a Higgs boson. The Higgs could be the link between our world and most of the matter in the universe.

The Higgs portal

This feature—interacting via Higgs exchange—turns out to be common in many theories of physics beyond the Standard Model. You have a whole bunch of new particles in what’s known as a “hidden sector,” and they don’t interact very noticeably with the particles we’ve already studied. The Higgs is a little more sociable than the known fermions and gauge bosons, which means that it’s more likely to interact with the new particles. That’s the sense in which our discovery of the Higgs is both the completion of one grand project—constructing the Standard Model—but also the beginning of the next—finding hidden worlds beyond that model. Wilczek and his collaborator Brian Patt have dubbed this possibility the “Higgs portal” between the Standard Model and hidden sectors of matter.

In discussing Higgs detection in Chapter Nine, I drew attention to the decay of the Higgs into two photons, which was mediated by a loop of virtual particles. The actual rate at which such a process occurs depends on all the different particles that can possibly appear in that loop—that is, particles that couple both to the Higgs and to photons. In the Standard Model itself, this rate is completely fixed once we know the Higgs mass. Therefore, if we carefully measure this decay and find that it proceeds more rapidly than we expect, that serves as strong evidence for the existence of new particles, even if we don’t see them directly. The LHC data from 2011 and early 2012 seemed to indicate that more photons were being produced than the Standard Model predicts, even though the difference was not extremely significant. That’s certainly something to be watching for as more data are collected.

In the WIMP scenario, dark matter is all around us, even right where you’re sitting this moment. In our local environment, we expect there to be roughly one dark-matter particle per coffee-cup-size volume of space. But the particles are moving quite rapidly, typically at hundreds of kilometers per second. As a result, billions of WIMPs pass through your body every second. Because they interact so weakly, you hardly notice; most WIMPs literally go right through you without ever interacting. But although the interactions are small, they’re not quite zero. By exchanging a Higgs boson, a WIMP can bump into one of the quarks contained in the protons and neutrons inside your body. Physicists Katherine Freese and Christopher Savage have calculated that in reasonable models, we expect about ten dark-matter particles to interact with the atoms in a typical human body every year. The effects of every individual interaction are pretty negligible, so don’t worry about getting a dark matter stomachache.

A Feynman diagram representing a dark-matter particle scattering off a quark by exchanging a Higgs boson.

We can, however, use this kind of interaction to search for dark matter. Just like in the LHC, a crucial task is to separate signals from background noise. Dark matter isn’t the only thing that can bump into a nucleus; radioactivity and cosmic rays do it all the time. Physicists therefore go deep underground, into mine shafts and specially built facilities, where they are as shielded from these pesky backgrounds as possible. They then build detectors that patiently wait for the faint signal of a dark-matter particle passing through and perturbing a nucleus. Two types of detectors are popular: cryogenic, where the detector registers the heat created when a dark-matter particle collides with a nucleus inside a low-temperature crystal, and liquid noble gas, where the detector measures light produced through scintillation when a dark-matter particle interacts with liquid xenon or argon.

The strategy of going deep underground and searching for interactions with ambient dark-matter particles is known as “direct detection,” and is an ongoing high-priority research frontier. A number of experiments have already ruled out some of the possible models. Knowing the mass of the Higgs boson will help relate the theoretical predictions for WIMP properties to the possible signatures these experiments might see. With the sensitivity already impressive and rapidly improving, nobody should be surprised if we finally detect dark matter once and for all sometime in the next five years. Nor should anyone be surprised if we don’t; nature always has surprises.

Naturally, if there is a technique called “direct detection,” there is a different technique called “indirect detection.” The idea here is to wait for WIMPs in our galaxy or others to collide with one another and annihilate. Among the particles produced in such an interaction will be gamma rays (high-energy photons), which can be searched for using satellite observatories. Currently, NASA’s Fermi Gamma-ray Space Telescope is scanning the sky, observing gamma rays, and building up a database of high-energy phenomena. Once again, the problem of separating signal from noise is severe. Astronomers are working hard to understand what kind of gamma-ray signature should be produced by annihilating dark matter, in the hope of being able to pick it out from the many conventional astrophysical processes that produce this kind of radiation. It’s also possible that dark matter could annihilate into a Higgs boson (instead of into other particles via a Higgs boson), a scenario that has naturally been dubbed “Higgs in Space.”