Knocking on Heaven's Door (44 page)

So the Higgs mechanism explains why it is the photon and not the other force carriers that has zero mass. It also explains one other property of masses. This next lesson is even a bit more subtle, but gives us deep insights into why the Higgs mechanism allows masses that are consistent with sensible high-energy predictions. If we think of the Higgs field as a fluid, we can imagine that its density is also relevant to particle masses. And if we think of this density as arising from charges with a fixed spacing, then these particles—which travel such small distances that they never hit a weak charge—will travel as if they had zero mass, whereas particles that travel over larger distances would inevitably bounce off weak charges and slow down.

This corresponds to the fact that the Higgs mechanism is associated with
spontaneous breaking
of the symmetry associated with the weak force—and that symmetry breaking is associated with a definite scale.

Spontaneous breaking of a symmetry occurs when the symmetry it-self is present in the laws of nature—as with any theory of forces—but is broken by the actual state of a system. As we’ve argued, symmetries must exist for reasons connected to the high-energy behavior of particles in the theory. The only solution then is that the symmetries exist—but they are spontaneously broken so that the weak gauge bosons can have mass, but not exhibit bad high-energy behavior.

The idea behind the Higgs mechanism is that the symmetry is indeed part of the theory. The laws of physics act symmetrically. But the actual state of the world doesn’t respect the symmetry. Think of a pencil that originally stood on end and then falls down and chooses one particular direction. All of the directions around the pencil were the same when it was upright, but the symmetry is broken once the pencil falls. The horizontal pencil thereby spontaneously breaks the rotational symmetry that the upright pencil preserved.

The Higgs mechanism similarly spontaneously breaks weak force symmetry. This means that the laws of physics preserve the symmetry, but it is broken by the state of the vacuum that is suffused with weak force charge. The Higgs field, which permeates the universe in a way that is not symmetric, allows elementary particles to acquire mass, since it breaks the weak force symmetry that would be present without it. The theory of forces preserves a symmetry associated with the weak force, but that symmetry is broken by the Higgs field that suffuses the vacuum.

By putting charge into the vacuum, the Higgs mechanism breaks the symmetry associated with the weak force. And it does so at a particular scale. The scale is set by the distribution of charges in the vacuum. At high energies, or equivalently—via quantum mechanics—small distances, particles won’t encounter any weak charge and therefore behave as if they have no mass. At small distances, or equivalently high energies, the symmetry therefore appears to be valid. At large distances, however, the weak charge acts in some respects like a frictional force that would slow the particles down. Only at low energies, or equivalently large distances, does the Higgs field seem to give particles mass.

And this is exactly as we need it to be. The dangerous interactions that wouldn’t make sense for massive particles apply only at high energies. At low energies particles can—and must, according to experiments—have mass. The Higgs mechanism, which spontaneously breaks the weak force symmetry, is the only way we know to accomplish this task.

Although we have not yet observed the particles responsible for the Higgs mechanism that is responsible for elementary particle masses, we do have experimental evidence that the Higgs mechanism applies in nature. It has already been seen many times in a completely different context—namely, in
superconducting
materials. Superconductivity occurs when electrons pair up and these pairs permeate a material. The so-called
condensate
in a superconductor consists of electron pairs that play the same role that the Higgs field does in our example above.

But rather than carry weak charge, the condensate in a superconductor carries electric charge. The condensate therefore gives mass to the photon that communicates electromagnetism inside the superconducting material. The mass
screens
the charge, which means that inside a superconductor, electric and magnetic fields do not reach very far. The force falls off very quickly over a short distance. Quantum mechanics and special relativity tell us that this
screening distance
inside a superconductor is the direct result of a photon mass that exists only inside the superconducting substrate. In these materials, electric fields can’t penetrate farther than the screening distance because in bouncing off the electron pairs that permeate the superconductor, the photon acquires a mass.

The Higgs mechanism works in a similar fashion. But rather than electron pairs (carrying electric charge) permeating the substance, we predict there is a Higgs field (that carries weak charge) that permeates the vacuum. And instead of a photon acquiring mass that screens electric charge, we find the weak gauge bosons acquire mass that screens weak charge. Because weak gauge bosons have nonzero mass, the weak force is effective only over very short distances of subnuclear size.

Since this is the only consistent way to give gauge bosons masses, physicists are fairly confident that the Higgs mechanism applies in nature. And we expect that it is responsible not just for the gauge boson masses, but for the masses of all elementary particles. We know of no other consistent theory that permits the Standard Model weakly charged particles to have mass.

This was a difficult section with several abstract concepts. The notions of a Higgs mechanism and a Higgs field are intrinsically linked to quantum field theory and particle physics and are remote from phenomena we can readily visualize. So let me briefly summarize some of the salient points. Without the Higgs mechanism, we would have to forfeit sensible high-energy predictions or particle masses. Yet both of these are essential to the correct theory. The solution is that symmetry exists in the laws of nature, but can be spontaneously broken by the nonzero value of a Higgs field. The broken symmetry of the vacuum allows Standard Model particles to have nonzero masses. However, because spontaneous symmetry breaking is associated with an energy (and length) scale, its effects are relevant only at low energies—the energy scale of elementary particle masses and smaller (and the weak length scale and bigger). For these energies and masses, the influence of gravity is negligible and the Standard Model (with masses taken into account) correctly describes particle physics measurements. Yet because symmetry is still present in the laws of nature, it allows for sensible high-energy predictions. Plus, as a bonus, the Higgs mechanism explains the photon’s zero mass as a result of its not interacting with the Higgs field spread throughout the universe.

However, successful as they are theoretically, we have yet to find experimental evidence that confirms these ideas. Even Peter Higgs has acknowledged the importance of such tests. In 2007, he said that he finds the mathematical structure very satisfying but “if it’s not verified experimentally, well, it’s just a game. It has to be put to the test.”
⁶⁰
Since we expect that Peter Higgs’ proposal is indeed correct, we anticipate an exciting discovery within the next few years. The evidence should appear at the LHC in the form of a particle or particles, and, in the simplest implementation of the idea, the evidence would be the particle known as the
Higgs boson
.

THE SEARCH FOR EXPERIMENTAL EVIDENCE

“Higgs” refers to a person and to a mechanism, but to a putative particle as well. The Higgs boson is the key missing ingredient of the Standard Model.
⁶¹
It is the anticipated vestige of the Higgs mechanism that we expect that LHC experiments will find. Its discovery would confirm theoretical considerations and tell us that a Higgs field indeed permeates the vacuum. We have good reasons to believe the Higgs mechanism is at work in the universe, since no one knows how to construct a sensible theory with fundamental particle masses without it. We also believe that some evidence for it should soon appear at the energy scales the LHC is about to probe, and that evidence is likely to be the Higgs boson.

The relationship between the Higgs field, which is part of the Higgs mechanism, and the Higgs boson, which is an actual particle, is subtle—but is very similar to the relationship between an electromagnetic field and a photon. You can feel the effects of a classical magnetic field when you hold a magnet close to your refrigerator, even though no actual physical photons are being produced. A classical Higgs field—a field that exists even in the absence of quantum effects—spreads throughout space and can take a nonzero value that influences particle masses. But that nonzero value for the field can also exist even when space contains no actual particles.

However, if something were to “tickle” the field—that is, add a little energy—that energy could create fluctuations in the field that lead to particle production. In the case of an electromagnetic field, the particle that would be produced is the photon. In the case of the Higgs field, the particle is the Higgs boson. The Higgs field permeates space and is responsible for electroweak symmetry breaking. The Higgs particle, on the other hand, is created from a Higgs field where there is energy—such as at the LHC. The evidence that the Higgs field exists is simply that elementary particles have mass. The discovery of a Higgs boson at the LHC (or anywhere else it could be produced) would confirm our conviction that the Higgs mechanism is the origin of those masses.

Sometimes the press calls the Higgs boson the “God particle,” as do many others who seem to find the name intriguing. Reporters like the term because people pay attention, which is why the physicist Leon Lederman was encouraged to use it in the first place. But the term is just a name. The Higgs boson would be a remarkable discovery, but not one whose moniker should be taken in vain.

Although it might sound overly theoretical, the logic for the existence of a new particle playing the role of the Higgs boson is very sound. In addition to the theoretical justification mentioned above, consistency of the theory with massive Standard Model particles requires it. Suppose only particles with mass were part of the underlying theory, but there was no Higgs mechanism to explain the mass. In that case, as the earlier part of the chapter explained, predictions for the interactions of high-energy particles would be nonsensical—and even suggest probabilities that are greater than one. Of course we don’t believe that prediction. The Standard Model with no additional structures has to be incomplete. The introduction of additional particles and interactions is the only way out.

A theory with a Higgs boson elegantly avoids high-energy problems. Interactions with the Higgs boson not only change the prediction for high-energy interactions, they exactly cancel the bad high-energy behavior. It’s not a coincidence, of course. It’s precisely what the Higgs mechanism guarantees. We don’t yet know for sure that we have correctly predicted the true implementation of the Higgs mechanism in nature, but physicists are fairly confident that some new particle or particles should appear at the weak scale.

Based on these considerations, we know that whatever saves the theory, be it new particles or interactions, cannot be overly heavy or happen at too high an energy. In the absence of additional particles, flawed predictions would already emerge at energies of about 1 TeV. So not only should the Higgs boson (or something that plays the same role) exist, but it should be light enough for the LHC to find. More precisely, it turns out that unless the Higgs boson is less than about 800 GeV, the Standard Model would make impossible predictions for high-energy interactions.

In reality, we expect the Higgs boson to be a good deal lighter than that. Current theories favor a relatively light Higgs boson—most theoretical clues point to a mass just barely in excess of the current mass bound from the LEP experiments of the 1990s, which is 114 GeV. That was the highest-mass Higgs boson LEP could possibly produce and detect, and many people thought they were on the verge of finding it. Most physicists today expect the Higgs boson mass to be very close to that value, and probably no heavier than about 140 GeV.

The strongest argument for this expectation of a light Higgs boson is based on experimental data—not simply searches for the Higgs boson itself, but measurements of other Standard Model quantities. Standard Model predictions accord with measurements spectacularly well, and even small deviations could affect this agreement. The Higgs boson contributes to Standard Model predictions through quantum effects. If it’s overly heavy, those effects would be too large to get agreement between theoretical predictions and data.

Recall that quantum mechanics tells us that virtual particles contribute to any interaction. They briefly appear and disappear from whatever state you started with and contribute to the net interaction. So even though many Standard Model processes don’t involve the Higgs boson at all, Higgs particle exchange influences all the Standard Model predictions, such as the rate of decay of a
Z
gauge boson to quarks and leptons and the ratio between the
W
and
Z
masses. The size of the Higgs’s virtual effects on these
precision electroweak
tests depends on its mass. And it turns out the predictions work well only if the Higgs mass is not too big.

The second (and more speculative) reason to favor a light Higgs boson has to do with a theory called supersymmetry that we’ll turn to shortly. Many physicists believe that supersymmetry exists in nature, and according to supersymmetry, the Higgs boson mass should be close to that of the measured
Z
gauge boson’s and hence relatively light.

So given the expectation that the Higgs boson is not very heavy, you can reasonably ask why we have seen all the Standard Model particles but we have not yet seen the Higgs boson. The answer lies in the Higgs boson’s properties. Even if a particle is light, we won’t see it unless colliders can make it and detect it. The ability to do so depends on its properties. After all, a particle that didn’t interact at all would never be seen, no matter how light it was.