Warped Passages - Lisa Randall [104]
The Higgs Mechanism
The Higgs mechanism involves a field that physicists call the Higgs field. As we have seen, the fields of quantum field theory are objects that can produce particles anywhere in space. Each type of field generates its own particular type of particle. An electron field is the source of electrons, for example. Similarly, a Higgs field is the source of Higgs particles.
As with heavy quarks and leptons, Higgs particles are so heavy that they aren’t found in ordinary matter. But unlike heavy quarks and leptons, no one has ever observed the Higgs particles that the Higgs field would produce, even in experiments performed at high-energy accelerators. This doesn’t mean that Higgs particles don’t exist, just that Higgs particles are too heavy to have been produced with the energies that experiments have explored so far. Physicists expect that if Higgs particles exist, we’ll create them in only a few years’ time, when the higher-energy LHC collider comes into operation.
Nevertheless, we are fairly confident the Higgs mechanism applies to our world, since it is the only known way to give Standard Model particles their masses. It is the only known solution to the problems that were posed in the previous section. Unfortunately, because no one has yet discovered the Higgs particle, we still don’t know precisely what the Higgs field (or fields) actually is.
The nature of the Higgs particle is one of the most hotly debated topics in particle physics. In this section, I will present the simplest of many candidate models—possible theories that contain different particles and forces—that demonstrates how the Higgs mechanism works. Whatever the true Higgs field theory turns out to be, it will implement the Higgs mechanism—spontaneously breaking the weak force symmetry and giving masses to elementary particles—in the same manner as the model I’m about to present.
In this model, a pair of fields experience the weak force. It will be useful later to think of these two Higgs fields, which are subject to the weak force, as carrying weak force charge. The Higgs mechanism terminology is sometimes sloppy, with “the Higgs” sometimes denoting the two fields together, and at other times one of the individual fields (and often the Higgs particles we hope to find). Here I will distinguish the possibilities and refer to the individual fields as Higgs1 and Higgs2.
Both Higgs1 and Higgs2 have the potential to produce particles. But they can also take nonzero values even when no particles are present. We haven’t encountered such nonzero values for quantum fields up to this point. So far, aside from the electric and magnetic fields, we have considered only quantum fields that create or destroy particles but take zero value in the absence of particles. But quantum fields can also have nonzero values, just like the classical electric and magnetic fields. And according to the Higgs mechanism, one of the Higgs fields takes a nonzero value. We will now see that this nonzero value is ultimately the origin of particle masses.17
When a field takes a nonzero value, the best way to think about it is to imagine space manifesting the charge that the field carries, but not containing any actual particles. You should think of the charge that the field carries as being present everywhere. This is, alas, a rather abstract notion because the field itself is an abstract object. But when the field takes a nonzero value, its consequences are concrete: the charge that a nonzero field would carry exists in the real world.
A nonzero Higgs field, in particular, distributes weak charge throughout the universe. It is as if the nonzero weak-charge-carrying Higgs field paints weak charge throughout space. A nonzero value for the Higgs fields means that the weak charge that Higgs1 (or Higgs2) carries is everywhere, even when there are no particles present. The vacuum—the state of the universe with no particles present—itself carries weak charge when one