Warped Passages - Lisa Randall [108]
Caveat
The Higgs mechanism works beautifully, and gives quarks, leptons, and weak gauge bosons their masses without making nonsensical high-energy predictions—and, furthermore, explains how the photon came to be. However, there is one essential property of the Higgs particle that physicists don’t yet fully understand.
Electroweak symmetry must be broken at about 250 GeV to give particles their measured masses. Experiments show that particles with energy greater than 250 GeV look as if they are massless, whereas particles with energy below 250 GeV act as if they have mass. However, the electroweak symmetry will break at 250 GeV only if the Higgs particle (sometimes also called the Higgs boson)20 itself has roughly this mass (again, using E = mc2); the theory of the weak force wouldn’t work if the Higgs mass were much greater. If the Higgs mass were greater, symmetry breaking would happen at a higher energy and the weak gauge bosons would be heavier—contradicting experimental results.
However, in Chapter 12, I will explain why a light Higgs particle poses a major theoretical problem. Calculations that take quantum mechanics into account point to a heavier Higgs particle, and physicists don’t yet know why the Higgs particle mass should be so low. This quandary is critical to motivating new particle physics ideas and some of the extra-dimensional models that we’ll consider later on.
Even without knowing the precise nature of the Higgs particle and the reason why it is so light, the mass requirement tells us that the Large Hadron Collider, which will start operating at CERN in Switzerland within the decade, should discover one or more crucial new particles. Whatever breaks electroweak symmetry must have a mass that is around the weak scale mass. And we expect that the LHC will find out what it is. When it does, this critically important discovery will greatly advance our knowledge of the underlying structure of matter. And it will also tell us which (if any) of the proposals for explaining the Higgs particle is correct.
But before we get to those proposals, we’ll look at one possible extension of the Standard Model that was suggested purely in the interest of simplicity of nature. The next chapter explores virtual particles, the distance dependence of forces, and the fascinating topic of grand unification.
What to Remember
Despite the importance of symmetries for making the right predictions about high-energy particles, the masses of quarks, leptons, and weak gauge bosons tell us that the weak force symmetry must be broken.
Because we still have to guard against false predictions, the weak force symmetry must nonetheless be maintained at high energy. Therefore, the weak force symmetry must be broken only at low energy.
Spontaneous symmetry breaking occurs when all physical laws preserve a symmetry but the actual physical system does not. Spontaneously broken symmetries are symmetries that are preserved at high energies but broken at low energies. The weak force symmetry is spontaneously broken.
The process by which weak force symmetry is spontaneously broken is the Higgs mechanism. For the Higgs mechanism to spontaneously break the weak force symmetry, there has to be a particle with a mass of about the weak scale mass, which is 250 GeV (remember, special relativity relates energy and mass through E = mc2).
11
Scaling and Grand Unification: Relating Interactions at Different Lengths and Energies
I hope someday you’ll join us
And the world will live as one.
John Lennon
Athena often felt like she was the last to be told anything interesting. She didn’t even hear about Ike’s adventures with his car until after he had owned it for over a month. And she