Warped Passages - Lisa Randall [107]
The Higgs mechanism functions in exactly the way that is needed to ensure that any theory that incorporates it can have massive weak gauge bosons (as well as massive quarks and leptons) and nonetheless will make the correct predictions for high-energy behavior. Specifically, for high-energy weak gauge bosons—those with energy larger than 250 GeV—symmetry appears to be preserved, so there are no incorrect predictions. At high energies the internal symmetry associated with the weak force still filters out the problematic polarization of the weak gauge boson that would cause interactions at too high a rate. But at low energies, where the mass is essential to reproducing the measured short-range interactions of the weak force, the weak force symmetry is broken.
This is why the Higgs mechanism is so important. No other theory that assigns these masses has these properties. Other approaches fail either at low energies, where the mass will be wrong, or at high energies, where interactions will be predicted incorrectly.
Bonus
There is one more successful feature of the Standard Model that I have not yet explained. Although the Higgs field will be relevant to the next few chapters, this particular aspect of the Higgs mechanism will not. Yet it is so surprising and fascinating that it’s worth mentioning.
The Higgs mechanism tells us about more than just the weak force. Surprisingly, it also gives new insight into why electromagnetism is special. Until the 1960s, no one would have believed that there was more to learn about the electromagnetic force, which had been so well studied and understood for over a century. In the 1960s, however, the electroweak theory proposed by Sheldon Glashow, Steven Weinberg, and Abdus Salam showed that when the universe began its evolution at high temperature and energy, there were three weak gauge bosons, plus a fourth, independent, neutral boson with a different interaction strength. The photon, ubiquitous and important as it is today, was not a member of this list. The authors of the electroweak theory deduced the nature of the four weak gauge bosons from both mathematical and physical clues, which I won’t go into here.
The remarkable thing is that the photon was originally nothing special. In fact, the photon we talk about today is actually a mixture of two of the original four gauge bosons. The reason that the photon got singled out is that it is the only gauge boson involved in the electroweak force that is impervious to the weak charge of the vacuum. The chief distinguishing feature of the photon is that it travels unfettered through the weakly charged vacuum and therefore has no mass.
Photon travel, unlike that of the W and Z, is not obstructed by the nonzero value of a Higgs field. That’s because although the vacuum carries weak charge, it does not carry electric charge. The photon, which communicates the electromagnetic force, interacts only with electrically charged objects. For this reason, the photon can communicate a long-range force without any interference from the vacuum. It is therefore the only gauge boson that remains massless even in the presence of the nonzero Higgs field.
The situation closely resembles the speed traps with which Ike had to contend (although this part of the analogy is admittedly a little more tenuous). The speed traps let dull cars pass through scot-free. Photons, like the dull neutral cars, always travel unimpeded.
Who would have thought it? The photon, the thing that physicists for years thought they understood completely, has an origin that can be understood only in terms of a more complex theory that combines the weak and electromagnetic forces into a single theory. This theory is therefore generally referred to as the electroweak theory, and the relevant symmetry is called electroweak symmetry. The electroweak theory and the Higgs mechanism are