Preposterous Universe

Thursday, June 10, 2004
He ain't heavy, he's my boson

It looks like the Higgs boson, the only part of the Standard Model of particle physics that has not yet been directly detected, might be heavier than we thought, and correspondingly even more difficult to detect. This, anyway, is the claim of a new analysis from the D0 experiment, one of the two (along with CDF) large general-purpose detectors at the Tevatron particle accelerator at Fermilab. (See also comments from David Harris.) What they've actually done is to improve their measurement of the mass of the top quark, the penultimate particle of the Standard Model, discovered at Fermilab in 1995. Through the miracle of quantum mechanics, the properties of all the different particles of the Standard Model come into calculating the rates of various interactions; so given what we know about certain interactions, we can infer the mass of the Higgs if we very accurately know the mass of the top.

Of course, there are a lot of assumptions that go into such an inference. Personally, I tend not to trust them; the history of physicists predicting the mass of particles like the top and the Higgs is not filled with spectacular successes. Usually we don't come up with a convincing explanation until after we've finally measured it.

Fermilab's Tevatron is currently the highest-energy particle accelerator in the world, and two of its major goals are to find the Higgs and to find something completely outside the Standard Model, such as supersymmetry or extra dimensions. Competition is on its way, from the Large Hadron Collider at CERN in Geneva, which is scheduled to turn on in 2007. The LHC will have a significantly higher energy than the Tevatron, and should easily be able to detect something new and interesting, whatever that may be. There is a "nightmare scenario" in which the LHC discovers the Standard-Model Higgs and nothing else, giving us no clues about new physics; I'll put long odds against it, though.

Particle physics is desperate for an experimental discovery that isn't already accommodated by the Standard Model; no particle accelerator has given us such a surprise since, oh, the mid-Seventies. This drought has caused a great deal of anxiety and hand-wringing, but it is certainly the exception rather than the rule; more often in physics the experiments are running far ahead of the theories, and we're scrambling to explain all of the new phenomena being uncovered. I suspect we'll be rapidly back into such a phase once the LHC comes online, if not sooner. (For any physics undergraduates or beginning graduate students out there who are worried about job prospects and wondering what field to go into, my advice would be particle phenomenology. In the five-year timescale to get a Ph.D., the field will go from being listless and anticipatory to rambunctious and contemporary, and good young people will be in huge demand.)

Ideas on culture, science, politics.
Sean Carroll

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