Penguins Attack Standard Model

Rara avis. The "penguin diagram" describes B-meson decay according to the Standard Model, but new measurements suggest that nonstandard particles are involved.

BATAVIA, ILLINOIS--Will a rogue penguin demolish the Standard Model of particle physics? Scientists at the Lepton Photon 2003 meeting here hope so. Last week, researchers from Japan told startled colleagues that a peculiar type of particle alchemy known as a "penguin" may have revealed holes in the reigning theory.

The experiment investigates the decay of a middleweight particle called a B meson, which contains a "bottom" quark. Once in a long while, the bottom quark spontaneously decays into two very massive objects: a "top" quark, the heaviest of the fundamental components of matter, and a W boson, a massive particle that carries the "weak" force. The W and top quark are transformed into three "strange" quarks; when diagrammed with a standard physics shorthand, this transformation looks vaguely like a penguin (see figure). The Standard Model makes firm predictions about how often and in which contexts these penguins appear.

Since it began operating 4 years ago, the KEK B factory in Tsukuba, which smashes electrons into their antimatter counterparts, has created more than 150 million B mesons. By studying how these B mesons decay, particle physicists in Japan and at a similar facility at the Stanford Linear Accelerator Laboratory (SLAC) in California measure numbers that dictate how quarks transform into one another, a key aspect of the Standard Model of particle physics. One of those pillars is the expression sin 2b. In 2001, the BaBar experiment at SLAC announced evidence that sin 2b was lower than the Standard Model predicted, but measurements from KEK B seemed to bring the value back in line (Science, 23 February 2001, p. 1471). No longer.

To the audience's surprise, KEK collaborator Thomas Browder of the University of Hawaii, Manoa, announced that the latest value of sin 2b was much lower even than BaBar's value. The difference might indicate that undiscovered particles subtly alter the penguin for this kind of B decay, changing the effective value of sin 2b. And this would probably mean that scientists have gotten the first glimpse of a shadowy mirror realm where each particle in the Standard Model has a "supersymmetric" partner; the size of the particle zoo would have to be doubled.

"A lot of people are not just stunned by this, but shocked," says Ikaros Bigi of the University of Notre Dame, Indiana. Before declaring that physics has pushed beyond the Standard Model, however, scientists must collect more B decays to make sure that the effect is real. The picture should become much clearer by this time next year, says Marcello Giorgi, a physicist at INFN in Pisa, Italy, and member of the BaBar group. "By next summer, there could be an announcement of new physics."

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