Physicists at Brookhaven National Laboratory have observed an exceedingly rare decay of a particle called a charged kaon, ending a 15-year quest. The decay, which occurs just once for every 10 billion ordinary decays, is the rarest ever detected and seems, so far, to mark yet another victory for the so-called Standard Model of particle interactions. The rate of the strange decay, reported in today's issue of Physical Review Letters, should reveal important information about the decay of the recently discovered top quark, which plays a role behind the scenes.
Charged kaons are subatomic particles that typically decay in about 10 billionths of a second, creating other exotic particles called muons and muon neutrinos. This breakdown takes places when the particle sheds either a single W or Z boson, particles that carry the weak force responsible for radioactive decays. But theory also allows for a complex dance in which a kaon momentarily creates two bosons (two W's, or a W and a Z) in addition to a top quark, which all come together again to make a pi meson and pair of neutrinos. "It's a very elusive decay," says Laurence Littenberg, a spokesman for the international group of 50 researchers, "because you can't detect the neutrinos," and the pi meson is easy to mistake for particles from more common events.
So the team built a detector specialized for measuring the energy and momentum of the pi meson and its own decay products. After observing the decays of about 1.5 trillion kaons from Brookhaven's Alternating Gradient Synchrotron, the team sifted through a carefully chosen subset and found one oddball. A lone pi meson had traveled through the detectors, giving off a diagnostic double pulse of energy in the scintillators. "It's an absolutely beautiful event," says spokesman Douglas Bryman of TRIUMF, Canada's National Meson Research Facility in Vancouver, British Columbia.
While the decay will certainly shine some light on the mysteriously heavy top quark, Bryman says, the rate they are seeing now looks a little high--opening up the intriguing possibility that a new, unseen particle may be facilitating the decay. But Bob Hsiung, a physicist who studies rare kaon decays at Fermilab, has his doubts: If the decay had happened much more frequently, "that would leave a lot of room for new physics." The truth may come out when the Brookhaven group analyzes the remaining three-quarters of its data.