A Fermilab experiment that began as an attempt to detect CP violation has made an observation that may signify major advances in physicists' knowledge of supersymmetry. In a paper published in Physical Review Letters and a presentation at Fermilab by HyangKyu Park of the University of Michigan, the HyperCP experiment at Fermilab (E871)--a collaboration of 33 physicists and 10 institutions--reports that it has found evidence for the rare decay of a Sigma-plus hyperon into a proton and two muons. This decay is allowed according to the Standard Model of particles and interactions. However, the properties of the three decay events recorded by the HyperCP collaboration may imply that this decay proceeds through a neutral intermediate state, which--if it exists--would be consistent with a supersymmetric particle called a sgoldstino.

"If this is confirmed, every type of high energy physics will be affected," said Park. "A new physics model would change everything, and although we have investigated several possibilities for what the intermediate particle could be, it seems to fit with the framework of supersymmetric model."

By observing the decay of the Sigma-plus (made from a strange quark and two up quarks) into a proton and two muons, the HyperCP physicists measured the smallest-ever branching ratio of a baryon decay. The mass distribution of the two muons was much narrower (1 MeV difference) than what the Standard Model predicts (40 MeV difference, or between 210-250 MeV), creating a buzz among experimenters at Fermilab. The dimuon's narrow mass distribution range suggests an intermediate particle state between the transition from the Sigma-plus to the two muons, which could be the sgoldstino. If the intermediate state exists, its mass would be 214.3 MeV, published in the HyperCP paper on January 21.

"The probability of finding such a narrow dimuon mass distribution is less than one percent," said Mike Longo, physics professor at the University of Michigan and head of the Michigan group. "If the sgoldstino is the correct particle, then this is the first observation and first experimental indication of supersymmetry."

The Sigma-plus had been observed in the '60s and '70s using bubble chambers. The HyperCP experiment uses new technology--a spectrometer--that exceeds the capability of the early experiments to identify Sigma-plus particles and their decay products. In order to achieve the goal of a sensitivity of 2x10^-4 in the search for CP violation in Lambda and Xi-minus decays, many billions of hyperons were produced in the HyperCP spectrometer, including an enormous sample of Sigma-plus hyperons. The spectrometer recorded 100,000 events per second, and the 120 terabytes of data it collected was 25 times the data on the entire Internet at the beginning of the analysis in 2001. However, this vast amount of data made it extremely challenging to identify individual Sigma-plus hyperons.

"HyangKyu came up with a brilliant way to do the normalization that minimized the uncertainty in counting the number of Sigmas we'd produced," said Kam-Biu Luk, Physics Professor at the University of California at Berkeley and co-leader of the experiment. Sigma-plus hyperons often prefer to decay to a proton and neutral pion, but HyperCP's detector couldn't observe neutral particles. Park realized that the short-lived pion sometimes alternatively decays into an electron, positron and gamma, and the first two of these particles could be observed. This decay gave the physicists a normalization mode so that they could infer the number of Sigma-plus hyperons by their parent particles.

Because the collaboration observed only three events from Sigma-plus decays, it hopes other experiments will provide better statistics to confirm that the sgoldstino exists. Although Park searched literature of other experiments and noticed consistencies that point to the sgoldstino, the team hasn't made any measurement besides the mass of the three sgoldstino candidate events.

"It's as if three students walk into a room, and you ask them what their birthdays are, and they all have the same birthday," said Craig Dukes, physics professor at the University of Virginia and co-leader of the experiment. "You think, are they triplets, or what? Even though we've only detected three events, their similarities seem to be more than a coincidence."

The HyperCP group can not repeat their experiment at Fermilab because the lab can no longer deliver 800 GeV protons to the Meson Detector Area, where fixed-target experiments such as HyperCP are mounted. However, Fermilab's KTeV project and experiments at other labs will search for the sgoldstino in their data.

"If the new physics in the unusual Sigma decay is related to strange quarks, then we should be able to see it, too," said Bob Tschirhart, co-spokesperson for the KTeV experiment. Kaons (made from an anti-strange quark and either an up or down quark or their antiparticles) can decay into two muons and either one or two pions. The KTeV experiment has the capability of looking at rare kaon decays to one in 10 billion, which is why kaons are sometimes called "the sensitivity frontier."

Other experiments that will look for similar events include Belle at KEK and BaBar at SLAC (using D and B mesons), and CLEO-c at Cornell (examining J/psi particles).

-Lisa Zyga