Rare particle decay could mean new physics

The work, which is generating buzz because of its possible implications for the existence of new physics, has been submitted to Physical Review Letters by an international team of scientists, among them Julia Thom-Levy, Cornell assistant professor of physics, and graduate student Walter Hopkins. The paper is available on arXiv.

Thom-Levy studies the decay of particles formed in high-energy collisions, with particular focus on a class of particles called strange B-mesons that consist of a beauty quark bound to a strange quark. The Standard Model of Physics predicts the rate of strange B-mesons decaying into a pair of oppositely charged muons as exceedingly rare, with only a few decays out of 350 trillion collisions expected.

In the new data from Fermilab's Tevatron particle accelerator, the researchers found four of these decays for every one expected. While the upward fluctuation could be a statistical fluke, it is attracting widespread attention in the field because it might possibly indicate the presence of new particles and lead to an entirely new model of physics.

Over the years, as scientists were able to collide more particles at higher energies, they were able to set upper limits on the probability of such decays. The Fermilab data set has allowed scientists to further narrow the probability of the rate of decay to be between 0.46 and 3.9 x 10^-9.

One possible explanation for the observed excess is the existence of supersymmetric particles, for which no evidence yet exists, but which can help explain the origin of dark matter, which makes up about one fourth of the universe.

"So this could mean an excess, and it could mean the presence of new particles, but what is needed is a lot more data to really have enough numbers to constrain this more strongly," Thom-Levy said.

Such a larger data set is being worked on now by scientists, including Thom-Levy, at the Large Hadron Collider at CERN, which runs at higher energies than the Tevatron and may eventually confirm the excess in the Fermilab data.

More information: Search for B_s --> mu+mu- and B_d --> mu+mu- Decays with CDF II, arXiv:1107.2304v1 [hep-ex] http://arxiv.org/a … /1107.2304v1

Abstract
A search has been performed for B_s --> mu+mu- and B_d --> mu+mu- decays using 7/fb of integrated luminosity collected by the CDF II detector at the Fermilab Tevatron collider. The observed number of B_d candidates is consistent with background-only expectations and yields an upper limit on the branching fraction of BF(B_d-->mu+mu-) < 6.0E-9 at 95% confidence level. We observe an excess of B_s candidates. The probability that the background processes alone could produce such an excess or larger is 0.27%. The probability that the combination of background and the expected standard model rate of B_s --> mu+mu- could produce such an excess or larger is 1.9%. These data are used to determine BF(B_s-->mu+mu-) = (1.8^{+1.1}_{-0.9})E-8 and provide an upper limit of BF(B_s -->mu+mu-) < 4.0E-8 at 95% confidence level.

Provided by Cornell University