Rare or medium rare?

The figure shows limits on the Bs decay rates at the Tevatron. CDF found at a 90 percent confidence level the rate is between 0.46 and 3.9 x 10-8. The central value is more than five times than predicted by the Standard Model.

These physicists were responsible for this analysis. Top row, from left: Satoru Uozumi and Daejung Kong, Kyungpook National University, Korea; Teruki Kamon, Texas A&M/KNU; Matthew Herndon and David Sperka, Wisconsin University. Bottom row, from left: Walter Hopkins and Julia Thom, Cornell University; Doug Glenzinski, Fermilab; Slava Krutelyov, University of California – Santa Barbara; and Cheng-Ju Kin, Lawrence Berkeley National Laboratory.

When particles decay, they frequently do so in only a few different ways. However, once in a while, particles can decay in an unusual way. It is in these rare instances that scientists can catch a glimpse of something that they normally wouldn’t otherwise see.

These decays are important because they can shed light on subatomic processes that scientists cannot observe directly, either here at the Tevatron or at the LHC. One example of such a rare decay is the decay of a Bs meson, which is composed of a b quark and an s quark, into a pair of muons (Bs→ µ+ and µ). The Standard Model predicts that the rate of this decay is so infrequent (3.2*10-9) that it would take more than 350 trillion collisions for scientists to see it.

So why look for it? The presence of new particles or new interactions can substantially increase how often these rare decays occur, making them worth studying. In fact, the Bs decay (Bs→ µ+ and µ) is sensitive to contributions from a wide variety of new physics. This makes this rare decay an excellent place to look for deviations from the Standard Model.

The earlier results of this important experiment appeared in Fermilab Today in March 2004 and September 2007 and in International Science Grid This Week in February 2008. In 2009, CDF set the upper limit on these rare Bs decays at 43 out of a billion. With the newest result, CDF has further reduced that upper limit to 39 decays per billion and has set for the first time a lower limit of more than 4.6 Bs meson decays per billion.

To get this result, a team of CDF physicists sifted through 7 inverse femtobarns of data searching for Bs mesons decaying into muon pairs. CDF physicists saw a slight excess in the data, which may provide us with the first hints of this elusive decay. If the excess is real, it would correspond to a decay rate that is somewhat larger than, but not inconsistent with the Standard Model prediction.

With the Tevatron and LHC experiments collecting more data, we’ll soon see if this excess stands the test of time.

A special Wine & Cheese seminar on this topic will take place at 2 p.m. today in the auditorium.

Click here to learn more about today’s CDF result.