Rare and Beautiful Decays

September 10, 2004

The aim of particle physics is to understand the fundamental forces governing Nature and discover the basic constituents of matter. The best description to date is contained in a theory now called the Standard Model.
We already know now that the Standard Model is not the final answer to all our questions since there are phenomena in particle physics for which the Standard Model makes predictions that contradict basic physical principles. In addition, it describes many phenomena but does not explain why these phenomena occur. Many people have come up with new ideas and new theories which try to answer these questions.

One example where a large deviation from the Standard Model could occur is a rare decay of the so-called Bs particle that consists of an anti-beauty (b(bar)) quark and a strange(s) quark. These two quarks cannot combine directly to form a muon and anti-muon; however, our Standard Model predicts that it is still possible to have such a decay, but only through a more involved web of intermediate particles. All this occurs in such a short time that Heisenberg's Uncertainty Principle allows the intermediate particles to be more massive than either the Bs or muons. These additional steps involving heavy particles lead to this particular decay being very rare, i.e., the Standard Model predicts that roughly only one out of 300 million Bs particles will decay this way! If other new massive particles due to new physics not described by the Standard Model exist, then they can possibly participate in these intermediate steps and dramatically increase the probability that the Bs can decay into a muon and anti-muon.

The left plot below shows the mass spectrum collected from individual proton-antiproton collisions recorded with the Dě detector. Pairs of muons are created in many of these collisions. The peaks seen in the data indicate the presence of known particles decaying into a muon pair.

The particle of lowest mass in the plot is the omega meson, which consists of a combination of up, antiup, down and antidown quarks. The phi (consisting of strange-antistrange quarks), J/psi and psi'(charm-anticharm quark pair), and the Upsilon(1S,2S,3S) (bottom-antibottom pair) can also be observed. The plot also shows a hypothetical signal peak for the B_s decay to muon pairs, if the theoretical value calculated from the Standard Model would be 100,000 larger.

After combining data taken during years 2002-2004 with the DZero detector, four candidates were observed as shown above on the left side, while 3.7 +/- 1.1 candidates were predicted to come from background, i.e., random muons that when combined accidentally appear to come from Bs decays. From this observation, we know that less than 1 in roughly 2.2 million Bs particles decay into a pair of muons. Although this measurement is not yet sensitiv enough to observe the tiny rate predicted by the Standard Model, it still rules out many theoretical models predicting particles that would result in a rate larger than we observe in our data.

For further information contact Dr. Frank Lehner, or Ralf Bernhard, University of Zurich, email: lehnerf@fnal.gov, ralf@fnal.gov.