Search for the Rare Decays Bs(d) → &mu+ &mu-

This page summarizes an analysis which searches for the rare decays Bs → μμ and Bd → μμ . These results were finalized on August 9th, 2007 and use a dataset of 1.9 fb-1.
This analysis supersedes the analyses performed with 365 pb-1 PRL 95, 221805 2005 (webpage) and 171pb-1 PRL 93, 032001 2004 (webpage).


A complete description is available at CDF Public note


Searches for the decays Bs → μμ and Bd → μμ can be a powerful tool to probe for physics beyond the SM. The SM predicts a BR( Bs → μμ ) of about 3.8x10-9. There are various extensions to the SM that predict an enhancement of this branching ratio by 1 to 3 orders of magnitude. Many of these models are, presently, theoretically very interesting since some have some relevance to the deviation of the muon (g-2), neutrino oscillations, and dark matter/dark energy results.

Here are two plots from Phys.Lett. B624, 47 2005 showing the excluded region for BR( Bs → μμ ) < 2.0x10-7 @95% CL and BR( Bs → μμ ) < 0.5x10-7 @95% CL in the (M0,M1/2) plane for a Constrained MSSM model. Blue regions are excluded by BR( Bs → μμ ), green regions by BR( B → s γ), and cyan regions by relic density constraints.

A second interesting class of models that predicts enhanced rates of Bs → μμ decay are SO(10) symmetry breaking models. This plot from (hep-ph/0507233) shows BR(Bs->&mu&mu) in the (&mu,M1/2) plane. Dashed blue lines are are contours of constant BR( Bs → μμ ), dashed black lines are are contours of constant higgs mass, green areas are regions of parameter space consistent with the observed relic density of dark matter and pink ares are excluded. A listing of relevant theory papers is below.

Result: World's best limits

BR( Bs → μμ ) < 5.8x10-8 @95% CL
BR( Bd → μμ ) < 1.8x10-8 @95% CL

BR( Bs → μμ ) < 4.7x10-8 @90% CL
BR( Bd → μμ ) < 1.5x10-8 @90% CL

The signal windows (Bs in red and Bd in blue) for the most sensitive NN bin (NN>0.995) are shown.


Below the eps and gif versions of all figures from the public conference note available for download. This analysis is divided into two channels for events found using only the central muon system(CMU), U-U events, and events where one muon is found in the forward muon extension(CMX), U-X events.

Experimental Papers and Status

Below is a comparison of this result to relevant experimental papers:

Theory Papers

Below are links to some relevant theory papers: Example Feynman diagrams: