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

This page summarizes an analysis which searches for the rare decays Bs → μμ and Bd → μμ . These results were finalized on March 16th, 2006 and use a dataset of 780 pb^-1. A public conference note on the analysis can be found at CDF Public note 8176.
This analysis supersedes the analyses performed with 365 pb^-1 PRL 95, 221805 2005 (webpage) and 171pb^-1 PRL 93, 032001 2004 (webpage).

Motivation

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, while the best current limit is: CDF Run 2, BR( B_s → μμ ) < 2.0x10^-7 @95% CL (PRL 95, 221805 2005). 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( B_s → μμ ) < 1.0x10^-7 @95% CL
BR( B_d → μμ ) < 3.0x10^-8 @95% CL

BR( B_s → μμ ) < 8.0x10^-8 @90% CL
BR( B_d → μμ ) < 2.3x10^-8 @90% CL

Figures

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.

Figure 1 : eps gif : Dimuon candidates signal and background comparison
Figure 2a: eps gif : U-U dimuon candidates invariant mass distribution
Figure 2b: eps gif : U-X dimuon candidates invariant mass distribution
Figure 3 : eps gif : U-U and U-X dimuon candidates invariant mass distribution
Figure 4 : eps gif : U-U dimuon candidates signal and background likelihood ratio PDFs comparison
Figure 5 : eps gif : U-X dimuon candidates signal and background likelihood ratio PDFs comparison
Figure 6 : eps gif : Dimuon candidates signal and background likelihood ratio comparison
Figure 7a: eps gif : U-U B &rarr J/ψK candidates invariant mass distribution
Figure 7b: eps gif : U-X B &rarr J/ψK candidates invariant mass distribution
Figure 8 : eps gif : U-U and U-X B &rarr J/ψK candidates invariant mass distribution
Figure 9 : eps gif : B &rarr J/ψK candidates reweighed signal vs MC pt comparison
Figure 10: eps gif : B &rarr J/ψK candidates signal vs MC vertex and lifetime distribution comparison
Figure 11: eps gif : U-U B &rarr J/ψK candidates signal vs raw selection variable comparison
Figure 12: eps gif : U-X B &rarr J/ψK candidates signal vs raw selection variable comparison
Figure 13: eps gif : B_s &rarr J/ψ&phi candidates raw signal vs MC isolation comparison
Figure 14a: eps gif : U-U dimuon candidates sideband and signal region likelihood ratio comparison
Figure 15b: eps gif : U-X dimuon candidates sideband and signal region likelihood ratio comparison
Figure 15a: eps gif : U-U dimuon candidates invariant mass distribution with final selection
Figure 15b: eps gif : U-X dimuon candidates invariant mass distribution with final selection
Figure 16a: eps gif : U-U dimuon candidates invariant mass vs. likelihood ratio distribution with final selection
Figure 16b: eps gif : U-X dimuon candidates invariant mass vs. likelihood ratio distribution with final selection
Figure 17: eps gif : U-U and U-X dimuon candidates invariant mass vs. likelihood ratio distribution with final selection

Experimental Papers and Status

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

CDF Run 2	(780 pb^-1)	BR( B_s → μμ ) < 1.0x10^-7 @95% CL	This result
Comb CDF/D0		BR( B_s → μμ ) < 1.5x10^-7 @95% CL	hep-ex/0508058
CDF Run 2	(364 pb^-1)	BR( B_s → μμ ) < 2.0x10^-7 @95% CL	Phys. Rev. Letters 95, 221805 2005
D0 Run 2	(300 pb^-1)	BR( B_s → μμ ) < 3.7x10^-7 @95% CL	D0-Note 4733-Conf, Preliminary
D0 Run 2	(240 pb^-1)	BR( B_s → μμ ) < 5.0x10^-7 @95% CL	Phys. Rev. Letters 94, 071802 2005
CDF Run 2	(171 pb^-1)	BR( B_s → μμ ) < 7.5x10^-7 @95% CL	Phys. Rev. Letters 93, 032001 2004

CDF Run 2	(780 pb^-1):	BR( B_d → μμ ) < 2.3x10^-8 @90% CL	This result
CDF Run 2	(364 pb^-1):	BR( B_d → μμ ) < 3.9x10^-8 @90% CL	Phys. Rev. Letters 95, 221805 2005
BaBar	(111 fb^-1):	BR( B_d → μμ ) < 8.3x10^-8 @90% CL	Phys. Rev. Letters 94, 221803 2005
CDF Run 2	(171 pb^-1):	BR( B_d → μμ ) < 1.5x10^-7 @90% CL	Phys. Rev. Letters 93, 032001 2004
Belle	(78 fb^-1):	BR( B_s → μμ ) < 1.6x10^-7 @90% CL	Phys. Rev. D RC 68, 111101 2003

Theory Papers

Below are links to some relevant theory papers:

hep-ph/0604121New Constraints on SUSY Flavour Mixing in Light of Recent Measurements at the Tevatron
(i.e. B_s Mixing and B_s → μμ)
Phys.Lett. B538 121 2002 Detection of Bs->mu mu at the Tevatron Run II and Constraints on the SUSY Parameter Space
JHEP 0306 054 2003 Updated Constraints on the Minimal Supergravity Model
JHEP 0509 029 2005 Dark Matter And Bs to mu+mu- With Minimal SO(10) Soft SUSY Breaking II
Phys.Lett. B600 261 2004 Bounding the MSSM Higgs sector from above with the Tevatron's Bs -> mu mu
Phys.Lett. B624 47 2005 On the Interpretation of Bs -> mu mu in the CMSSM
hep-ph/0510422 Current and Future Limits on General Flavour Violation in b-->s Transitions in Minimal Supersymmetry
hep-ph/0603106 Constraints on B and Higgs Physics in Minimal Low Energy Supersymmetric Models

Example Feynman diagrams:

Standard Model diagrams: Box and Penguin
2HDM diagrams: Box and Penguin
SUSY diagrams: Loop
Higgs mediated: Tree - where multiple Higgs can allow flavor violating diagrams
R-Parity violating: Tree