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 April 7th, 2005 and correspond to a dataset of 364 pb-1. A PRL paper on the analysis can be found at hep-ex/0508036.

Motivation

The SM predicts a BR( Bs → μμ ) of about 3.8x10^-9, while the best current limit was:
D0 Run 2, BR( B_s → μμ ) < 3.5x10^-7 @95% CL (D0-Note 4733-Conf, Preliminary).

There are various extensions to the SM 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's a plot from (hep-ph/0108037), which shows BR( Bs → μμ ) in the (M0,M1/2) plane in the MSuGra model. The solid black lines are contours of constant BR( Bs → μμ ), the dotted green lines are contours of constant deviations of the muon (g-2) (in units of E-10) relative to SM expectations and the dash-dot red lines are contours of constant (lowest-lying) higgs mass in this model (in units of GeV). The shaded red-regions are excluded.

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/0304101) 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. This model with the choice of parameters shown in the plot is disfavored. A listing of relevant theory papers is below.

Description

A complete description of the analysis is available here: pdf. The final version published in PRL is here: hep-ex/0508036

Result: World's best limits

BR( B_s → μμ ) < 2.0x10^-7 @95% CL
BR( B_d → μμ ) < 5.1x10^-8 @95% CL

BR( B_s → μμ ) < 1.5x10^-7 @90% CL
BR( B_d → μμ ) < 3.9x10^-8 @90% CL

Figures

Below the eps versions of all figures available for download.

Figure: Central-Central(CMU-CMU) muon candidates invariant mass distribution
Figure: Central-Forward(CMU-CMX) muon candidates invariant mass distribution
Figure: Central-Central muon B &rarr J/&Psi K invariant mass distribution
Figure: Central-Forward muon B &rarr J/&Psi K invariant mass distribution
Figure: Pythia MC B &rarr J/&Psi K: &mu&mu K invariant mass distribution
Figure: J/&Psi &rarr &mu&mu, from B &rarr J/&Psi K: &mu&mu invariant mass distribution
Figure: Pythia MC J/&Psi &rarr &mu&mu, from B &rarr J/&Psi K: &mu&mu invariant mass distribution
Figure: Pythia MC B_s &rarr &mu&mu: &mu&mu invariant mass distribution
Figure: Transvers momentum, P_T B distribution
Figure: Shape comparison between Pythia MC Signal and Background for discriminating variables
Figure: Central-Central muon correlation plots for background
Figure: Central-Forward muon correlation plots for background
Figure: Central-Central muon likelihood PDFs
Figure: Central-Forward muon likelihood PDFs
Figure: Central-Central muon background likelihood distribution and comparison to Toy MC simulation
Figure: Central-Forward muon background likelihood distribution and comparison to Toy MC simulation
Figure: Central-Central muon B &rarr J/&Psi K Shape comparison between Phythia MC and data Signal for discriminating variables
Figure: Central-Central muon B &rarr J/&Psi K Shape comparison between Phythia MC and data Signal for discriminating variables
Figure: Likelihood distribution comparison between Pythia MC Signal and background
Figure: Central-Central muon invariant mass distribution of events passing all cuts
Figure: Central-Forward muon invariant mass distribution of events passing all cuts
Figure: Central-Central muon likelihood distribution of events passing all cuts
Figure: Central-Forward muon likelihood distribution of events passing all cuts
Figure: Central-Central muon invariant mass vs. likelihood distribution of events passing all cuts
Figure: Central-Forward muon invariant mass vs. likelihood distribution of events passing all cuts
Web page : Di-muon trigger plots

Documentation

If you have access to internal CDFnotes, more details about the various acceptances and efficiencies can be found in these cdfnotes:

A full description of the analysis is in cdfnote 7463

geometric and kinematic acceptance: cdf-6204
trigger efficiencies: cdf-7314
muon efficiencies: cdf-6347, cdf-6612
muon chi2: cdf-6114, cdf-6835
tracking efficiencies: cdf-6318, cdf-6394
muon fake-rates for pions and kaons in D* data sample: cdf-6273

The pre-blessing talk and blessing talk at the B Physics meeting also offers a summary of the analysis.

Experimental Papers and Status

Below are links the relevant experimental papers:

Combination CDF and D0:	BR( B_s → μμ ) < 1.5x10^-7 @95% CL	(hep-ex/0508058)
D0 Run 2:	BR( B_s → μμ ) < 3.7x10^-7 @95% CL	(D0-Note 4733-Conf, Preliminary)
D0 Run 2:	BR( B_s → μμ ) < 5.0x10^-7 @95% CL	(hep-ex/0410039, Phys. Rev. Letters 94, 071802 (2005))
CDF Run 2:	BR( B_s → μμ ) < 7.5x10^-7 @95% CL	(hep-ex/0403032 , Phys. Rev. Letters 93, 032001 2004)
BaBar:	BR( B_d → μμ ) < 8.3x10^-8 @90% CL	(hep-ex/0408096)
CDF Run 2:	BR( B_d → μμ ) < 1.5x10^-7 @90% CL	(hep-ex/0403032 , Phys. Rev. Letters 93, 032001 2004)
Belle:	BR( B_s → μμ ) < 1.6x10^-7 @90% CL	(hep-ex/0309069 , Phys. Rev. D 68, 111101 (R) (2003) )

hep-ex/0401008, Phys.Rev.Lett. 92 (2004)

page

Theory Papers

Below are links to some relevant theory papers:

hep-ph/0108037 Correlation of Bs -> mu+mu- and (g-2)mu in Minimal Supergravity
hep-ph/0207026 Trilepton Events and Bs -> mu+mu- : No-lose for mSUGRA at the Tevatron?
hep-ph/0203069 Detection of Bs->mu mu at the Tevatron Run II and Constraints on the SUSY Parameter Space
hep-ph/0304303 Updated Constraints on the Minimal Supergravity Model
hep-ph/0310042 Bs -> mumu as a Probe of Tan(beta) at the Tevatron
hep-ph/0304101 Dark Matter And Bs to mu+mu- With Minimal SO(10) Soft SUSY Breaking
hep-ph/0507233 Dark Matter And Bs to mu+mu- With Minimal SO(10) Soft SUSY Breaking II
hep-ph/0302155 Yukawa Coupling Unification in Supersymmetric Models
hep-ph/0501230 Non-Decoupling Effects of the Heavy T in the B{d,s} Mixing and Rare K and B Decays
hep-ph/0407285 Bounding the MSSM Higgs sector from above with the Tevatron's Bs -> mu mu
hep-ph/0504196 On the Interpretation of Bs -> mu mu in the CMSSM

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