• Di-muon invariant mass distributions in the barrel (left) and endcap (right) channels. The signal windows for Bs and B0 are indi

Read the paper: BPH-11-002

The observed numbers of events in both decay modes are consistent with those expected in the Standard Model.
Upper limits on the branching fraction have been determined at 90% and 95% confidence levels (CL).

The Standard Model predicts which fraction of B-mesons will decay into which specific particles, these fractions being known as branching fractions. Several extensions of the Standard Model predict different branching fractions for these decays, some enhanced and some lowered. Any observed deviation from the Standard-Model values could hint towards new physics.

According to some models, the branching fractions for Bs → μ+μ and B0 → μ+μ can be enhanced by different factors. Therefore, for this paper, both modes were studied in parallel.

Group: B Physics (BPH)
Collisions: pp
Centre-of-mass energy: 7 TeV
Data sample: 1.14 fb–1
Dates of data collection: 2011
Obtained upper limits on the branching fractions at 95% CL:

  1. B(Bs → μ+μ) < 1.9 x 10–8
  2. B(B0 → μ+μ) < 4.6 x 10–9

Obtained upper limits on the branching fractions at 90% CL:

  1. B(Bs → μ+μ) < 1.6 x 10–8
  2. B(B0 → μ+μ) < 3.7 x 10–9

Compared with the median upper limits expected (in the context of the Standard Model) on the branching fractions at 95% CL:

  1. B(Bs → μ+μ) < 1.8 x 10–8
  2. B(B0 → μ+μ) < 4.8 x 10–9

Background-only p-value: 0.11 (0.40) for Bs → μ+μ (B0 → μ+μ) corresponding to 1.2 (0.27) standard deviations.
p-value is 0.053 assuming a Bs → μ+μ signal at 5.6 times the SM value.
Submitted to: Physical Review Letters