Observation of the rare $B^0_s\toμ^+μ^-$ decay from the combined analysis of CMS and LHCb data (1411.4413v2)

Abstract: A joint measurement is presented of the branching fractions $B^0_s\to\mu^+\mu^-$ and $B^0\to\mu^+\mu^-$ in proton-proton collisions at the LHC by the CMS and LHCb experiments. The data samples were collected in 2011 at a centre-of-mass energy of 7 TeV, and in 2012 at 8 TeV. The combined analysis produces the first observation of the $B^0_s\to\mu^+\mu^-$ decay, with a statistical significance exceeding six standard deviations, and the best measurement of its branching fraction so far. Furthermore, evidence for the $B^0\to\mu^+\mu^-$ decay is obtained with a statistical significance of three standard deviations. The branching fraction measurements are statistically compatible with SM predictions and impose stringent constraints on several theories beyond the SM.

• The paper reports the first statistically definitive observation of the B_s0 → μ+μ− decay with a 6.2-sigma significance.
• The analysis combined data from CMS and LHCb experiments, enhancing statistical precision using proton-proton collisions at 7 and 8 TeV.
• The measured branching fractions align with Standard Model predictions, tightening constraints on theories beyond the Standard Model.

Observation of Rare B Meson Decays in the CMS and LHCb Experiments

The paper presents a significant advancement in the field of particle physics by detailing the observation of the rare decays of B mesons into muon pairs, a topic of high relevance for testing the predictions of the Standard Model (SM) of particle physics. The research involved a combined analysis of data from the CMS and LHCb experiments, which operate at the Large Hadron Collider (LHC) at CERN. The primary aim was to investigate the decay of B^0_s and B^0 mesons into two oppositely charged muons, an investigation pivotal for understanding potential extensions to the SM.

Key Results and Methodology

• Branching Fractions: The analysis successfully observed the decay B^0_s → μ^+μ^- with a branching fraction significant enough to exceed six standard deviations, marking it as a statistically definitive observation. The decay B^0 → μ^+μ^-, though detected, was at an evidence level of three standard deviations.
• Statistical Analysis: The data from proton-proton collisions collected at 7 and 8 TeV during 2011 and 2012 allowed for a robust statistical analysis. The dual use of CMS and LHCb data enhanced statistical precision, evidenced by a 6.2-sigma significance for the B^0_s decay, consistent with SM predictions.
• SM Compatibility: The measured branching fractions for both decay modes align with the Standard Model expectations. Importantly, they establish stringent constraints on theories beyond the Standard Model (BSM), highlighting the probable accuracy of the current theoretical framework while leaving room for further BSM exploration.

Implications for Particle Physics

The confirmation of these rare decay processes serves multiple roles. It provides a stringent test of the SM, reinforcing its robustness and limiting the parameter space for new physics. This finding places severe restrictions on BSM theories that predict enhancements in flavor-changing neutral current processes, such as those involving additional Higgs bosons or supersymmetric particles.

Future Prospects

As the LHC continues to operate at higher energies, such as the 13 TeV planned for succeeding runs, the production rates of B mesons are expected to increase, enabling more precise measurements. These forthcoming analyses will further probe the validity of the Standard Model, allowing for the possibility of discovering new phenomena that might point to physics beyond the SM.

This research exemplifies the power of collaborative experimental physics across major detector experiments. The combined efforts of CMS and LHCb not only optimize the use of available data but also set a new precedent for how high-energy physics collaborations might address similarly fundamental questions in the future. The constraints provided by these results are expected to guide future theoretical and experimental explorations into the parameter space of BSM physics.