Rare B-meson decays to two muons

Studies of rare B-meson decays at the LHC provide a sensitive probe of physics beyond the Standard Model (SM) and allow us to explore energy scales much higher than those directly accessible. A key factor in the success of these studies is the availability of precise theoretical predictions that can be compared with experimentally accessible processes. The dimuon decays B⁰_S → μ⁺μ^– and B⁰ → μ⁺μ^– are a case in point. In particular, studies of these decays could help researchers to understand the nature of several anomalies seen in other rare B-meson decays.

The CMS collaboration recently reported a new measurement of the B⁰_S → μ⁺μ^– branching fraction and effective lifetime, as well as the result of a search for the B⁰ → μ⁺μ^– decay, using data recorded during LHC Run 2. This new study benefits not only from a large event sample but also from advanced machine-learning algorithms, which are used to uncover the rare signal events out of the overwhelming background. The B⁰_S → μ⁺μ^– signal is very clearly seen (see figure 1), leading to more precise measurements than previously achieved. The B⁰_S → μ⁺μ^– branching fraction is measured to be (3.8 ± 0.4) × 10^–9, the relative uncertainty of 11% being a remarkable improvement with respect to that of the previous CMS result, 23%.

This measured value is consistent with the SM prediction of (3.7 ± 0.1) × 10^–9, and reduces a previous tension between theory and experiment, which was based on the combination of the previous CMS result with the ATLAS and LHCb values. The variation in the central value of the CMS measurements is mostly driven by the use of a larger data sample and by the change of the B-hadron fragmentation fraction ratio (by about 8%). The measured effective lifetime of the B⁰_S → μ⁺μ^–  decay, 1.8 ± 0.2 ps, is also consistent with the SM prediction. The precision of this measurement is approaching the level necessary to probe the CP properties of B⁰_S → μ⁺μ^–, which could differ from the SM prediction. Finally, the B⁰ → μ⁺μ^– decay remains unseen.

CMS physicists are looking forward to continuing these rare-decay studies with the large data samples to be collected during LHC Run 3. Besides the improved precision expected for B⁰_S → μ⁺μ^– measurements, seeing the first evidence of B⁰ → μ⁺μ^– is high on their wish list.