LHCb constrains ultra-rare muonic B decay

A report from the LHCb experiment

Measurements of b-hadron decays with neutrinos in the final state are one of the best ways to understand how quarks decay, and in particular how they couple to leptons. With recent results from LHCb, BaBar and Belle raising questions about whether the Standard Model (with its assumption of lepton-flavour universality) is able to explain these couplings fully, further experimental results are needed.

At first glance, measuring fully leptonic decays such as B⁺ → τ⁺ν_τ and B⁺ → μ⁺ν_μ seems a step too far, since there is only one charged particle as a signature and no reconstructed B-decay vertex.

However, studying these decays is notoriously tricky at a hadron collider, where the busy collision environment makes it challenging to control the background. Despite this, the LHCb collaboration has made unexpected progress in this area over the last few years, with a comparison of decays with taus and muons, and measurements the CKM element ratio |V_ub/V_cb| that originally seemed impossible.

At first glance, measuring fully leptonic decays such as B⁺ → τ⁺ν_τ and B⁺ → μ⁺ν_μ seems a step too far, since there is only one charged particle as a signature and no reconstructed B-decay vertex. The key to accessing these processes is to allow additional particles to be radiated, while preserving the underlying decay amplitude. The decay B⁺ → μ⁺μ⁻μ⁺ν_μ is a good example of this, where a hard photon is radiated and converts immediately into two additional muons. Such a signature is significantly more appealing experimentally: there is a vertex to reconstruct and the background is low, as there are not many B decays that produce three muons.

B decays with a well-defined vertex and only one missing neutrino are becoming LHCb’s “bread and butter” thanks to the so-called corrected mass technique. The idea behind the corrected mass is that if you are only missing one neutrino, then adding the momentum perpendicular to the B flight direction is enough to recover the B mass. This technique is only possible thanks to the precise vertex resolution provided by the LHCb’s innermost detector, the VELO. Using this technique, LHCb expects to have a very good sensitivity for this decay, at a branching fraction level of 2.8 × 10⁻⁸ (equivalent to around one in 40 million B⁺ decays) with the 2011–2016 data sample.

The LHCb collaboration searched for this decay using 5 fb^–1 of data (see figure). The main backgrounds come from reconstructed muons that originate from different decays (“combinatorial”) or from hadrons misidentified as muons (“misidentified”). No evidence for the signal is seen and an upper limit on the branching fraction of 1.6 × 10⁻⁸ is set at a confidence level of 95%.

The figure also shows a projected signal expected from a recent Standard Model prediction, which is based on the vector meson dominance model. This prediction includes two contributions to the decay: one in which two muons originate from a photon, and another in which they originate from the annihilation of a hadron (such as ρ⁰ → μ⁺μ⁻ or ω → μ⁺μ⁻). As can be seen, the data disfavour this prediction, which motivates further theoretical work to understand the discrepancy. The good sensitivity for this decay is encouraging, and raises interesting prospects for observing the signal with future datasets collected at the upgraded LHCb detector.