The analysis of the B⁰→K^*μμ decay is considered as a very promising way to search for effects of yet undiscovered particles. Unfortunately, the analysis of this decay is also complicated; the best sensitivity to the new particles comes from the study of the angular distribution of the muons and the kaon and pion from the K^* meson decay. Physicists from the LHCb experiment have studied different angular observables as functions of the mass of the muon pair. One of these observables, "P₅^'" shows a local deviation with respect to the Standard Model calculation at a level of 2.5 to 2.9σ in the mass squared of the muon pair region q² from 4.0-6.0 and 6.0-8.0 Gev²/c⁴. However, the exact significance of this discrepancy depends on the choice of theory nuisance parameters.

There is particular interest in these observables because their theoretical prediction is much less dependent on a good understanding of the hadronic physics involved in turning a B meson into a K^* meson (so called form-factors). These observables are therefore ideal for searching for the effects of new particles in this decay.

The image shows the distribution of the P₅^’ observable as functions of the mass squared of the muon pair q². The black points show the LHCb results, the Standard Model predictions are presented as orange boxes.

[1] Updated angular analysis of the B⁰→K^*μ⁺μ^- decay - LHCb Collaboration (LHCb-PAPER-2020-002)

https://arxiv.org/abs/2003.04831