Testing lepton flavour universality in excited strange mesons
Our understanding of all known particles and their interactions is successfully described in the Standard Model (SM). It is, however, known to be incomplete since it does not explain e.g. dark matter, or the matter-antimatter asymmetry in the universe. Recently, several experiments found hints of physics beyond the SM, which must be further investigated.

A key assumption of the SM is that leptons (electrons and their heavier siblings, muons and taus) have the exact same couplings to forces, called lepton universality. When testing this assumption using decays of so-called B mesons, three independent experiments found increased values for their decay rate to a D meson and tau lepton, compared to a D meson and a muon. This could be explained by the existence of new particles, such as leptoquarks, which couple more strongly to leptons with a higher mass.

This proposal concerns a similar measurement using strange B mesons (Bs), which decay into strange D mesons (Ds). The D mesons that were measured before are extremely difficult to distinguish from their so-called resonance states, which are excitations of the D meson. Misidentifying these could wrongly increase the rate of tau decays of all three experiments. Ds mesons, on the other hand, have a different structure of resonances, and fewer exist that can mimic the signal. Additionally, these can be constrained using their well-defined kinematics. As such, an observation of non-universality in the Bs meson decays would provide conclusive evidence for the existence of physics beyond the SM.

In 2021, the LHCb experiment will recommence data-taking with a completely new detector and reconstruction software. Dr. Klaver plays a key role in studying the impact of misaligned or misplaced detectors and detector defects of the upgrade detector. This proposal describes how a validation framework is to be set up to aid the commissioning of the new experiment. This secures future tests of lepton universality with high precision.