Lepton universality, flavour and number violation in $B$ and $\tau$ decays at LHCb

The LHCb experiment has explored lepton universality violation in b → c ` − ν ` and b → s ` + ` − transitions through the measurement of the ratios R ( D ∗ ) and R K . These ratios are found to be compatible with the Standard Model expectation at the level of 2.1 σ and 2.6 σ , respectively. Additionally, searches for lepton flavour violation in B , D and τ decays have been performed. No signal for such decays is found, and upper limits on their branching fractions are set