Probes of Lepton Flavor Universality in b → u Transitions

Anomalies recently observed in semileptonic b → c l - ν ¯ l and b → s l + l - transitions point to violation of Lepton Flavour Universality. Strategies for new analyses of different modes are required, in particular for the modes induced by the b → u transition. We describe the purely leptonic B decay, the B ¯ → π l - ν ¯ l channel and the B semileptonic modes to ρ ( 770 ) and a 1 ( 1260 ) in extensions of the Standard Model involving Lepton Flavour Universality violating b → u operators. In particular, we review the observables in the four-dimensional angular B ¯ → ρ ( π π ) l - ν ¯ l and B ¯ → a 1 ( ρ π ) l - ν ¯ l distributions, suitable to pin down deviations from the Standard Model. We discuss the complementarity among the various modes for New Physics searches.