First observation of the decay Lambda_b to Lambda_c tau nu and tests of the Lepton Flavour Universality1h
Several hints of a potential Lepton Flavour Universality (LFU) violation have been recently reported both in charged-current and neutral-current B decays. Such a violation would constitue a definite proof of New Physics beyond the The Standard Model and provide information upon its nature. In this context, the first observation of the semileptonic decay Lambda_b to Lambda_c tau nu, using 3fb-1 of data taken at the LHC by the LHCb experiment will be reported. The tau is reconstructed through the hadronic three-prong decay to pi+pi-pi+.
The subsequent measurement of R(Lambda_c), the ratio between the semileptonic decay rates to a tau lepton and to a muon will be presented. Because of the half-integer spin of the baryons, this LFU is quite complementary to those performed with mesons and can provide constraints on some New Physics models, which will be discussed.