Tests of lepton flavour universality in semileptonic 𝑩 meson decays at LHCb

	The Standard Model (SM) of particle physics treats the lepton generations in a similar manner, except for the difference in their masses. This is extensively tested by experiments using semileptonic 𝐵 meson decays of the type 𝑏 → 𝑐ℓ𝜈ℓ . One of the important observables is the ratio of branching fractions 𝑅(𝐷 ∗ ) ≡ ℬ(𝐵 0 → 𝐷 ∗ 𝜏𝜈𝜏)/ℬ(𝐵 0 → 𝐷 ∗ℓ𝜈ℓ), with ℓ = 𝑒, 𝜇. They have been measured by the BaBar, Belle, and LHCb collaborations. The discrepancy between the experimental world-average values of 𝑅(𝐷 ∗ ) and 𝑅(𝐷) and their theoretical predictions is at the level of three standard deviations. This seminar presents the Lepton Flavour Universality measurements from LHCb, with a focus on the latest 𝑅(𝐷 ∗ ) using 𝐵 0 → 𝐷 ∗−𝜏 +𝜈𝜏 decays with hadronic 𝜏 + channels. The excellent vertexing capabilities of the LHCb detector allow for the exploration of interesting features of the three-prong 𝜏 decay, which will be described in this talk. The measurement of 𝜏 lepton polarisation will also be discussed, since exploring more observables in addition to the branching fraction ratios is key to the understanding of New Physics beyond the SM.