Constraint on the branching ratio of Bc- →τ ν from LEP1 data and consequences for R (D (∗)) anomaly

Recently there has been interest in the correlation between R(D∗) and the branching ratio (BR) of Bc-→τν in models with a charged scalar H±. Any enhancement of R(D∗) by H± alone (in order to agree with current data) also enhances BR(Bc-→τν), for which there has been no direct search at hadron colliders. We show that LEP data taken at the Z peak requires BR(Bc-→τν) 10%, and this constraint is significantly stronger than the recent constraint BR(Bc-→τν) 30% from considering the lifetime of Bc. In order to respect this new constraint, any explanation of the R(D) and R(D∗) anomaly in terms of H± alone would require the future measurements of R(D∗) to be even closer to the Standard Model prediction. A stronger limit on BR(Bc-→τν) (or its first measurement) would be obtained if the L3 Collaboration used all its data taken at the Z peak.