Constraint on the branching ratio of B_c \to tau nu from LEP1 and consequences for R(D(*)) anomaly

TL;DR

This paper constrains the branching ratio of B_c to tau nu using LEP1 data, impacting models that explain R(D*) anomalies with charged scalars, and emphasizes the need for future measurements to clarify these effects.

Contribution

It establishes a new, stronger upper limit on BR(B_c to tau nu) from LEP data, challenging models that attribute R(D*) anomalies solely to charged scalar contributions.

Findings

LEP data constrains BR(B_c to tau nu) to less than 10%.

This limit is stronger than the 30% bound from B_c lifetime considerations.

Future LEP data could further tighten this constraint.

Abstract

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