Measurement of the Ratios of Branching Fractions ℛ⁡(𝐷^*) and ℛ⁡(𝐷⁰)

The ratios of branching fractions ℛ⁡(𝐷^*)≡ℬ⁡(¯𝐵→𝐷^*⁢𝜏⁻⁢¯𝜈_𝜏)/ℬ⁡(¯𝐵→𝐷^*⁢𝜇^−⁢¯𝜈_𝜇) and ℛ⁡(𝐷⁰)≡ℬ⁡(𝐵⁻→𝐷0⁢𝜏⁻⁢¯𝜈_𝜏)/ℬ⁡(𝐵⁻→𝐷0⁢𝜇⁻⁢¯𝜈_𝜇) are measured, assuming isospin symmetry, using a sample of proton-proton collision data corresponding to 3.0  fb−1 of integrated luminosity recorded by the LHCb experiment during 2011 and 2012. The tau lepton is identified in the decay mode 𝜏⁻→𝜇−⁢𝜈_𝜏⁢¯𝜈_𝜇. The measured values are ℛ⁡(𝐷^*)=0.281±0.018±0.024 and ℛ⁡(𝐷⁰)=0.441±0.060±0.066, where the first uncertainty is statistical and the second is systematic. The correlation between these measurements is 𝜌 =−0.43. The results are consistent with the current average of these quantities and are at a combined 1.9 standard deviations from the predictions based on lepton flavor universality in the standard model.