Precision Measurement of the R(D*) at the Belle II Experiment

Measurement of the R(D*) is defined as the ratio of Br (anti-B0 → D*+ τ- anti-ν) over

Br (anti-B0 → D*+ ℓ− anti-ν), where the lepton ℓ− denotes an electron or a muon.

The τ lepton is reconstructed in the leptonic decay of τ− → e− anti-νe ντ and τ− → μ− anti-νμ ντ.

The decay of anti-B0 → D*+ ℓ− anti-ν and anti-B0 → D*+ τ- anti-ν are reconstructed partially without reconstructing the intermediate decay of D0 meson. The semileptonic decay of anti-B0 → D*+ℓ−anti-ν is mediated by W boson. The W boson couples equally to the three lepton generations: e, μ, and τ. However, decays involving the τ lepton in the semitauonic decay of anti- B0 → D*+ τ− anti-ν provide a sensitive probe to additional amplitudes that involving an intermediate Higgs boson or a leptoquark being mediated in the transition of b → c τ ν. The partially reconstructed method may result in gain of as much as a factor 25 in statistics compared to the fully reconstructed method.