Closing the gap on RD* by including longitudinal effects

Abstract

Measurements of the RD * ≡ Br(B → τ νD * )/Br(B → eνD * ) parameter remain in tension with the standard model prediction, despite recent results helping to close the gap. The standard model prediction it is compared with considers the D * as an external particle, even though what is detected in experiments is a Dπ pair it decays into, from which it is reconstructed. We argue that the experimental result must be compared with the theoretical prediction considering the full 4-body decay (B → lνD * → lνDπ). We show that the longitudinal degree of freedom of the off-shell D * helps to further close the disagreement gap with experimental data. We find values for the ratio R l Dπ ≡ Br(B → τ ντ Dπ)/Br(B → lν l Dπ) of R e Dπ = 0.274 ± 0.003 and R µ Dπ = 0.275 ± 0.003, where the uncertainty comes from the uncertainty of the form factors parameters. Comparing against RDπ reduces the gap with the latest LHCb result from 1.1σ to 0.48σ, while the gap with the latest Belle result is reduced from 0.42σ to just 0.10σ and with the world average results from 3.7σ to 2.1σ. Erratum added at the end of the file.

Review of lepton universality tests in B decays

“…[ results on light-lepton decays, Light-Cone Sum Rules, and lattice inputs show very little dispersion in the predictions for R D and R D * [94]. These predictions do not take into account the width of the D * -meson, whose effect is currently under investigation and could have a noticeable impact of a few percent on the measurement of R D * [95,96].…”

Semileptonic decays of B(*) , D(*) into Dai1, Zhang2, Oset3 2018Phys. Rev. D222440View full textAdd to dashboardCite

“…[7]. This latter case is increased to 0.27 in [32] by taking into account in the theoretical evaluation that D Ã → Dπ, the mode where the D Ã is observed experimentally.…”