Test of lepton flavor universality with measurements of <i>R</i>(<i>D</i><SUP>+</SUP>) and <i>R</i>(<i>D</i>* plus ) using semileptonic <i>B</i> tagging at the Belle II experiment
Description
We report measurements of the ratios of branching fractions R(D(*)+) = B((B) over bar (0) -> D(*)+ tau(-)(nu) over bar (tau)/B((B) over bar)(0) -> D(*)+ l(-) (nu) over bar (l)), where l denotes either an electron or a muon. These ratios test the universality of the charged-current weak interaction. The results are based on a 365 fb(-1) data sample collected with the Belle II detector at the SuperKEKB e(+)e(-) collider, which operates at a center-of-mass energy corresponding to the Upsilon(4S) resonance, just above the threshold for B (B) over bar production. Signal candidates are reconstructed by selecting events in which the companion B meson from the (Upsilon(4S) -> B (B) over bar decay is identified in semileptonic modes. The tau lepton is reconstructed via its leptonic decays. We obtain R(D+) = 0.418(-0.073)(+0.075) (stat)(-0.056)(+0.049) (syst) and R(D*(+)) = 0.306(-0.033)(+0.035) (stat)(-0.018)(+0.016) (syst), which are consistent with world average values. Accounting for the correlation between them, these values differ from the Standard Model expectation by a collective significance of 1.7 standard deviations.
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