Observation of the Leptonic Decay D+→τ+ντ

Abstract

We report the first observation of D + → τ + ντ with a significance of 5.1σ. We measure B(D + → τ + ντ ) = (1.20 ± 0.24stat. ± 0.12syst.) × 10 −3 . Taking the world average B(D + → µ + νµ) = (3.74 ± 0.17)×10 −4 , we obtain R τ /µ = Γ(D + → τ + ντ )/Γ(D + → µ + νµ) = 3.21±0.64stat. ±0.43syst., which is consistent with the Standard Model expectation of lepton flavor universality. Using external inputs, our results give values for the D + decay constant f D + and the CKM matrix element |V cd | that are consistent with, but less precise than, other determinations.

First Observation of D+→ημ+νμ and Measurement of Its Decay Dy

“…[29] leads to jV cd j ¼ 0.242 AE 0.022 stat AE 0.006 syst AE 0.033 theory . This result is consistent with our previous measurements of jV cd j via D þ → l þ ν l [57,58] and D 0ðþÞ → π −ð0Þ e þ ν e [43,54]. Conversely, using our product of f η þ ð0ÞjV cd j and the jV cd j ¼ 0.22438 AE 0.00044 given by CKMFitter [21] yields f η þ ð0Þ ¼ 0.39 AE 0.04 stat AE 0.01 syst .…”

Testing lepton flavour universality with (semi)-leptonic $$\varvec{D_{(s)}}$$ decays

“…where we have utilized the recent first observation of the decay D + → τ + ν τ by the BESIII collaboration [36]. By comparing the experimental value with the corresponding theoretical expression, we determine the allowed regions in the C µ P -C τ P plane.…”

“…By comparing our theoretical expressions with the experimental information in Eqs. (36) and (37), we determine the allowed regions in the C µ T -C e T plane. The constraints from R e µ;π 0 and R e µ;π − are shown in the panels on the left-and right-hand sides of Fig.…”

“…+ → e + ν e ) (9.16 ± 0.22) × 10 −9 < 8.8 × 10 −6 [30] B(D + → µ + ν µ ) (3.89 ± 0.09) × 10 −4 (3.74 ± 0.17) × 10 −4 [30] B(D + → τ + ν τ ) (1.04 ± 0.03) × 10 −3 (1.20 ± 0.27) × 10 −3[36] …”

Tau and charm decays

“…The unique tagging technique and the excellent performance of the BESIII detector allows to reconstruct these decays. Recently, the decay D + → τ + ν τ was observed for a first time by the BESIII collaboration [66]. Its branching fraction was determined to be B (D + → τ + ν τ ) = (1.20±0.24±0.12)×10 −4 .…”