Çetin, Serkant Ali2021-05-052021-05-052019-11-220031-9007https://hdl.handle.net/11411/3635https://doi.org/10.1103/PhysRevLett.123.211802We report the first observation of D+??+?? with a significance of 5.1?. We measure B(D+??+??)=(1.20±0.24stat±0.12syst)×10-3. Taking the world average B(D+??+??)=(3.74±0.17)×10-4, we obtain R?/?=?(D+??+??)/?(D+??+??)=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 fD+ and the Cabibbo-Kobayashi-Maskawa matrix element |Vcd| that are consistent with, but less precise than, other determinations.eninfo:eu-repo/semantics/openAccessArticle2-s2.0-8507560344110.1103/PhysRevLett.123.21180231809130Q1WOS:000498887900005