Ablikim, M.Achasov, M. N.Adlarson, P.Ahmed, S.Albrecht, M.Aliberti, R.Çetin, Serkant Ali2024-07-182024-07-1820210031-90071079-7114https://doi.org/10.1103/PhysRevLett.127.131801https://hdl.handle.net/11411/7720Using 2.93 fb(-1) of e(+)e(-) collision data taken with the BESIII detector at a center-of-mass energy of 3.773 GeV, the observation of the D-0 -> K-1 (1270)(-)e(+)nu(e) semileptonic decay is presented. The statistical significance of the decay D-0 -> K-1 (1270)e(+)nu(e) is greater than 10 sigma. The branching fraction of D-0 -> K-1 (1270)(-)e(+)nu(e) is measured to be (1.09 +/- 0.13(-0.16)(+0.09) +/- 0.12) x 10(-3). Here, the first uncertainty is statistical, the second is systematic, and the third originates from the assumed branching fraction of K-1 (1270)(-) -> K-pi(+)pi(-). The fraction of longitudinal polarization in D-0 -> K-1 (1270)(-)e(+)nu(e) is determined for the first time to be 0.50 +/- 0.19(stat )+/- 0.08(syst).eninfo:eu-repo/semantics/openAccessSemileptonic-BTau-DecaysMesonsModelArticle2-s2.0-8511828605110.1103/PhysRevLett.127.1318011334623854Q1127Q1WOS:000704680000008