Measurement of the branching fraction for the decay K → πμν with the KLOE detector
D. Babusci, M. Berłowski, C. Bloise, F. Bossi, P. Branchini, A. Budano, B. Cao, F. Ceradini, P. Ciambrone, F. Curciarello, E. Czerwiński, G. D’Agostini, E. Danè, V. De Leo, E. De Lucia, A. De Santis, P. De Simone, A. Di Cicco, A. Di Domenico, D. Domenici, A. D’Uffizi, A. Fantini, P. Fermani, S. Fiore, A. Gajos, P. Gauzzi, S. Giovannella, E. Graziani, V. L. Ivanov, T. Johansson, X. Kang, D. Kisielewska-Kamińska, E.A. Kozyrev, W. Krzemień, A. Kupść, P.A. Lukin, G. Mandaglio, M. Martini, R. Messi, S. Miscetti, D. Moricciani, P. Moskal, S. Parzych, A. Passeri, V. Patera, E. Pérez del Río, P. Santangelo, M. Schioppa, A. Selce, M. Silarski, F. Sirghi, E. P. Solodov, L. Tortora, G. Venanzoni, W. Wiślicki, M. Wolke
Abstract
Based on a sample of 300 million K S mesons produced in ϕ → K L K S decays recorded by the KLOE experiment at the DAΦNE e + e − collider we have measured the branching fraction for the decay K S → π μ ν . The K S mesons are identified by the interaction of K L mesons in the detector. The K S → π μ ν decays are selected by a boosted decision tree built with kinematic variables and by a time-of-flight measurement. Signal efficiencies are evaluated with data control samples of K L → π μ ν decays. A fit to the reconstructed muon mass distribution finds 7223 ± 180 signal events. Normalising to the K S → π + π − decay events the result for the branching fraction is B ( K S → π μ ν ) = ( 4.56 ± 0.11 stat ± 0.17 syst ) × 10 − 4 . It is the first measurement of this decay mode and the result allows an independent determination of | V u s | and a test of the lepton-flavour universality.