Search for the DecayK+→π+μ+e

Abstract

We report here on a search for the lepton-family-number-violating decay K + -• x + fi + e ~. We found no evidence for this decay mode and we establish an upper limit on its branching ratio of 1.1 x 10 ~9 at the 90% confidence level (C.L.) for a uniform phase-space decay distribution. We also use this result to establish an upper limit for the branching ratio of the decay 7r°-• JJ, +e ~ of 7.8 x 10 ~8 (90% C.L.).PACS numbers: 13.20. Eb, 12.15.Mm, 13.20.Cz The level to which lepton-family number (LFN) is conserved is one of the outstanding issues of particle physics. Conservation of this quantity was hypothesized in the 1950's 1 and is now incorporated into the standard model. More recently, however, theoretical models addressing issues not resolved by the standard model have suggested that LFN may be violated at levels observable with present technology. 2,3 For many of these models, the decays ATj?->p, e and K + -• n + /i + e ~ are good candidates for such observations. For example, while violating LFN, these decays conserve "generation" number, as defined by Cahn and Harari, 3 and thus may be less inhibited than other processes, e.g., //-• eee, p 4 A -+ e + A, or ft-• ey. New measurements of K®-• jxe have recently been published. 4 We are reporting in this paper the first results of a new search for the decay