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Evidence of massive neutrinos was sought in the .rr+-+e+v decay spectrum with the background from the .rr-+p-+e decay chain highly suppressed. Upper limits (90% C.L.) were set on the neutrino mixing parameter / U,, / < lo-' in the mass region 50 < m, < 130 M e~/ c *.PACS number(s): 14.60. Gh, 12.15.Ff, 13.20.C~ Based on the Z O width, the experiments [ l ] at the CERN e + e -collider LEP have demonstrated that there are only three light neutrinos. The existence of conventional neutrinos with masses between the upper bound of the r neutrino mass m, I 35 M~V / C [2] and mZo/2 -45 G~V / C has therefore become unlikely. Nevertheless, some extensions of the standard model such as ones involving left-handed neutrino singlets (vX,,vX,, . . . , vXk )[3], in addition to the three generations bf left-handed lepton doublets and corresponding right-handed neutrinos, lead to neutrino mixing without affecting the Z O width. The weak eigenstates vxk of such neutrinos are related to the mass eigenstates vi by a unitary matrix, v, where I =~, , L~, I -, X~, X~,. . . , x k . Such mixings would produce additional peaks in the positron energy spectrum from two-body meson decays such as a + -+ e + v 7 as discussed by Shrock [4]. The contribution to the a + -e + v decay from the mixing of a massive neutrino vi can be written as where v, is the conventional massless neutrino and p, is a kinematic factor, with 6, =m:/m : , and Si = m t, / m 2,. The sensitivity to I U,, l2 (or p,) tends to increase as the helicity suppression effect relaxes with the increase of m v i 7 but tends to decrease above m,, -80 MeV/c2 due to the diminishing phase space. Previous direct searches for the production of massive neutrinos coupling to the electron in the mass region above the r neutrino mass limit have been carried out using the decays a + +e + v [5,6] and K +-e + v [7]. There have also been indirect searches for such mixing using neutrino beams [8,9]. Furthermore, limits on neutrinoless double-B decay can be used to constrain the mass and *present address: the mixing parameters of possible Majorana neutrinos [lo].The present experiment is a refinement of the previous work performed at TRIUMF [5]. Figure 1 shows a schematic diagram of the experimental setup. Positive pions of momentum Pv+ =83 MeV/c from the TRIUMF M13 channel were stopped at a rate of lo5 s-I in a 12.7-mm-thick scintillator target B3 sandwiched between two 1.6-mm-thick scintillation counters B2 and B4. Positrons from the decay n-+-e+v ( Te+=69.3 MeV with the branching ratio -lop4) and from the decay ,~i+-e'va following the decay a + -,~i + v (the ~+ -~+ -e + chain, Te + =0-52.3 MeV) were detected by counters T1 -T4 (1.6-3.2 mm thick) and energy-analyzed by a 51-cmlong X 46-cm-diameter NaI (TI) crystal TINA placed 25 cm from the target. The detection solid angle defined by T4 was -3%. Figure 2(a) shows the energy spectrum of positrons in an early time window 5-30 ns after the pion stop time. The pedestal peak at channel 850 ( T e + = 4 . 0 MeV after a correction for the energy loss of...

Three events for the decay K+-->pi+ nunu have been observed in the pion momentum region below the K+-->pi+pi0 peak, 140 < Ppi < 199 MeV/c, with an estimated background of 0.93+/-0.17(stat.) -0.24+0.32(syst.) events. Combining this observation with previously reported results yields a branching ratio of B(K+-->pi+ nunu) = (1.73(-1.05)+1.15) x 10(-10) consistent with the standard model prediction.

This chapter of the report of the "Flavor in the era of the LHC" Workshop discusses the theoretical, phenomenological and experimental issues related to flavor phenomena in the charged lepton sector and in flavor conserving CPviolating processes. We review the current experimental limits and the main theoretical models for the flavor structure of fundamental particles. We analyze the phenomenological consequences of the available data, setting constraints on explicit models beyond the standard model, presenting benchmarks for the discovery potential of forthcoming measurements both at the LHC and at low energy, and exploring options for possible future experiments.