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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.

A detailed discussion is given of the analysis of recent data to obtain improved upper bounds on the couplings |Ue4| 2 and |Uµ4| 2 for a mainly sterile neutrino mass eigenstate ν4. Using the excellent agreement among Ft values for superallowed nuclear beta decay, an improved upper limit is derived for emission of a ν4. The agreement of the ratios of branching ratios R (π) e/µ = BR(π + → e + νe)/BR(π + → µ + νµ), R (K) e/µ , R (Ds) e/τ , R (Ds) µ/τ , and R (D)e/τ , and the branching ratios BR(B + → e + νe) and BR(B + → µ + νµ) decays with predictions of the Standard Model, is utilized to derive new constraints on ν4 emission covering the ν4 mass range from MeV to GeV. We also discuss constraints from peak search experiments probing for emission of a ν4 via lepton mixing, as well as constraints from pion beta decay, CKM unitarity, µ decay, leptonic τ decay, and other experimental inputs.

A search for massive neutrinos has been made in the decay π þ → e þ ν. No evidence was found for extra peaks in the positron energy spectrum indicative of pion decays involving massive neutrinos (π → e þ ν h ).Upper limits (90% C.L.) on the neutrino mixing matrix element jU ei j 2 in the neutrino mass region 60-135 MeV=c 2 were set and are an order of magnitude improvement over previous results.

A new measurement of the branching ratio, R e/µ = Γ(π + → e + ν + π + → e + νγ)/Γ(π + → µ + ν + π + → µ + νγ), resulted in R exp e/µ = (1.2344 ± 0.0023(stat) ± 0.0019(syst)) × 10 −4 . This is in agreement with the standard model prediction and improves the test of electron-muon universality to the level of 0.1 %.PACS numbers: 13.20. Cz, 14.40.Be, 14.60.St, The standard model (SM) assumes equal electro-weak couplings of the three lepton generations, a hypothesis known as lepton universality which is studied in high precision measurements of π, K, τ, B, and W decays. A recent measurement of, where l represents e or µ, hinted at a possible violation of e-µ universality in second order weak interactions that involve neutral and charged currents. The branching ratio of pion decays, R e/µ = Γ(π → eν(γ))/Γ(π → µν(γ)), where (γ) indicates inclusion of associated radiative decays, has been calculated in the SM with extraordinary precision to be R [5], has provided one of the best tests of e-µ universality in weak interactions for the charged current, at the 0.2 % level giving sensitivity to new physics beyond the SM up to mass scales of O(500) TeV [3]. Examples of new physics probed include R-parity violating SUSY [6], extra leptons [7] and leptoquarks [8]. In this paper, we present the first results from the PIENU experiment, which improve on the precision of R exp e/µ and the test of e-µ universality.The branching ratio R e/µ is obtained from the ratio of positron yields from the π + → e + ν(γ) decay (total positron energy E e + = 69.8 MeV) and the π + → µ + ν(γ) decay followed by the µ + → e + νν(γ) decay (π + → µ + → e + , E e + = 0.5 − 52.8 MeV) using pions at rest. Figure 1 shows a schematic view of the apparatus [9] in which a 75-MeV/c π + beam from the TRIUMF M13 channel [10] was degraded by two thin plastic scintillators B1 and B2 and stopped in an 8-mm thick scintillator target (B3) at a rate of 5 × 10 4 π + /s. Pion tracking was provided by wire chambers (WC1 and WC2) at the exit of the beam line and two (x,y) sets of single-sided 0.3-mm thick planes of silicon strip detectors, S1 and S2, located immediately upstream of B3.