A search for the rare decay K L → π 0 νν was performed. With the data collected in 2015, corresponding to 2.2 × 10 19 protons on target, a single event sensitivity of ð1.30 AE 0.01 stat AE 0.14 syst Þ × 10 −9 was achieved and no candidate events were observed. We set an upper limit of 3.0 × 10 −9 for the branching fraction of K L → π 0 νν at the 90% confidence level (C.L.), which improved the previous limit by almost an order of magnitude. An upper limit for K L → π 0 X 0 was also set as 2.4 × 10 −9 at the 90% C.L., where X 0 is an invisible boson with a mass of 135 MeV=c 2 .

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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.

High-precision 1H(e,e'p)pi(0) measurements at Q2 = 0.126 (GeV/c)2 are reported, which allow the determination of quadrupole amplitudes in the gamma*N-->Delta transition; they simultaneously test the reliability of electroproduction models. The derived quadrupole-to-dipole ( I = 3/2) amplitude ratios, R(SM) = (-6.5+/-0.2(stat+sys)+/-2.5(mod))% and R(EM) = (-2.1+/-0.2(stat+sys)+/-2.0(mod))%, are dominated by model error. Previous R(SM) and R(EM) results should be reconsidered after the model uncertainties associated with the method of their extraction are taken into account.

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