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The PIENU experiment at TRIUMF aims to measure the branching ratio of pion decays R = Γ(π+ → e+νe + π+ → e+ νeγ) Γ(π+→μ+νμ + π+ → μ+νμγ) with precision <0.1%, providing a stringent test of the Standard Model hypothesis of electron-muon universality and a search for new physics.

The branching ratio, Re/μ Γ(π → eν + eνγ)/Γ(π → μν + μνγ), provides a sensitive test of muon-electron universality in weak interactions. The status of the PIENU experiment at TRIUMF, which aims to improve the precision of the Re/μ measurement by a factor of > 5, is presented.

Heavy neutrinos were sought in pion decays \\(\\pi^+ \\rightarrow \\mu^+ \\nu\\) by examining the observed muon energy spectrum for extra peaks in addition to the expected peak for a massless neutrino. No evidence for heavy neutrinos was observed. Upper limits were set on the neutrino mixing matrix \\(|U_{\\mu i}|^2\\) in the neutrino mass region of 15.7--33.8 MeV/c\\(^2\\), improving on previous results by an order of magnitude.

Measurements of the response function of the PIENU NaI(T\\(\\ell\\)) and CsI crystal calorimeter using a monochromatic 70 MeV/c positron beam at various incidence angles are described. The experimental setup and relevant physical processes involved were simulated using Geant4 to reproduce positron energy spectra. Monte Carlo simulations were compared with experimental data across ten calorimeter-beam angles and showed good agreement. This allowed the validation of simulation parameters that were essential for precise measurements of pion decays.

The multi-Photomultiplier Tube (mPMT) photosensors will be used in the Water Cherenkov Test Experiment (WCTE) to efficiently detect the photons produced in the whole detector. One of the aims behind the development of WCTE is to test the technology and implement it in future water Cherenkov experiments such as the Hyper-Kamiokande experiment and its Intermediate Water Cherenkov Detector. Each mPMT is built using nineteen 3-inch PMTs arranged on a semi-spherical support matrix. In this paper, we describe the design and manufacture of the mechanical components, the procedures for casting an optical gel between PMTs and acrylic cover, and the overall assembly procedure of the mPMTs. Details of the electronics used in the mPMT modules are not included in this paper and will be presented in a separate publication. We also report on the R&D performed on the selection of the optical gel ratio along with transmittance measurements and the reflectance measurements performed on the aluminium reflector. We also present the optical tests performed on the mPMT module using a 405 nm LED and the resulting increase in the effective photosensitive area by surrounding the PMTs with a reflector. A summary of the production and installation of the mPMTs for the WCTE is also presented in this paper.

PIONEER is a rapidly developing effort aimed to perform a pristine test of lepton flavour universality (LFU) and of the unitarity of the first row of the CKM matrix by significantly improving the measurements of rare decays of the charged pion. In Phase I, PIONEER aims to measure the charged-pion branching ratio to electrons vs.\\ muons \\(R_{e/\\mu}\\) to 1 part in \\(10^4\\), improving the current experimental result \\(R_{e/\\mu}\\,\\text{(exp)} =1.2327(23)\\times10^{-4}\\) by a factor of 15. This precision on \\(R_{e/\\mu}\\) will match the theoretical accuracy of the SM prediction allowing for a test of LFU at an unprecedented level, probing non-SM explanations of LFU violation through sensitivity to quantum effects of new particles up to the PeV mass scale. Phase II and III will aim to improve the experimental precision of the branching ratio of pion beta decay, \\(\\pi^+\\to \\pi^0 e^+ \\nu (\\gamma)\\), currently at \\(1.036(6)\\times10^{-8}\\), by a factor of three and six, respectively. The improved measurements will be used to extract \\(V_{ud}\\) in a theoretically pristine manner. The ultimate precision of \\(V_{ud}\\) is expected to reach the 0.05\\,\\% level, allowing for a stringent test of CKM unitarity. The PIONEER experiment will also improve the experimental limits by an order of magnitude or more on a host of exotic decays that probe the effects of heavy neutrinos and dark sector physics. This input to the 2026 update of the European Strategy for Particle Physics Strategy describes the physics motivation and the conceptual design of the PIONEER experiment, and is prepared based on the PIONEER proposal submitted to and approved with high priority by the PSI program advisory committee (PAC). Using intense pion beams, and state-of-the-art instrumentation and computational resources, the PIONEER experiment is aiming to begin data taking by the end of this decade.

The three body pion decays \\({\\pi}^+{\\rightarrow}l^+{\\nu}X~(l=e,{\\mu})\\), where \\(X\\) is a weakly interacting neutral boson, were searched for using the full data set from the PIENU experiment. An improved limit on \\({\\Gamma}({\\pi}^+{\\to}e^+{\\nu}X)/{\\Gamma}({\\pi}^+{\\to}{\\mu}^+{\\nu}_{\\mu})\\) in the mass range \\(0

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