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"Britton, D I"
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The PIENU experiment at TRIUMF : a sensitive probe for new physics
Study of rare decays is an important approach for exploring physics beyond the Standard Model (SM). The branching ratio of the helicity suppressed pion decays, R = , is one of the most accurately calculated decay process involving hadrons and has so far provided the most stringent test of the hypothesis of electron-muon universality in weak interactions. The branching ratio has been calculated in the SM to better than 0.01% accuracy to be RSM = 1.2353(1) × 10 . The PIENU experiment at TRIUMF, which started taking physics data in September 2009, aims to reach an accuracy five times better than the previous experiments, so as to confront the theoretical calculation at the level of ±0.1%. If a deviation from the RSM is found, \"new physics\" beyond the SM, at potentially very high mass scales (up to 1000 TeV), could be revealed. Alternatively, sensitive constraints on hypotheses can be obtained for interactions involving pseudoscalar or scalar interactions. So far, 4 million π+ → e+ νe ue events have been accumulated by PIENU. This paper will outline the physics motivations, describe the apparatus and techniques designed to achieve high precision and present the latest results.
Journal Article
Status of the PIENU experiment at TRIUMF
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.
Journal Article
Status of the PIENU experiment
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.
Journal Article
Search for Heavy Neutrinos in \\(\\pi \\to \\mu\\nu\\) Decay
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.
Paper
Measurement of the response function of the PIENU calorimeter
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.
Paper
Search for three body pion decays \\({\\pi}^+{\\to}l^+{\\nu}X\\)
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
Paper
Search for the rare decays \\(\\pi^+ \\to \\mu^+\\nu_\\mu\\nu\\bar\\nu\\) and \\(\\pi^+ \\to e^+\\nu_e\\nu\\bar\\nu\\)
The rare pion decays \\({\\pi}^+{\\rightarrow}{\\mu}^+{\\nu}_{\\mu}{\\nu}\\bar{\\nu}\\) and \\({\\pi}^+{\\rightarrow}e^+{\\nu}_{e}{\\nu}\\bar{\\nu}\\) are allowed in the Standard Model but highly suppressed. These decays were searched for using data from the PIENU experiment. A first result for \\({\\Gamma}({\\pi}^+{\\rightarrow}{\\mu}^+{\\nu}_{\\mu}{\\nu}\\bar{\\nu})/{\\Gamma}({\\pi}^+{\\rightarrow}{\\mu}^+{\\nu}_{\\mu})<8.6{\\times}10^{-6}\\), and an improved measurement \\({\\Gamma}({\\pi}^+{\\rightarrow}{e}^+{\\nu}_{e}{\\nu}\\bar{\\nu})/{\\Gamma}({\\pi}^+{\\rightarrow}{\\mu}^+{\\nu}_{\\mu})<1.6{\\times}10^{-7}\\) were obtained.
Paper
Improved search for two body muon decay \\({\\mu}^+{\\rightarrow}e^+X_H\\)
Charged lepton flavor violating muon decay \\({\\mu}^+{\\rightarrow}e^+X_H\\), where \\(X_H\\) is a massive neutral boson, was sought by searching for extra peaks in the muon decay \\({\\mu}^+{\\rightarrow}e^+{\\nu}\\bar{\\nu}\\) energy spectrum in the \\(m_{X_H}\\) mass region \\(47.8-95.1\\) MeV/\\(c^2\\). No signal was found and 90% confidence level upper limits were set on the branching ratio \\({\\Gamma}({\\mu}^+{\\rightarrow}e^+X_H)/{\\Gamma}({\\mu}^+{\\rightarrow}e^+{\\nu}\\bar{\\nu})\\) at the level of \\(10^{-5}\\) for this region.
Paper
Improved Search for Heavy Neutrinos and a Test of Lepton Universality in the Decay \\(\\pi^+ \\rightarrow \\mbox{e}^+ \\nu\\)
Two results from the PIENU Experiment are presented reporting a test of lepton universality in pion decay and improved limits on heavy neutrinos coupling to positrons. The status of the full analysis for the \\(\\pi^+ \\rightarrow \\mbox{e}^+ \\nu\\) branching ratio measurement is summarized.
Paper
Improved Search for Heavy Neutrinos in the Decay \\(\\pi\\rightarrow e\\nu\\)
A search for massive neutrinos has been made in the decay \\(\\pi\\rightarrow e^+ \\nu\\). No evidence was found for extra peaks in the positron energy spectrum indicative of pion decays involving massive neutrinos (\\(\\pi\\rightarrow e^+ \\nu_h\\)). Upper limits (90 \\% C.L.) on the neutrino mixing matrix element \\(|U_{ei}|^2\\) in the neutrino mass region 60--135 MeV/\\(c^2\\) were set, which are %representing an order of magnitude improvement over previous results.
Paper