In the present work of the PIENU experiment, heavy neutrinos were sought in pion decays π + →µ + ν.No evidence for extra peak was found in the muon kinetic energy spectrum and 90% confidence level upper limits were set on the neutrino mixing matrix |U µi | 2 in the mass range of 15.7 to 33.8 MeV/c 2 , improving an order of magnitude over previous experiments.Current status of lepton universality test is also reported.

(PIENU Collaboration) Instituto de Ciencias Nucleares, Universidad Nacional Autónoma de México, CDMX 04510, México Department of Physics, Graduate School of Science, Osaka University, Toyonaka, Osaka, 560-0043, Japan Virginia Tech., Blacksburg, Virginia 24061, USA SUPA School of Physics and Astronomy, University of Glasgow, Glasgow, G12-8QQ, United Kingdom Department of Physics and Astronomy, University of British Columbia, Vancouver, British Columbia V6T 1Z1, Canada TRIUMF, 4004 Wesbrook Mall, Vancouver, British Columbia V6T 2A3, Canada Department of Engineering Physics, Tsinghua University, Beijing, 100084, China Physics Department, Arizona State University, Tempe, AZ 85287, USA Universidad Autónoma de Sinaloa, Culiacán, México PRISMA Cluster of Excellence and Institut für Kernphysik, Johannes Gutenberg-Universität Mainz, Johann-Joachim-Becher-Weg 45, D 55128 Mainz, Germany University of Northern British Columbia, Prince George, British Columbia V2N 4Z9, Canada KEK, 1-1 Oho, Tsukuba-shi, Ibaraki, 300-3256, Japan Physics Department, Osaka University, Toyonaka, Osaka, 560-0043, Japan Brookhaven National Laboratory, Upton, NY, 11973-5000, USA (Dated: February 10, 2021)

The rare pion decays π^+→μ^+ν_μνν̅ and π^+→e^+ν_eνν̅ are allowed in the Standard Model but highly suppressed. These decays were searched for using data from the PIENU experiment. A first result for Γ(π^+→μ^+ν_μνν̅)/Γ(π^+→μ^+ν_μ)<8.6×10^-6, and an improved measurement Γ(π^+→e^+ν_eνν̅)/Γ(π^+→μ^+ν_μ)<1.6×10^-7 were obtained.

In the present work of the PIENU experiment, heavy neutrinos were sought in pion decays (cid:25) + ! (cid:22) + (cid:23) . No evidence for extra peak was found in the muon kinetic energy spectrum and 90% confidence level upper limits were set on the neutrino mixing matrix j U (cid:22) i j 2 in the mass range of 15.7 to 33.8 MeV / c 2 , improving an order of magnitude over previous experiments. Current status of lepton universality test is also reported.

(The PIENU Collaboration) Instituto de Ciencias Nucleares, Universidad Nacional Autónoma de México, CDMX 04510, México Physics Department, Osaka University, Toyonaka, Osaka, 560-0043, Japan Virginia Tech., Blacksburg, VA, 24061, USA SUPA School of Physics and Astronomy, University of Glasgow, Glasgow, United Kingdom Department of Physics and Astronomy, University of British Columbia, Vancouver, B.C., V6T 1Z1, Canada TRIUMF, 4004 Wesbrook Mall, Vancouver, B.C., V6T 2A3, Canada Department of Engineering Physics, Tsinghua University, Beijing, 100084, China Physics Department, Arizona State University, Tempe, AZ 85287, USA Universidad Autónoma de Sinaloa, Culiacán, México PRISMA Cluster of Excellence and Institut für Kernphysik, Johannes Gutenberg-Universität Mainz, Johann-Joachim-Becher-Weg 45, D 55128 Mainz, Germany University of Northern British Columbia, Prince George, B.C., V2N 4Z9, Canada KEK, 1-1 Oho, Tsukuba-shi, Ibaraki, Japan Brookhaven National Laboratory, Upton, NY, 11973-5000, USA (Dated: February 24, 2020)

The rare pion decays pi(+)->mu(+)nu(mu)nu(nu) over bar and pi(+)-> e(+)nu(e)nu(nu) over bar 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(+)->mu(+)nu(mu)nu(nu) over bar)/Gamma(pi(+)->mu(+)nu(mu)) < 8.6 x 10(-6) and an improved measurement Gamma ( pi(+)-> e(+)nu(e)nu(nu) over bar)/Gamma(pi(+)->mu(+)nu(mu) ) < 1.6 x 10(-7) were obtained.

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.

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.

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.

The rare pion decays ${\ensuremath{\pi}}^{+}\ensuremath{\rightarrow}{\ensuremath{\mu}}^{+}{\ensuremath{\nu}}_{\ensuremath{\mu}}\ensuremath{\nu}\overline{\ensuremath{\nu}}$ and ${\ensuremath{\pi}}^{+}\ensuremath{\rightarrow}{e}^{+}{\ensuremath{\nu}}_{e}\ensuremath{\nu}\overline{\ensuremath{\nu}}$ are allowed in the Standard Model but highly suppressed. These decays were searched for using data from the PIENU experiment. A first result for $\mathrm{\ensuremath{\Gamma}}({\ensuremath{\pi}}^{+}\ensuremath{\rightarrow}{\ensuremath{\mu}}^{+}{\ensuremath{\nu}}_{\ensuremath{\mu}}\ensuremath{\nu}\overline{\ensuremath{\nu}})/\mathrm{\ensuremath{\Gamma}}({\ensuremath{\pi}}^{+}\ensuremath{\rightarrow}{\ensuremath{\mu}}^{+}{\ensuremath{\nu}}_{\ensuremath{\mu}})<8.6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$ and an improved measurement $\mathrm{\ensuremath{\Gamma}}({\ensuremath{\pi}}^{+}\ensuremath{\rightarrow}{e}^{+}{\ensuremath{\nu}}_{e}\ensuremath{\nu}\overline{\ensuremath{\nu}})/\mathrm{\ensuremath{\Gamma}}({\ensuremath{\pi}}^{+}\ensuremath{\rightarrow}{\ensuremath{\mu}}^{+}{\ensuremath{\nu}}_{\ensuremath{\mu}})<1.6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}$ were obtained.

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.

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

TRIUMF, 4004 Wesbrook Mall, Vancouver, B.C., V6T 2A3, Canada Inst. de Ciencias Nucl., Univ. Nacional Autónoma de México, CDMX 04510, México Physics Department, Osaka University, Toyonaka, Osaka, 560-0043, Japan Virginia Tech., Blacksburg, VA, 24061, USA SUPA School of Physics and Astronomy, Univ. of Glasgow, Glasgow, United Kingdom Dept. of Phys. and Astronomy, Univ. of British Columbia, Vancouver, V6T 1Z1, Canada Department of Engineering Physics, Tsinghua University, Beijing, 100084, China Physics Department, Arizona State University, Tempe, AZ 85287, USA Institut für Kernphysik, Johannes Gutenberg-Universität Mainz,

The PIENU experiment at TRIUMF aims to measure the branching ratio of the pion decay modes $R^{\pi}=[{\pi}^+{\rightarrow}e^+{\nu}_e({\gamma})]/[{\pi}^+{\rightarrow}{\mu}^+{\nu}_{\mu}({\gamma})]$ with precision of $<0.1\%$. Precise measurement of $R^{\pi}$ provides a stringent test of electron-muon universality in weak interactions. The current status of the PIENU experiment and future prospects are presented.

The PIENU experiment aims to measure the branching ratio of the charged pion decay with precision of < 0.1 %. This measurement is much sensitive to search for massive neutrinos coupled to electrons in \({\pi }^{+}{\rightarrow }e^{+}{\nu }_{e}\) decay. The initial analysis was completed and the upper limit on the neutrino mixing parameter |U e i |2 in the neutrino mass range of 0 to 55 MeV/c 2 was improved by a factor of 1.5, and the sensitivity for the mass range of 68 to 129 MeV/c 2 was improve by a factor of up to 4.

The PIENU experiment at TRIUMF aims to measure the branching ratio of the pion decay modes Rπ=[π+→e+νe(γ)]/[π+→μ+νμ(γ)] with precision of <0.1%. Precise measurement of Rπ provides a stringent test of electron-muon universality in weak interactions. The current status of the PIENU experiment and future prospects are presented.

The pion branching ratio, \(R_{\pi } = \frac { {\Gamma }(\pi ^{+} \rightarrow e^{+} \nu _{e} + \pi ^{+}\rightarrow e^{+} \nu _{e} \gamma )}{\Gamma (\pi ^{+} \rightarrow \mu ^{+} \nu _{\mu } + \pi ^{+} \rightarrow \mu ^{+} \nu _{\mu } \gamma )}\), provides a sensitive test of lepton universality and constraints on many new physics scenarios. The theoretical uncertainty on the Standard Model prediction of R π is 0.02 %, a factor of twenty smaller than the experimental uncertainty. The analysis of a subset of data taken by the PIENU experiment will be presented. The result, R π = (1.2344 ± 0.0023(s t a t) ± 0.0019(s y s t)) ⋅ 10−4 [1], is consistent with the Standard Model prediction and represents a 0.1 % constraint on lepton non-universality.

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_{e/μ}^{exp}=[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%.