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Register a new userImproved Search for Heavy Neutrinos and a Test of Lepton Universality in the Decay $pi^+ rightarrow mbox{e}^+ u$
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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 $pi^+ rightarrow mbox{e}^+ u$ branching ratio measurement is summarized.
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A new measurement of the branching ratio, $R_{e/mu} =Gamma (pi^+ rightarrow mbox{e}^+ u + pi^+ rightarrow mbox{e}^+ u gamma)/ Gamma (pi^+ rightarrow mu^+ u + pi^+ rightarrow mu^+ u gamma)$, resulted in $R_{e/mu}^{exp} = (1.2344 pm 0.0023 (stat) pm 0.0019 (syst)) times 10^{-4}$. This is in agreement with the standard model prediction and improves the test of electron-muon universality to the level of 0.1 %.
A search for massive neutrinos has been made in the decay $pirightarrow e^+ u$. No evidence was found for extra peaks in the positron energy spectrum indicative of pion decays involving massive neutrinos ($pirightarrow e^+ u_h$). 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.
Heavy neutrinos were sought in pion decays $pi^+ rightarrow mu^+ u$ 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.
We present the results of searches for nucleon decay via bound neutron to antineutrino plus pizero and proton to antineutrino plus piplus using data from a combined 172.8 kiloton-years exposure of Super-Kamiokande-I, -II, and -III. We set lower limits on the partial lifetime for each of these modes. For antineutrino pizero, the partial lifetime is >1.1x10^{33} years; for antineutrino piplus, the partial lifetime is >3.9x10^{32} years at 90% confidence level.
The trilepton nucleon decay modes $p rightarrow e^+ u u$ and $p rightarrow mu^+ u u$ violate $|Delta (B - L)|$ by two units. Using data from a 273.4 kiloton year exposure of Super-Kamiokande a search for these decays yields a fit consistent with no signal. Accordingly, lower limits on the partial lifetimes of $tau_{p rightarrow e^+ u u} > 1.7 times 10^{32}$ years and $tau_{p rightarrow mu^+ u u} > 2.2 times 10^{32}$ years at a $90 % $ confidence level are obtained. These limits can constrain Grand Unified Theories which allow for such processes.
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