Search for Heavy Neutrinos in $\pi \to \mu\nu$ Decay

The PIENU collaboration Aguilar-Arevalo, A. ; Aoki, M. ; Blecher, M. ; et al.
Phys.Lett. B798 (2019) 134980, 2019.
Inspire Record 1728771 DOI 10.17182/hepdata.91134

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.

2 data tables

90% C.L. upper limits in Tmu<1.2MeV

90% C.L. upper limits in Tmu>1.2MeV


Improved search for heavy neutrinos in the decay $\pi\rightarrow e\nu$

The PIENU collaboration Aguilar-Arevalo, A. ; Aoki, M. ; Blecher, M. ; et al.
Phys.Rev. D97 (2018) 072012, 2018.
Inspire Record 1642434 DOI 10.17182/hepdata.91133

A search for massive neutrinos has been made in the decay π+→e+ν. No evidence was found for extra peaks in the positron energy spectrum indicative of pion decays involving massive neutrinos (π→e+νh). Upper limits (90% C.L.) on the neutrino mixing matrix element |Uei|2 in the neutrino mass region 60–135  MeV/c2 were set and are an order of magnitude improvement over previous results.

1 data table

90% C.L. upper limits to |U_{ei}|^2