Abstract
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.
Highlights
In the original Standard Model (SM) [1], neutrinos are included as massless particles
Depending on the choices of parameters and mass hierarchy in the νMSM, the two heaviest sterile neutrino states may occur in the range probed by meson decays [5], while the lightest state can play a role as a dark matter particle in the keV=c2 mass range
Most systematic and acceptance effects canceled to first order, since the upper limit was proportional to the ratio of the fitted number of signal events and the number of πþ → eþν events Nðπ → eνeÞ, which were estimated by fitting a Monte Carlo (MC) generated spectrum to the data for Eeþ > 52 MeV
Summary
In the original Standard Model (SM) [1], neutrinos are included as massless particles. Heavy neutrinos with MeV=c2 to GeV=c2 masses can have measurable effects in meson decays that can be explored by precisely measuring their decay branching ratios or by searching for extra peaks in the energy spectrum of their leptonic two-body decays (e.g. π; K; B → lν) [9]. The presence of heavy neutrinos relaxes the helicity suppression; comparing the experimental value with the theoretical SM calculation RSM 1⁄4 ð1.2352 Æ 0.0002Þ × 10−4 [14,15,16], limits on jUeij have been obtained for masses below 60 MeV=c2 [13]. We present a search for additional peaks in the low-energy region of the backgroundsuppressed πþ → eþν spectrum using the full data set collected by the PIENU experiment, representing a sample larger than [19] by an order of magnitude
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