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

A search for massive neutrinos has been made in the decay ${\pi }^{+}\to {\mathrm{e}}^{+}\nu$. No evidence was found for extra peaks in the positron energy spectrum indicative of pion decays involving massive neutrinos ($\pi \to e^{+}{\nu }_h$). Upper limits (90% C.L.) on the neutrino mixing matrix element $|U_{ei}{|}^2$ in the neutrino mass region $60–135\mathrm{MeV}/c^2$ were set and are an order of magnitude improvement over previous results.

Figure 1

Schematic illustration of the PIENU detectors with the pion stopping region shown in the inset.

Figure 2

(a) Energy sum $E_s$ measured in B1, B2, S1, S2, and B3. (b) Z vertex for events with positron energy $E_{e^{+}}<52\mathrm{MeV}$ (shaded histogram) and $E_{e^{+}}>52\mathrm{MeV}$ (blue full line). The two distributions are normalized to contain the same number of events, and cuts applied are indicated by the red vertical dashed lines.

Figure 3

Background-suppressed positron energy spectrum (black histogram). Fitted components include muon decays in flight (thick blue line, from MC), ${\pi }^{+}\to {\mathrm{e}}^{+}\nu$ (green, dot-dashed line, fit to MC), and ${\pi }^{+}\to {\mu }^{+}\to {\mathrm{e}}^{+}$ (red dashed line, from late-time data events). The insert shows the (rebinned) residuals (Data–Fit) with statistical error bars and the signal shape in the case of $E_{e^{+}}=40\mathrm{MeV}$ and $|U_{ei}{|}^2={10}^{-8}$.

Figure 4

Acceptance correction $\text{Acc}(E_{e^{+}})$ as a function of the positron energy as determined with the MC simulation. The error bars are statistical only.

Figure 5

90% C.L. upper limits on the square of the mixing matrix elements $|U_{ei}{|}^2$ of heavy neutrinos coupled to electrons (thick red line). The black dashed line shows the results from Ref. [18].