Improved constraints on sterile neutrinos in the MeV to GeV mass range

Improved upper bounds are presented on the coupling $|U_{e4}{|}^2$ of an electron to a sterile neutrino ${\nu }_4$ from analyses of data on nuclear and particle decays, including superallowed nuclear beta decays, the ratios $R_{e/\mu }^{(\pi )}=\mathrm{BR}({\pi }^{+}\to e^{+}{\nu }_e)/\mathrm{BR}({\pi }^{+}\to {\mu }^{+}{\nu }_{\mu })$, $R_{e/\mu }^{(K)}$, $R_{e/\tau }^{(D_s)}$, and $B_{e2}^{+}$ decay, covering the mass range from MeV to GeV.

Figure 1

The 90% C.L. upper limits on $|U_{e4}{|}^2$ vs $m_{{\nu }_4}$ from various sources: PIBETA, pion beta decay BD1, previous limits from beta decay [29]; BD2, beta decay with the two dashed horizonal lines based on our analysis using [42] and [43]; PIENU and PIENU-H, the ratio $\frac{\mathrm{BR}({\pi }^{+}\to e^{+}{\nu }_e)}{\mathrm{BR}({\pi }^{+}\to {\mu }^{+}{\nu }_{\mu })}$ in the kinematically allowed and forbidden regions for ${\nu }_4$ emission; $Pie2$, ${\pi }^{+}\to e^{+}{\nu }_{e4}$ peak search [47]; KENU and KENU-H, the ratio $\frac{\mathrm{BR}(K^{+}\to e^{+}{\nu }_e)}{\mathrm{BR}(K^{+}\to {\mu }^{+}{\nu }_{\mu })}$ in the kinematically allowed and forbidden regions for ${\nu }_4$ emission; $Ke2$, $K^{+}\to e^{+}{\nu }_{e4}$ peak search [48]; $Dse2$, $D_s^{+}\to e^{+}{\nu }_{e4}$; and $Be2$, $B^{+}\to e^{+}{\nu }_{e4}$.