Searching for sterile neutrinos fromπandKdecays

Abstract

The production of heavy sterile neutrinos from ${\ensuremath{\pi}}^{\ensuremath{-}}$, ${K}^{\ensuremath{-}}$ decay at rest yields charged leptons with negative helicity (positive for ${\ensuremath{\pi}}^{+}$, ${K}^{+}$). We obtain the branching ratio for this process and argue that a Stern-Gerlach filter with a magnetic field gradient leads to spatially separated domains of both helicity components with abundances determined by the branching ratio. Complemented with a search of the monochromatic peak, this setup can yield both the mass and mixing angles for sterile neutrinos with masses in the range $3\text{ }\text{ }\mathrm{MeV}\ensuremath{\lesssim}{m}_{s}\ensuremath{\lesssim}414\text{ }\text{ }\mathrm{MeV}$ in next generation high intensity experiments. We also study oscillations of light Dirac and Majorana sterile neutrinos with ${m}_{s}\ensuremath{\simeq}\mathrm{eV}$ produced in meson decays including decoherence aspects arising from lifetime effects of the decaying mesons and the stopping distance of the charged lepton in short baseline experiments. We obtain the transition probability from production to detection via charged current interactions including these decoherence effects for $3+1$ and $3+2$ scenarios, also studying $|\ensuremath{\Delta}L|=2$ transitions from $\overline{\ensuremath{\nu}}\ensuremath{\leftrightarrow}\ensuremath{\nu}$ oscillations for Majorana neutrinos and the impact of these effects on the determination of $CP$-violating amplitudes. We argue that decoherence effects are important in current short baseline accelerator experiments, leading to an underestimate of masses, mixing and $CP$-violating angles. At MiniBooNE/SciBooNE we estimate that these effects lead to an $\ensuremath{\sim}15%$ underestimate for sterile neutrino masses ${m}_{s}\ensuremath{\gtrsim}3\text{ }\text{ }\mathrm{eV}$. We argue that reactor and current short baseline accelerator experiments are fundamentally different and suggest that in future high intensity experiments with neutrinos produced from $\ensuremath{\pi}$, $K$ decay at rest, stopping the charged leptons on distances much smaller than the decay length of the parent meson suppresses considerably these decoherence effects.