Direct measurement of the helicity of the muonic neutrino

(eng) We describe a simple experiment for measuring the helicity of the muonic neutrino, hνμ, directly. This experiment is the muonic analog of the celebrated Brookhaven experiment for the electron neutrino. Both are based on measuring the longitudinal polarization PL of the recoils in a 0→1 lepton capture; here we exploit 12C(μ−, νμ)12B(g.s.), determining the polarization of 12B through its β‐decay asymmetry. Recoils going into a specific hemisphere are chosen by the trick of ’’selective implantation.’’ The paper includes an elementary derivation of the requisite theoretical expressions. We find hνμ = −1.06(11), i.e., −1.0 with considerable accuracy.