Novel Search for Heavy [Formula presented] Mixing from the [Formula presented] Decay of [Formula presented] Confined in an Atom Trap

A new technique, full neutrino momentum reconstruction, is used to set limits on the admixture of heavy neutrinos into the electron neutrino. We measure coincidences between nuclear recoils and positrons from the beta decay of trapped radioactive atoms and deduce the neutrino momentum. A search for peaks in the reconstructed recoil time-of-flight spectrum as a function of positron energy is performed. The admixture upper limits range from [Formula presented] to [Formula presented] and are the best direct limits for neutrinos (as opposed to antineutrinos) for the mass region of 0.7 to 3.5 MeV.