Search for Neutral Heavy Leptons in a High-Energy Neutrino Beam

A search for neutral heavy leptons (NHLs) has been performed using an instrumented decay channel at the NuTeV (E-815) experiment at Fermilab. The data were examined for NHLs decaying into muonic final states ( $\mu \mu \nu$, $\mu e\nu$, $\mu \pi$, and $\mu \rho$); no evidence has been found for NHLs in the 0.25–2.0 GeV mass range. This analysis places limits on the mixing of NHLs with standard light neutrinos at a level up to an order of magnitude more restrictive than previous search limits in this mass range.