A Search for Longlived Neutral Heavy Leptons in 400-GeV/c Proton - Nucleon Interactions

We report a null search for neutral heavy leptons produced unaccompanied by muons in 400-GeV/c proton-nucleon collisions in a magnetized beam dump and decaying in vacuum downstream. The upper limit on sigmaB for L/sup 0/'s with Feynman x > or approx. = 0.2 and theta/sub lab/ 9 or approx. = 10 mrad with branching ratio B into two charged particles is sigmaB 9 or approx. = 2.8 x 10/sup -35/ cm/sup 2//nucleon at the 90% confidence level for masses below 1.0 GeV/c/sup 2/ and lifetimes between 10/sup -10/ and 10/sup -8/ sec.