Search for Long-Lived Neutral Heavy Leptons in 400-GeV/cProton 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 $\sigma B$ for $L^0\text{'}\mathrm{s}$ with Feynman $x\gtrsim 0.2$ and ${\theta }_{\mathrm{lab}}\lesssim 10$ mrad with branching ratio $B$ into two charged particles is $\sigma B\lesssim 2.8\times {10}^{-35\mathrm{}}$ ${\mathrm{cm}}^2$/nucleon at the 90% confidence level for masses below 1.0 GeV/${\mathit{c}}^2$ and lifetimes between ${10}^{-10\mathrm{}}$ and ${10}^{-8\mathrm{}}$ sec.