Search for long-lived massive neutrinos inZdecays

We search for events in the Mark II detector at SLAC Linear Collider with the topology of a Z boson decaying into a pair of long-lived massive particles. No events that are consistent with the search hypothesis are found. Interpreting the long-lived particle as a sequential Dirac neutrino ${\nu }_4$ of the fourth generation, we exclude at the 95% confidence level a significant range of mixing-matrix elements of ${\nu }_4$ to other-generation neutrinos for a ${\nu }_4$ mass from 10 to 43 GeV/${\mathit{c}}^2$.