Constraints on a massive Dirac neutrino model.

We examine constraints on a simple neutrino model in which there are three massless and three massive Dirac neutrinos and in which the left handed neutrinos are linear combinations of doublet and singlet neutrinos. We examine constraints from direct decays into heavy neutrinos, indirect effects on electroweak parameters, and flavor changing processes. We combine these constraints to examine the allowed mass range for the heavy neutrinos of each of the three generations. a This work is supported in part by funds provided by the U.S. Department of Energy (DOE) under contract #DE-AC02-76ER03069 and in part by the Texas National Research Laboratory Commission under grant #RGFY92C6. b National Science Foundation Young Investigator Award. Alfred P. Sloan Foundation Research Fellowship. Department of Energy Outstanding Junior Investigator Award.