Sterile Neutrinos in a Grand Unified Model

The recent experimental results indicating the neutrino oscillations maystrongly suggest that at least one more light neutrino species is required inorder to reconcile the existing data. In the simple GUT frameworks, this factseems difficult to preserve the parallelism between quarks and leptons. In thisletter, we investigate an SO(10) grand unified model with a pair of extragenerations in addition to the known three ones. Using the GUT relations, theobtained neutrino mass matrix naturally indicates that one of the SU(2) singlet(sterile) neutrino is very light and has large mixing with muon neutrino, whichcan explain the atmospheric neutrino anomaly, and the hot dark matter neutrinois also provided. The solar neutrino problem can be solved by the mixing withmuon neutrino consistently with quark mixing, namely, the Cabibbo angle.Comment: 15 pages, 5 eps figure