Neutrino Masses Without Seesaw Mechanism in a SUSY SU(5) Model With Additional bar{5}'_L+5'_L

A radiatively-induced neutrino mass matrix with a simple structure is proposed on the basis of an SU(5) SUSY GUT model with R-parity violation. The model has matter fields $\bar{5}'_{L}+5'_{L}$ in addition to the ordinary matter fields $\bar{5}_{L}+10_{L}$ and Higgs fields $H_u+\bar{H}_d$. The R-parity violating terms are given by $\bar{5}_{L} \bar{5}_{L} 10_{L}$, while the Yukawa interactions are given by $\bar{H}_d \bar{5}'_{L} 10_{L}$. Since the matter fields $\bar{5}'_L$ and $\bar{5}_L$ are different from each other at the unification scale, the R-parity violation effects at a low energy scale appear only through the $\bar{5}'_L \leftrightarrow \bar{5}_{L}$ mixings. In order to make this R-parity violation effect harmless for proton decay, a discrete symmetry Z_3 and a triplet-doublet splitting mechanism analogous to the Higgs sector are assumed.