Proton Decay and the Dimopoulos-Wilczek Mechanism in Minimal SO(10)   Models
  Proton decay is examined within the framework of certain SO(10) models which have low dimension representations compatible with the recently found constraints coming from string theory, and which use the Dimopoulos-Wilczek mechanism to solve the doublet-triplet splitting problem. It is found that the mass parameter that controls proton decay rate is intimately related to the low energy data via the coupling unification condition, and that a suppression of proton decay is achieved at the cost of large threshold effects. Furthermore, in cases where there are states with intermediate masses, the threshold effects further enhance proton decay. The experimental bound then severely constrains the parameter space of these models. Some possible solutions are suggested. 
S. Urano, R. Arnowitt