Intermediate mass scale in rank-six superstring models

The current experimental lower limit on proton lifetime from the decay mode p→nu-bar+K+ is used to obtain a limit on the intermediate mass scale in superstring-inspired models with rank-six groups. It is shown that the intermediate mass scale breaking in these models can be as low as 1 x 1014 GeV without violating current proton-decay data, provided the Yukawa couplings of superstring models are no larger than the corresponding couplings in SU(5) supergravity models. Scalar-quark masses are then larger than 1 TeV and the photino mass is no more than a few gigaelectronvolts