A Reexamination of Proton Decay in Supersymmetric Grand Unified Theories

We reconsider dimension-five proton decay operators, making semi-quantitative remarks which apply to a large class of supersymmetric GUTs in which the short-distance operators are correlated with the fermion Yukawa couplings. In these GUTs, which include minimal SU(5), the operators $(u^cd_i^c)^dagger(d_j u_tau)$, induced by charged Higgsino dressing, completely dominate for moderate to large $tanbeta$. The rate for $p rarr (K^+,pi^+) bar{ u}_tau$ grows rapidly, as $tanbeta^4$, and the K^+ to pi^+ branching ratio can often be precisely predicted. At small $tanbeta$ the operators $(u d_i)(d_j u)$ are dominant, while the operators $(u d_i)(u ell^-)$, with left-handed charged leptons, are comparable to the neutrino operators in the generic GUT and suppressed in minimal GUTs. Charged-lepton branching fractions are always small at large $tanbeta$. The electron to muon ratio is small in minimal GUTs but can be larger, even of order one, in other models. All other operators are very small. At small $tanbeta$ in non-minimal GUTs, gluino and neutralino dressing effects on neutrino rates are not negligible.