Flavour-changing neutral interactions in broken supersymmetric theories

We point out that in order to ensure an efficient 'super-GIM' suppression of flavour-changing neutral interactions, the supersymmetric partners of conventional fermions (squarks and sleptons) must be almost degenerate in mass. The strongest constraints on squark mass differences of Δmsub(sq)2/msub(sq)2 < O(10-3) come from the K1-K2 mass matrix, while the non-observation of μ → eγ imposes Δmsub(sl)2/msub(sl)2 < O(10-3) if the supersymmetric partners of the SU(2) and SU(1) bosons have masses O(100) GeV. These results help motivate a susy gauge theory with an extra U tilde(1) symmetry spontaneously broken at low energy, perhaps of a non-minimal type. (orig.)