Flavour-changing neutral interactions in broken supersymmetry theories

The authors 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 Delta m/sub sq//sup 2//m/sub sq//sup 2 /<0(10/sup -3/) come from the K/sub 1/-K/sub 2/ mass matrix, while the non-observation of mu to e gamma imposes Delta m/sub sl//sup 2//m/sub sl//sup 2/<0(10/sup -3/) if the supersymmetric partners of the SU(2) and SU(1) bosons have masses O(100) GeV. These results help motivate a busy gauge theory with an extra U(1) symmetry spontaneously broken at low energy, perhaps of a non-minimal type.