Higher Loop Effects in M(atrix) Orbifolds

Scattering of zero branes off the fixed point in R/Z2, as described by a super-quantum mechanics with eight supercharges, displays some novel effects relevant to Matrix theory in non-compact backgrounds. The leading long distance behaviour of the moduli space metric receives no correction at one loop in Matrix theory, but does receive a correction at two loops. There are no contributions at higher loops. We explicitly calculate the two-loop term, finding a non-zero result. It has the right dependence on v and b for the scattering of zero branes off the (fractional) two-brane charge, expected in M-theory, at the fixed point. We discuss this result in the light of the Matrix theory conjecture, taking into account possible N dependent rescalings of velocity and impact parameter. We also discuss scattering in the orbifolds, R/Z2 and R /Z2 where we find the predicted fractional charges.