Higher loop effects in M(atrix) orbifolds

Scattering of zero-branes off the fixed point in R8/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 2-loop term, finding a non-zero result. We find a discrepancy with M(atrix) theory. Although the result has the right dependence on v and b for the scattering of zero-branes off the fixed point the factors of N do not match. We also discuss scattering in the orbifolds, R5/Z2 and R9/Z2, where we find the predicted fractional charges. (orig.)