HOW PRECISELY CAN ONE DETERMINE M(U)/M(D)

Abstract

We point out that in order to precisely determine the quark mass ratio m(u)/m(d) one needs to theoretically calculate a QCD matrix element which receives a potentially large contribution from instantons. Within the semiclassical approximation, this instanton contribution can be large enough to have m(u) = 0 without any difficulty. We also discuss how instantons affect the chiral coupling constant L7 whose value was recently estimated by Leutwyler to determine m(u)/m(d).