Hi, There is a paper by Hoodbhoy, Jaffe and Sather, which says that higher twist (twist-4) effects on b1 at Q^2 = 1 GeV^2 are only 5%. So they would seem negligible at the kinematics of our proposal or HERMES. See http://dx.doi.org/10.1103/PhysRevD.43.3071 Cheers, Oscar