Positivity constraints and flavor dependence of higher twists

Abstract

The positivity bound for the transverse asymmetry A 2 may be improved and applied for each quark flavor separately. We use it to test the consistency of twist-two approximation for the transverse spin-dependent ( g T) and the longitudinal spin-averaged ( F L) structure functions. While it is satisfied in the case of u quarks, it might be violated for d quarks in a region of moderate x because of its negative polarization. We attribute this inconsistency to a stronger twist-three contribution, whose existence found by the QCD sum rule method is a long-standing puzzle.