Quark Helicity Flip and the Transverse Spin Dependence of Inclusive Dis *

Inclusive DIS with unpolarized beam exhibits a subtle dependence on the transverse target spin, arising from the interference of one–photon and two–photon exchange amplitudes in the cross section. We argue that this observable probes mainly the quark helicity–flip amplitudes induced by the non-perturbative vacuum structure of QCD (spontaneous chiral symmetry breaking). This conjecture is based on (a) the absence of significant Sudakov suppression of the helicity–flip process if soft gluon emission in the quark subprocess is limited by the chiral symmetry–breaking scale µ 2 chiral ≫ Λ 2 QCD ; (b) the expectation that the quark helicity–conserving twist–3 contribution is small. The normal target spin asymmetry is estimated to be of the order 10 −4 in the kinematics of the planned Jefferson Lab Hall A experiment. A fundamental property of QCD is that the quark helicity is conserved in perturbative interactions, because of chiral invariance in the light–quark sector. It is known, however, that non-perturbative interactions at distances of the order 1/µ chiral ∼ 0.3 fm cause the dynamical breaking of chiral symmetry and lead to the appearance of non-zero quark helicity–flip amplitudes. This effect essentially determines the structure and interaction of hadrons at large distances. An interesting question is what the presence of quark helicity–flip amplitudes implies for hard electromagnetic processes (invariant momentum transfer Q 2 ≫ 1 GeV 2). Such processes couple directly to the quark degrees of freedom and in certain cases can be described by factorization of the cross section/amplitude into " hard " and " soft " con