POSSIBLE VIOLATION OF SUM-RULES FOR HIGHER-TWIST PARTON DISTRIBUTIONS

Precise measurements of polarized electroproduction and Drell-Yan proc esses in the deep inelastic limit will soon provide the first informat ion on the higher-twist parton distributions g(T)(x) and h(L) (a:). Su m rules for higher-twist structure functions are only valid provided t he corresponding Compton amplitudes satisfy unsubtracted dispersion re lations. Subtracted dispersion relations have to be used when the (rea l part of the) forward scattering amplitudes do not fall off rapidly e nough for nu --> infinity (fixed Q(2)). Formally, such subtractions le ad to delta functions at the origin in the parton distributions, which are not accessible to experiment, and the integral over the data fail s to satisfy the sum rule. The delta functions in the parton distribut ions can be identified with the zero modes that appear in light-front quantization. An explicit infinite momentum boost identifies these sof t quark modes with low momentum contributions arising from self-energy -type interactions.