Higher twists in spin structure functions from a 'constituent quark' point of view

We discuss the implications of a ''constituent quark'' structure of the nucleon for the leading (1/Q2-) power corrections to the spin structure functions. Our basic assumption is the presence of quark-gluon correlations in the nucleon wave function, whose size, ρ ∼ 0.3 fm, is small compared to the nucleon radius, R (two-scale picture). We argue that in this picture the isovector twist-4 matrix element in the proton has a sizable negative value, MN2|f2u-d| ∼ ρ-2, while the twist-3 matrix elements are small, MN2d2 ∼ R-2. These findings are in agreement with the result of a QCD fit to g1 world data, including recent neutron data from HERMES and Jefferson Lab Hall A, which gives MN2f2u-d = -0.28 ± 0.08 GeV2