Twist-three fragmentation function contribution to the single spin asymmetry in pp collisions

We study the twist-three fragmentation function contribution to the single transverse spin asymmetries in inclusive hadron production in pp collisions, p↑p→h+X. In particular, we calculate the associated derivative terms which dominate the spin asymmetries in these processes. With certain parameterizations for the twist-three fragmentation function, we estimate its contribution to the single spin asymmetry of π0 production at RHIC energy. We find that the contribution is sizable and might be responsible for the big difference between the asymmetries in η and π0 productions observed by the STAR collaboration at RHIC.