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$p^{\uparrow }p\to h+X$. 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${\pi }^0$ 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${\pi }^0$ productions observed by the STAR collaboration at RHIC.