Twist-three Fragmentation Function Contribution to the Single Spin Asymmetry in pp Collisions Zhong-Bo Kang, 1, ∗ Feng Yuan, 2, 1, † and Jian Zhou 2, ‡ $^{1}$RIKEN BNL Research Center, Brookhaven National Laboratory, Upton, NY 11973 $^{2}$Nuclear Science Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720 (Dated: November 21, 2018) We study the twist-three fragmentation function contribution to the single transverse spin asym- metries in inclusive hadron production in pp collisions, p $^{↑}$p → h + X . In particular, we evaluate the so-called derivative contribution which dominates the spin asymmetry in the forward direction of the polarized proton. With certain parametrizations for the twist-three fragmentation function, we estimate its contribution to the asymmetry of π 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 π 0 productions observed by the STAR collaboration at RHIC. PACS numbers: 12.38.Bx, 13.88.+e, 12.39.St Abstract arXiv:1002.0399v1 [hep-ph] 2 Feb 2010 $^{∗}$Electronic address: zkang@bnl.gov $^{†}$Electronic address: fyuan@lbl.gov $^{‡}$Electronic address: jzhou@lbl.gov 1