Twist-three Fragmentation Function Contribution to the Single Spin Asymmetry in pp Collisions

Abstract

We study the twist-three fragmentation function contribution to the single transverse spin asymmetries in inclusive hadron production in pp collisions, pp->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 pi0 production at RHIC energy. We find that the contribution is sizable and might be responsible for the big difference between the asymmetries in eta and pi0 productions observed by the STAR collaboration at RHIC.

Figures (2)

• Figure 1: Generic Feynman diagrams to calculate the derivative term of the twist-three fragmentation function contributions to the single inclusive hadron production in $pp$ scattering, $p^\uparrow p\to hX$: $p_1$ and $p_2$ are the two incident partons' momenta, $k_1$ and $k_2$ are the outgoing partons' momenta, where the quark $k_1$ fragments into final-state hadron $P_h$. The expansion of the scattering amplitude in terms of transverse momentum component of $k_1=P_h/z+k_{1T}$ leads to the twist-three contributions from the fragmentation function $\hat{H}(z)$ defined in Eq. (\ref{['hzdef']}).
• Figure 2: Twist-three fragmentation contributions to the single spin asymmetries in hadron production in the forward direction of the polarized nucleon at RHIC at $\sqrt{s}=200GeV$, as functions of $x_F$, with the parameterization for the twist-three fragmentation function $\hat{H}(z)$ in Eq. (\ref{['parHz']}) with parameters $C_f=-0.4$ GeV and $a=1,2,4$ (from up to bottom), respectively.