Soft-Fermion-Pole Mechanism to Single Spin Asymmetry in Hadronic Pion Production

TL;DR

This paper derives a twist-3 formula for single spin asymmetry in pion production during proton-proton collisions, highlighting the significant role of soft-fermion-pole contributions in the observed asymmetries at RHIC.

Contribution

It introduces a novel twist-3 cross section formula including soft-fermion-pole effects, which were previously less understood in the context of SSA in hadronic pion production.

Findings

SFP contributions can be as large as SGP contributions due to large partonic cross sections.

The derivative of the SFP function does not affect the cross section.

The formula is relevant for interpreting RHIC experimental data.

Abstract

Single spin asymmetry (SSA) is a twist-3 observable in the collinear factorization approach. We present a twist-3 single-spin-dependent cross section formula for the pion production in pp-collision, p^\uparrow p\to\pi X, relevant to RHIC experiment. In particular, we calculate the soft-fermion-pole (SFP) contribution to the cross section from the quark-gluon correlation functions. We show that its effect can be as large as the soft-gluon-pole (SGP) contribution owing to the large SFP partonic hard cross section, even though the derivative of the SFP function does not participate in the cross section.