Twist-3 fragmentation effects for ALT in light hadron production from proton–proton collisions

We compute the contribution from the twist-3 fragmentation function for light hadron production in collisions between transversely and longitudinally polarized protons, i.e., p↑p→→hX, which can cause a double-spin asymmetry (DSA) ALTALT. This is a naïve T-even twist-3 observable that we analyze in collinear factorization using both Feynman gauge and lightcone gauge as well as give a general proof of color gauge invariance. So far only twist-3 effects in the transversely polarized proton have been studied for ALTALT in p↑p→→hX. However, there are indications that the naïve T-odd transverse single-spin asymmetry (SSA) ANAN in p↑p→hX is dominated not by such distribution effects but rather by a fragmentation mechanism. Therefore, one may expect similarly that the fragmentation contribution is important for ALTALT. Given possible plans at RHIC to measure this observable, it is timely to provide a calculation of this term.