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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 → h X, which can cause a double-spin asymmetry (DSA) A LT . 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 A LT in p ↑ p → h X. However, there are indications that the naïve T-odd transverse single-spin asymmetry (SSA) A N in p ↑ p → h X 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 A LT . Given possible plans at RHIC to measure this observable, it is timely to provide a calculation of this term.

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