Fragmentation of Transversely Polarized Quarks Probed in Transverse Momentum Distributions
John Collins
TL;DR
This work introduces a leading-twist spin-dependent fragmentation function that links transverse quark polarization to the azimuthal distribution of hadrons within jets, enabling direct probes of transverse spin in SIDIS and hadron-hadron processes. It develops a comprehensive framework for transverse-momentum dependent parton distributions and fragmentation functions, including polarization via helicity density matrices, and shows how factorization extends to intrinsic transverse momentum with polarization. By computing the spin-transfer in elastic e-q scattering, it derives a coefficient that governs spin transfer to observed hadrons through fragmentation, predicting a sinφ azimuthal asymmetry in SIDIS. The findings provide a concrete experimental pathway to study quark transversity and its connection to chiral-symmetry breaking in QCD, with clear implications for SIDIS and back-to-back hadron production measurements in hadronic collisions.
Abstract
It is shown that the azimuthal dependence of the distribution of hadrons in a quark jet is a probe of the transverse spin of the quark initiating the jet. This results in a new spin-dependent fragmentation function that acts at the twist-2 level. One example of a process where it contributes is semi-inclusive deeply inelastic scattering with a transversely polarized nucleon target but with an {\em unpolarized} electron beam. This process is treated in detail. Another process is the cross section for singly polarized hadron-hadron scattering when two high $p_{\perp}$ hadrons are measured in the final state and are close to back-to-back in azimuth. The new fragmentation function is sensitive to the coupling of the fragmentation process to (spontaneous) chiral symmetry breaking.