Speaker
Description
We report a preliminary measurement of the Collins azimuthal asymmetry for identified charged pions within jets in transversely polarized proton--gold collisions at $\sqrt{s_{NN}}=200$ GeV using the Relativistic Heavy Ion Collider (RHIC) and recorded by the Solenoidal Tracker At RHIC (STAR) detector. In the Transverse-Momentum-Dependent (TMD) framework, the Collins modulation $A_{UT}^{\sin(\phi_S-\phi_H)}$ provides access to the product of the quark transversity distribution and the Collins fragmentation function, and thus probes transverse spin--momentum correlations in QCD hadronization. On the one hand, general arguments have been put forth that there should be factorization breaking of TMDs in $pp$ collisions. But on the other hand, detailed calculations imply the Collins asymmetries in $pp$ collisions and semi-inclusive deep-inelastic scattering are the same. A central motivation of this measurement is to test the robustness and universality of the Collins mechanism and associated TMD factorization in a nuclear environment, where additional color interactions could in principle modify the observed asymmetry compared to that seen in $pp$ collisions.
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