2025/09/22 by The STAR collaboration
#FOS: Physical sciences #High Energy Physics - Experiment (hep-ex)
paper · doi:10.48550/arxiv.2509.17487
We report the first measurement of Λ and Λ transverse polarization inside jets in unpolarized proton-proton collisions, which is attributed to the polarizing fragmentation function (PFF). The polarization is measured as a function of the jet transverse momentum (pT), the fraction of the jet momentum carried by Λ(Λ) hyperons, and the transverse momentum of Λ (Λ) hyperons relative to the jet axis. Λ polarization shows a clear dependence on the jet pT, changing sign from negative to positive from low to high jet pT, while Λ polarization mostly remains negative. These data cover a wide range of jet energies, and will provide the first constraints on the gluon PFF, which is not constrained by e+e- data. Moreover, these results provide an opportunity to test the transverse momentum-dependent (TMD) evolution effect and its universality for PFF, thus are important input to a complete understanding of the transverse polarization mechanism.