Measurement of transverse polarization of $Λ$ and $\barΛ$ hyperons inside jets in $pp$ collisions at $\sqrt{s}=200$ GeV

Abstract: We report the first measurement of $\Lambda$ and $\overline{\Lambda}$ 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 ($p_{\text{T}}$), the fraction of the jet momentum carried by $\Lambda$($\overline{\Lambda}$) hyperons, and the transverse momentum of $\Lambda:(\overline{\Lambda})$ hyperons relative to the jet axis. $\Lambda$ polarization shows a clear dependence on the jet $p_{\text{T}}$, changing sign from negative to positive from low to high jet $p_{\text{T}}$, while $\overline{\Lambda}$ 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.