Speaker
Description
Recent measurements by the PHENIX collaboration of $\eta$ meson spin observables will be presented. The $\eta$ meson is a practical final-state for study: as a light neutral meson, it is abundantly produced in hadronic collisions, while its heavier mass compared to the $\pi^0$ helps mitigate detector effects that limit the reach and precision of its lighter counterpart. At forward rapidity, the cross section at $\sqrt{s} = 500$ GeV and transverse single spin asymmetry (TSSA) at $\sqrt{s} = 200$ GeV were measured. For $p_T \gtrsim 2$ GeV, the cross section is well-described by perturbative QCD calculations, confirming collinear factorization in this regime. This measurement is set to be included in an updated $\eta$ meson fragmentation function global analysis. The forward TSSA provides sensitivity to the twist-3 quark-gluon correlation functions, related by $k_T$ moments to the quark transverse-momentum-dependent distributions. At high Feynman-x values, large asymmetries exceeding 30% are observed. Comparisons to the $\pi^0$ TSSA indicate no major impact from final-state differences like mass or strange quark content. On the other hand, predictions of the twist-3 initial-state contribution to the TSSA underestimate the asymmetry, suggesting a key role played by twist-3 fragmentation terms. The status of the midrapidity cross section and longitudinal double spin asymmetry at $\sqrt{s} = 510$ GeV will also be presented. These measurements have the broadest reach in $p_T$ of any PHENIX inclusive measurement. The cross section will serve as an additional input to the global fragmentation function fit while the longitudinal spin asymmetry probes the gluon helicity distribution.
