Spin dependence of {\eta} meson production in proton-proton collisions close to threshold

TL;DR

This study measures the spin-dependent analyzing power in eta meson production during proton-proton collisions near threshold, providing precise data that challenge existing theoretical models of the production mechanism.

Contribution

It offers the first high-precision measurements of the analyzing power close to threshold, revealing discrepancies with current theoretical predictions.

Findings

Analyzing power is consistent with zero at Q=15 MeV.

Strong interference effects observed at Q=72 MeV.

Results challenge existing eta meson production theories.

Abstract

Taking advantage of the high acceptance and axial symmetry of the WASA-at-COSY detector, and the high degree of the polarized proton beam of COSY, the reaction pp{\to} pp{\eta} has been measured close to threshold to explore the analyzing power Ay. The angular distribution of Ay is determined with the precision improved by more than one order of magnitude with respect to previous results allowing a first accurate comparison with theoretical predictions. The determined analyzing power is consistent with zero for an excess energy of Q = 15 MeV signaling s wave production with no evidence for higher partial waves. At Q = 72 MeV the data reveals strong interference of P s and P p partial waves and cancellation of (P p) and Ss*Sd contributions. These results rule out the presently available theoretical predictions for the production mechanism of the {\eta} meson.