Well-established nucleon resonances revisited by double-polarization measurements

TL;DR

This study reports the first measurement of the double-polarization observable G in neutral pion photoproduction off protons, revealing significant discrepancies with existing models and providing new insights into baryon resonance contributions.

Contribution

It provides the first experimental data for the observable G in this reaction, challenging current partial wave analyses and improving understanding of nucleon resonances.

Findings

Large differences between data and predictions for N(1535) and N(1520) contributions.

Data in the third resonance region align reasonably with models.

Highlights the sensitivity of G to baryon resonance contributions.

Abstract

The first measurement is reported of the double-polarization observable G in photoproduction of neutral pions off protons, covering the photon energy range from 620 to 1120 MeV and the full solid angle. G describes the correlation between the photon polarization plane and the scattering plane for protons polarized along the direction of the incoming photon. The observable is highly sensitive to contributions from baryon resonances. The new results are compared to the predictions from SAID, MAID, and BnGa partial wave analyses. In spite of the long-lasting efforts to understand {\gamma}p -> p{\pi} 0 as the simplest photoproduction reaction, surprisingly large differences between the new data and the latest predictions are observed which are traced to different contributions of the N (1535) with spin-parity J^P = 1/2^- and N (1520) with J^P = 3/2^- . In the third resonance region, where N (1680) with J^P = 5/2^+ production dominates, the new data are reasonably close to the predictions.