Polarization Observables $T$ and $F$ in the $\gamma p \to \pi^0 p$ Reaction at CLAS

TL;DR

This study measures polarization observables in pion photoproduction at Jefferson Lab, providing data that refines partial-wave analyses and models of nucleon resonances, especially around higher energy ranges.

Contribution

First measurement of polarization observables $T$ and $F$ in this reaction at Jefferson Lab, improving nucleon resonance models and partial-wave analyses.

Findings

Data generally agree with existing models at lower energies.

Significant differences observed at higher energies, indicating the need for model updates.

Inclusion of data improves fit quality and alters resonance parameters.

Abstract

Pion photoproduction in the $\overrightarrow\gamma\overrightarrow p \to \pi^0 p$ reaction has been measured in the FROST experiment at the Thomas Jefferson National Accelerator Facility. In this experiment, circularly polarized photons with energies up to 3.082 GeV impinged on a transversely polarized frozen-spin target. Final-state protons were detected in the CEBAF Large Acceptance Spectrometer. Results of the polarization observables $T$ and $F$ have been extracted for $W$ from 1445 MeV to 2525 MeV. The data generally agree with predictions of present partial-wave analyses, but also show marked differences for higher $W$ ranges. By incorporating the present data into the databases, the SAID fits have been improved with relatively small $\chi^2$ and significant changes in the parameters of the $\Delta(1910)1/2^+$ and $N(2190)7/2^-$ have been found with the J\"uBo model.