Asymmetries for neutral pion photoproduction in the threshold region

TL;DR

This paper presents new measurements of neutral pion photoproduction near threshold, providing stringent tests of Chiral Perturbation Theory and exploring isospin conservation and chiral symmetry breaking in QCD.

Contribution

It reports novel polarization measurements in pion photoproduction, offering the most precise data to date for testing theoretical predictions and extracting pion-nucleon interaction information.

Findings

Linearly polarized beam asymmetry data constrains Chiral Perturbation Theory.

First extraction of target and beam-target asymmetries in photo-pion reactions.

Results support or challenge existing models of chiral symmetry breaking.

Abstract

We report on two pion-photoproduction measurements in the threshold region conducted in the A2 collaboration at MAMI with the almost 4{\pi} Crystal Ball detector. The first was with a linearly polarized photon beam and unpolarized liquid-hydrogen target. The data analysis is now complete and the linearly polarized beam asymmetry along with differential cross sections provide the most stringent test to date of the predictions of Chiral Perturbation Theory and its energy region of convergence. More recently, a measurement was performed using both circularly polarized photons and a transversely polarized butanol frozen-spin target, with the goal of extracting both the target and beam-target asymmetries. From these we intend to extract {\pi}N scattering sensitive information for the first time in photo-pion reactions. This will be used to test isospin conservation and further test dynamics of chiral symmetry breaking in QCD as calculated at low energies by Chiral Perturbation Theory.