Neutral Pion Production in the Threshold Region

TL;DR

This paper reviews recent experimental measurements of neutral pion photoproduction near threshold, testing predictions of Chiral Perturbation Theory and exploring chiral symmetry breaking in QCD through polarized photon and target experiments.

Contribution

It presents new high-precision measurements of differential cross sections and asymmetries using polarized beams and targets, providing stringent tests of theoretical models.

Findings

Linearly polarized beam asymmetry matches Chiral Perturbation Theory predictions.

First extraction of pi-N scattering information from photo-pion reactions.

Data constrains chiral symmetry breaking dynamics in QCD.

Abstract

We give an overview of the physics motivation and evolution of the neutral pion photoproduction measurements in the threshold region conducted in the A2 collaboration at MAMI. The latest two experiments have been performed 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.