Photoproduction of mesons and Compton scattering on the proton: Selected results from the A2 Collaboration at MAMI

Vahe Sokhoyan,R Beck,U Thoma,Y Wunderlich,A Thiel

doi:10.1051/epjconf/202024101017

Abstract

The A2 Collaboration performs a manifold research program using real photons in the Crystal Ball/TAPS experiment at the MAMI accelerator facility in Mainz. The experiments take advantage of high- intensity unpolarized, linearly or circularly polarized photon beams, and unpolarized or polarized targets. The detector setup provides almost complete coverage in solid angle and is well suited for the detection of multi particle final states. In order to probe the internal structure of the nucleon, the spectrum of baryon resonances is studied via measurements of unpolarized cross sections and various polarization observables in single and double meson photoproduction. The program aiming to determine the scalar and spin polarizabilities of the nucleons with high precision is performed with the Compton scattering experiments. In 2017, the focal plane detector used in the tagging system of the Crystal Ball/TAPS experiment was completely renewed, allowing new measurements with unprecedentedly high precision. This paper presents recent selected results from the A2 Collaboration at MAMI.

Highlights

The program carried out by the A2 Collaboration covers a broad range of topics in hadron physics
The experimental setup consisting of the Crystal Ball and TAPS calorimeters provides a nearly 4π acceptance coverage
The beam asymmetry Σ3 was measured for Compton scattering below pion threshold for the first time, in order to obtain complementary information for the extraction of the proton scalar polarizabilities, in addition to the unpolarized cross section

Summary

Introduction

The program carried out by the A2 Collaboration covers a broad range of topics in hadron physics. In addition to the proton targets, various experiments are carried out with light and heavy nuclei for the studies of mesons photoproduction on neutrons and search for inmedium modifications of hadrons in the nuclear medium. At energies below pion threshold (80–140 MeV), new data sets on Compton scattering on the proton were recently acquired. Based on these data sets, the scalar polarizabilities of the proton will be extracted with an unprecedented precision. The beam asymmetry Σ3 was measured for Compton scattering below pion threshold for the first time, in order to obtain complementary information for the extraction of the proton scalar polarizabilities, in addition to the unpolarized cross section. This paper includes recent results from the proton scalar polarizability measurement and the new results on the production of π0η pairs on a free proton and nuclear targets

Experimental Setup

Measurement of the proton scalar polarizabilities

Photoproduction of π0η pairs on the proton