Measurement of the proton scalar polarizabilities at MAMI

Johannes Gutenberg - Universität Mainz, Germany

^* e-mail: emornacc@uni-mainz.de

Published online: 1 February 2019

Abstract

The electric (α_E1) and magnetic (β_M1) scalar polarizabilities describe the response of the nucleon to an applied electric or magnetic field. They are not only fundamental properties related to the internal structure and dynamics of the nucleon, but they are important also in other areas of physics, such as atomic structure. The values of α_E1 and β_M1 quoted by the Particle Data Group were determined using data on the unpolarized differential cross-section of the Compton scattering $\gamma p\to \gamma p$. The measurement of the beam asymmetry Σ₃, provides an alternative approach to the extraction of the scalar polarizabilities, with different sensitivity and systematics compared to the unpolarized crosssection. This asymmetry was measured recently for the first time below the pion photoproduction threshold by the A2 Collaboration with the Crystal Ball/TAPS experiment at MAMI (Mainz, Germany). A new high precision measurement of both a unpolarized cross-section and beam asymmetry Σ₃a is ongoing at MAMI and the polarizabilities α_E1 and β_M1 will be extracted with unprecedented precision.