Measurement of the proton scalar polarizabilities at MAMI

Abstract

The electric (α E1 ) and magnetic (β M1 ) scalar polarizabilities are fundamental properties related to the internal structure of the nucleon. They play a crucial role not only in our understanding of the nucleon, but also in other areas, such as atomic physics. In the past, the values of α E1 and β M1 were determined from the unpolarized differential cross-section of Compton scattering γ p → γ p. The measurement of the beam asymmetry Σ 3 provides an alternative approach to the extraction of the scalar polarizabilities with different sensitivity and systematics compared to the unpolarized cross-section. This asymmetry was measured for the first time below the pion photoproduction threshold by the A2 Collaboration with the Crystal Ball/TAPS experiment at MAMI (Mainz, Germany). A linearly polarized photon beam impinged on a liquid hydrogen target and the scattered photons were detected with the Crystal Ball/TAPS setup, providing almost 4π coverage. A new high precision measurement of both unpolarized cross-section and beam asymmetry Σ 3 will be performed in the near future and polarizabilities α E1 and β M1 will be extracted with unprecedented precision.