Measurement of the Proton Scalar Polarizabilities at MAMI

The scalar polarizabilities, $\alpha_{E1}$ and $\beta_{M1}$, as fundamental properties of the nucleon, play a crucial role not only in our understanding of the nucleon, but also in other areas such as atomic physics, where they provide e.g. corrections to the Lamb Shift. To date, these observables were extracted in parallel from unpolarized cross-sections of Compton scattering on the proton. At the MAMI accelerator facility in Mainz, Germany, the proton polarizabilities were measured using a linearly polarized photon beam for the first time in a photon energy range from 110 to 150 MeV. The photon beam, produced in the Glasgow-Mainz Photon Tagger, impinged on a liquid Hydrogen target and the reaction products were detected in the Crystal Ball and TAPS $4\pi$ spectrometer setup. The beam asymmetry $\Sigma_{3}$ was measured for the first time below pion threshold. This measurement will allow the first independent extraction of the observables $\alpha_{E1}$ and $\beta_{M1}$. The current status of the $\Sigma_{3}$ measurement as function of incoming photon energy and polar angle of the outgoing photon will be presented and the $\alpha_{E1}$ and $\beta_{M1}$ extraction will be discussed.