Measurement of the proton scalar polarizabilities at MAMI

Abstract

The scalar polarizabilities are fundamental static properties of the nucleon, like the mass or the charge. Together with the four spin polarizabilities, they describe the response of the nucleon’s internal structure to an external electromagnetic field. An experimental determination of these parameters is possible via nuclear Compton scattering on protons and neutrons. Within this dissertation, Compton scattering on protons was studied in an experiment performed at the tagged photon facility of the Mainz Microtron (MAMI). A linearly polarized photon beam impinged on a liquid hydrogen target and the scattered photons were detected using the Crystal Ball/TAPS calorimeter, which provides large acceptance together with an excellent energy and angular resolution for photon detection. The unpolarized differential cross-section and photon beam asymmetry were measured with unprecedented precision for photon beam energies from 85 to 140 MeV . Using only these data, new values of the two proton scalar polarizabilities were extracted with a precision comparable to several current global extractions using different theoretical frameworks and combining all previous data. They will be crucial to resolve ambiguities in these existing extractions. The new data provide an important contribution to the experimental Compton scattering program at MAMI. They will be used in combination with already published results on single and double spin observables allowing for a combined extraction of all the six proton polarizabilities from experimental data measured at the MAMI tagged photon facility.