New Experimental Method for Investigation of the Nucleon Polarizabilities

The electric α and magnetic β polarizabilities are basic structure constants of the nucleon, which characterize the response of the internal nucleon structure to the action of external electric and magnetic fields. The knowledge of α and β provides a stringent test of models describing the quark-meson structure of the nucleon, e.g., in Chiral Perturbation Theory [1]. These polarizabilities are found by measuring Compton scattering cross-sections as a function of the photon energy and scattering angle and by a best fit using a theoretical description with the quantities of interest as open parameters. Experiments focus on photon energies below the pion mass because at these energies the Compton scattering cross-sections can be related model-independently to the polarizabilities through a Low Energy Theorem [2], which takes into account the proton structure in lowest order and shows that the scattering amplitude of Compton scattering off a system with spin 1⁄2 can be expanded in powers of Eγ. Recent experiments on Compton scattering to measure the polarizability of the proton and the deuteron were mostly performed using quasi-monoenergetic photons from bremsstrahlung tagging (see review of Shumacher [3]). The photons were scattered on liquid hydrogen or deuterium, and the scattered photons were detected under one or several scattering angles with NaI or BaF2 spectrometers. These experiments provided the four-vectors pi and the numbers of the incoming and scattered photons. A disadvantage of this method is a limited photon flux a tagger system can provide. To increase the luminosity, thick liquid targets were used. In contrast to the above experiments, in the method proposed in [4] and developed at the Superconducting Darmstadt Electron Accelerator S-DALINAC, bremsstrahlung photons produced by an electron beam are used, and not only the angle and energy of the scattered photons but also the angle and energy of the recoiling protons (deuterons) are measured. This approach (sketched in Fig. 1) increases the luminosity and lowers the background considerably, especially below 50 MeV.