Spin Polarizability of a Proton using Polarized Photon Beam and Polarized Butanol Target at Mainz Microtron

Abstract

Within this work, the data analysis and experimental results on the measurement of the double polarization observable (or beam-target asymmetry), Σ2z, for real Compton scattering off the proton is presented. The Σ2z values are measured via a circularly polarized photon beam incident upon a longitudinally polarized butanol target in the resonance region, Eγ = 250-310 MeV. This experiment was performed at the MAMI-A2 tagged photon facility in Mainz, Germany, during two experimental beamtimes in 2014 and 2015. This work also involves the extraction of proton spin polarizabilities, which are fundamental structure constants, similar to its charge and mass. These constants provide a measure of the global resistance of the nucleon's spin axis against displacement in an external electric or magnetic field, which makes them an excellent tool to study the structure of the nucleon. While the spin-independent, or scalar, electric (α_E1) and magnetic (β_M1) polarizabilities of the nucleon have been measured, little effort has been made to extract the spin-dependent polarizabilities. These four leading order spin dependent polarizabilities, γ_E1E1, γ_M1M1, γ_M1E2 and γ_E1M2, describe the spin response of a proton to electric and magnetic dipole and quadrupole interactions. This requires the precise measurement of the single and double polarization observables which are sensitive to these polarizabilities. Through this analysis, the spin polarizabilities have been extracted by performing a global analysis with the aid of two QCD-based models. The extracted four spin polarizabilities are: γ_E1E1= -4.06 ± 1.46, γ_M1M1= -3.24 ± 1.05, γ_E1M2= 0.47 ± 2.00 and γ_M1E2= 1.36 ± 1.43 in units of 10-4 fm4. The uncertainties in γ_E1E1, γ_M1M1 and γ_E1M2, were improved by a factor of two to four, but γ_M1E2 remained unchanged. These extracted spin polarizabilities are also in good agreement with dispersion, Heavy Baryon chiral perturbation and K-matrix theory predictions.