Abstract
The electric (αE1) and magnetic (βM1) scalar polarizabilities describe the response of the nucleon to an applied electric or magnetic field. They are not only fundamental properties related to the internal structure and dynamics of the nucleon, but they are important also in other areas of physics, such as atomic structure. The values of αE1 and βM1 quoted by the Particle Data Group were determined using data on the unpolarized differential cross-section of the Compton scattering $\gamma p \to \gamma p$. The measurement of the beam asymmetry Σ3, provides an alternative approach to the extraction of the scalar polarizabilities, with different sensitivity and systematics compared to the unpolarized crosssection. This asymmetry was measured recently for the first time below the pion photoproduction threshold by the A2 Collaboration with the Crystal Ball/TAPS experiment at MAMI (Mainz, Germany). A new high precision measurement of both a unpolarized cross-section and beam asymmetry Σ3a is ongoing at MAMI and the polarizabilities αE1 and βM1 will be extracted with unprecedented precision.
Highlights
The scalar polarizabilities αE1 and βM1 characterise the response of a nucleon to an external static electromagnetic field
They play a fundamental role in our understanding of the nucleon, but they limit the precision in many other areas of physics. They can be extracted by measuring the differential unpolarized cross section [1] and the beam asymmetry Σ3 [2] for Compton scattering
To account for the variation of the polarization degree along the beamtime for the two datasets with different orientation of the polarization plane, the number of Compton scattering events was weighted with the corresponding value of the linear polarization in the equation for Σ3 derived from equation 2
Summary
The scalar polarizabilities αE1 and βM1 characterise the response of a nucleon to an external static electromagnetic field. They play a fundamental role in our understanding of the nucleon, but they limit the precision in many other areas of physics They can be extracted by measuring the differential unpolarized cross section [1] and the beam asymmetry Σ3 [2] for Compton scattering. The current PDG [3] values for the scalar dipole polarizabilities are αE1 = (11.2 ± 0.4) × 10−4fm−3 and βM1 = (11.2 ± 0.4) × 10−4fm−3 These values were previously extracted only from the unpolarized cross-section in different Compton scattering experiments, and the largest data set was obtained by the TAPS collaboration at MAMI [4].
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