Measurement of Compton Scattering at MAMI for the Extraction of the Electric and Magnetic Polarizabilities of the Proton

First concurrent extraction of the leading-order scalar and spin proton polarizabilities

“…In particular, real Compton scattering (RCS) at low energies is parametrized by polarizabilities that describe the response of the charge and magnetization distributions inside the nucleon to an applied quasistatic electromagnetic field. These structure constants are fundamental properties of the nucleon and their determination has driven a relevant experimental effort in the last few years [1][2][3][4][5].…”

“…[6] for the definition of the different observables). In particular, the work from Mornacchi et al [4] provides the highest statistics proton RCS single dataset ever obtained below the pion photoproduction threshold, with 60 unpolarized differential cross section points and 36 beam asymmetry points measured over a large angular range and with small statistical and systematic errors. Therefore, it represents a significant improvement for a more accurate extractions of all the different polarizability values.…”

“…For the low-energy set, in addition to the new data from Refs. [4,5], we used the same selection extensively discussed in a previous work by Pasquini et al [14], which includes datasets from Refs. [28][29][30][31][32][33][34][35][36][37][38][39].…”

“…The polarization observables have enhanced sensitivity to the spin polarizabilities [23,51], hence are crucial for the extraction of these structure constants. The adopted datasets include three different polarization observables: Σ 2x [1], Σ 2z [3], and Σ 3 both below [2,4,5] and above [48] pion photoproduction threshold. Although the data for the photon asymmetry Σ 3 above threshold are from the same dataset of the LEGS unpolarized cross section, they are consistent with the available MAMI results for both the pπ 0 and the nπ + photoproduction channels [50].…”

“…In the first step of our analysis, we checked the consistency of the selected database by looking at the distribution of the normalized residual for each of the largest datasets (e.g., with more than 40 data points): the unpolarized cross section from the A2 [4], the TAPS [39], and the LARA [44] Collaborations; and the beam asymmetry from the LEGS [48] Collaboration. For this purpose, we performed a preliminary test including the entire database where all six polarizabilities were simultaneously fitted using the MAID-2021 [25,26] multipole solution.…”