Extracting the spin polarizabilities of the proton by measurement of Compton double-polarization observables

Abstract

The Compton double-polarization observable 2z has been measured for the first time in the (1232) resonance region using a circularly polarized photon beam incident on a longitudinally polarized proton target at the Mainz Microtron. This paper reports these results, together with the model-dependent extraction of four proton spin polarizabilities from fits to additional asymmetry data using either a dispersion relation calculation or a baryon chiral perturbation theory calculations with the weighted average of these two fits resulting in γ E 1E 1 = −2.87 ± 0.52, γ M1M1 = 2.70 ± 0.43, γ E 1M2 = −0.85 ± 0.72, and γ M1E 2 = 2.04 ± 0.43, in units of 10 −4 fm 4 .

Nucleon Polarizabilities and Compton Scattering as Playground for Chiral Perturbation Theory

“…With MAMI [133] and HIGS [134] experiments underway, the database will soon be greatly improved. It is worth mentioning that MAMI is also finalising a program to measure the CS double-polarization observables (Σ 2x , Σ 2z ) which will lead to an improved extraction of proton spin polarizabilities [86][87][88].…”

“…Equations ( 2) and (3). It follows from the ν 2 term in the low-energy expansion of the RCS amplitude f (ν): [88], and Σ 3 [89,90]. Krupina et al [91] performed a partial-wave analysis (PWA) of proton RCS data below pion-production threshold.…”

“…In the following, I want to discuss the nucleon polarizabilities, focusing on new empirical results from the last five years and comparisons to χPT predictions. References quoted in the summary figures are: PDG [106], MAID [104], experiments [86][87][88][89]107,108], dispersion relations [11,76,83,94,101,105,109,110], PWA [91], lattice QCD [111][112][113][114][115][116], HBχPT fit [62,64,66], BχPT fit [63], HBχPT [73,94,117], BχPT δ-expansion [41,[43][44][45] and BχPT -expansion [56,59].…”

“…Combining double-polarization observable and beam-asymmetry measurements, one is sensitive to the lowest-order spin polarizabilities, see Figure 9. For the extraction of the polarizabilities from the MAMI data for Σ 2x [86,87], Σ 2z [88] and Σ 3 [89], as well as the older LEGS data for Σ 3 [90], one can use dispersive models [28,94,124] or χPT fits [40].…”

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].…”