Measurements of Double-Polarized Compton Scattering Asymmetries and Extraction of the Proton Spin Polarizabilities

TL;DR

This paper reports measurements of double-polarized Compton scattering asymmetries on the proton to extract its spin polarizabilities, providing model-independent insights into nucleon spin structure.

Contribution

It presents the first separation of all four proton spin polarizabilities using polarized Compton scattering data analyzed with dispersion and chiral perturbation theory models.

Findings

Measured asymmetries in the $ ext{Δ}(1232)$ region

Extracted four proton spin polarizabilities with uncertainties

Compared results with theoretical models

Abstract

The spin polarizabilities of the nucleon describe how the spin of the nucleon responds to an incident polarized photon. The most model-independent way to measure the nucleon spin polarizabilities is through polarized Compton scattering. Double-polarized Compton scattering asymmetries on the proton were measured in the $\Delta(1232)$ region using circularly polarized incident photons and a transversely polarized proton target at the Mainz Microtron. Fits to asymmetry data were performed using a dispersion model calculation and a baryon chiral perturbation theory calculation, and a separation of all four proton spin polarizabilities in the multipole basis was achieved. The analysis based on a dispersion model calculation yields $\gamma_{E1E1} = -3.5 \pm 1.2$, $\gamma_{M1M1}= 3.16 \pm 0.85$, $\gamma_{E1M2} = -0.7 \pm 1.2$, and $\gamma_{M1E2} = 1.99 \pm 0.29$, in units of $10^{-4}$ fm$^4$.