Polarization Transfer in Wide-Angle Compton Scattering and Single-Pion Photoproduction from the Proton

Area of Science:

• Nuclear Physics
• Particle Physics
• Quantum Chromodynamics

Background:

• Proton spin structure is crucial for understanding fundamental particle interactions.
• Generalized Parton Distributions (GPDs) offer a framework to probe the proton's internal structure.
• Previous studies have explored various aspects of proton structure, but discrepancies remain.

Purpose of the Study:

• To investigate the proton's spin structure using wide-angle exclusive Compton scattering.
• To measure the polarization transfer from a circularly polarized photon beam to the recoil proton.
• To compare experimental results with theoretical predictions based on GPDs.

Main Methods:

• Measurement of polarization transfer in wide-angle Compton scattering at 3.7 GeV incident photon energy.
• Analysis of proton scattering at a center-of-mass angle of 70°.
• Calculation of the longitudinal transfer coefficient K_LL.

Main Results:

• The longitudinal transfer coefficient K_LL was measured to be 0.645 ± 0.059 (statistical) ± 0.048 (systematic).
• The measured K_LL has the same sign as predicted for a mechanism involving photon interaction with a single, spin-carrying quark.
• The experimental value of K_LL is approximately three times larger than predicted by GPD-based calculations.

Conclusions:

• The findings support the role of single-quark spin transfer in Compton scattering.
• The significant deviation from theoretical predictions indicates the presence of unknown contributions to the scattering amplitude.
• Further theoretical and experimental work is needed to fully understand the proton's spin structure.