Exploring the Structure of the Bound Proton with Deeply Virtual Compton Scattering.

Area of Science:

• Nuclear Physics
• Particle Physics
• Quantum Chromodynamics

Background:

• Deeply virtual Compton scattering (DVCS) probes the proton's internal structure.
• The EMC effect reveals modifications to bound nucleon structure in nuclei.
• Understanding bound nucleon structure is crucial for nuclear physics.

Purpose of the Study:

• To measure beam spin asymmetry (BSA) in exclusive DVCS off a proton bound in Helium-4.
• To investigate medium modifications of the bound proton's partonic structure.
• To compare BSA for bound and free protons across various kinematics.

Main Methods:

• Utilized a 6 GeV longitudinally polarized electron beam.
• Scattered electrons off a pressurized Helium-4 gas target.
• Measured the azimuthal angle dependence of BSA using the CLAS spectrometer.

Main Results:

• Reported the first BSA measurement for exclusive DVCS on a bound proton.
• Observed a 20% to 40% smaller BSA for bound protons compared to free protons.
• Studied BSA dependencies on Q², xB, and t in a wide kinematic range.

Conclusions:

• The reduced BSA suggests possible medium modifications of the bound proton's partonic structure.
• This finding provides new insights into the EMC effect and nucleon structure in nuclei.
• Further studies are needed to fully understand these modifications.