The g2p Experiment: A Measurement of the Proton's Spin Structure Functions

Author

Ryan Zielinski, University of New Hampshire, Durham

Date of Award

Fall 2017

Project Type

Dissertation

Program or Major

Physics

Degree Name

Doctor of Philosophy

First Advisor

Karl Slifer

Second Advisor

Elena Long

Third Advisor

Maurik Holtrop

Abstract

The E08-027 (g2p) experiment measured the spin structure functions of the proton at Jefferson Laboratory in Newport News, Va. Longitudinally polarized electrons were scattered from a transversely and longitudinally polarized solid ammonia target in Jefferson Lab's Hall A, with the polarized NH₃ acting as an effective proton target. Focusing on small scattering angle events at the electron energies available at Jefferson Lab, the experiment covered a kinematic phase space of 0.02 GeV² < Q² < 0.20 GeV² in the proton's resonance region. The spin structure functions, g₁^p(x,Q²) and g₂^p(x,Q²) , are extracted from an inclusive polarized cross section measurement of the electron-proton interaction.

Low momentum transfer measurements, such as this experiment, are critical to enhance the understanding of the proton because of its complex internal structure and finite size. These internal interactions influence the proton's global properties and even the energy levels in atomic hydrogen. The non-pertubative nature of the theory governing the interactions of the internal quarks and gluons, Quantum Chromodynamics (QCD), makes it difficult to calculate the effect from the internal interactions in the underlying theory. While not able to calculate the structure functions directly, QCD can make predictions of the spin structure functions integrated over the kinematic phase space.

This thesis will present results for the proton spin structure functions g₁(x,Q²) and g₂(x,Q²) from the E08-027 experimental data. Integrated moments of g₁(x,Q²) are calculated and compared to theoretical predictions made by Chiral Perturbation Theory. The g₁(x,Q²) results are in agreement with previous measurements, but include a significant increase in statistical precision. The spin structure function contributions to the hyperfine energy levels in the hydrogen atom are also investigated. The g₂(x,Q²) measured contribution to the hyperfine splitting is the first ever experimental determination of this quantity. The results of this thesis suggest a disagreement of over 100% with previously published model results.

Recommended Citation

Zielinski, Ryan, "The g2p Experiment: A Measurement of the Proton's Spin Structure Functions" (2017). Doctoral Dissertations. 2285.
https://scholars.unh.edu/dissertation/2285