Exclusive Backward-Angle Omega Meson Electroproduction

Li, Wenliang
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Faculty of Graduate Studies and Research, University of Regina

Exclusive meson electroproduction at different squared four-momenta of the exchanged virtual photon, Q^2 , and at different four-momentum transfers, t and u, can be used to probe QCD’s transition from hadronic degrees of freedom at the long distance scale to quark-gluon degrees of freedom at the short distance scale. Backward-angle meson electroproduction was previously ignored, but is anticipated to offer complimentary information to conventional forward-angle meson electroproduction studies on nucleon structure.

This work is a pioneering study of backward-angle omega cross sections through the exclusive 1H(e, e′ p)omega reaction using the missing mass reconstruction technique. The extracted cross sections are separated into the transverse (T), longitudinal (L), and LT, TT interference terms.

The analyzed data were part of experiment E01-004 (Fpi-2), which used 2.6-5.2 GeV electron beams and HMS+SOS spectrometers in Jefferson Lab Hall C. The primary objective was to detect coincidence pion in the forward-angle, where the backward-angle omega events were fortuitously detected. The experiment has central Q2 values of 1.60 and 2.45 GeV2 , at W = 2.21 GeV. There was significant coverage in phi and epsilon, which allowed separation of sigma_T,L,LT,TT . The data set has a unique u coverage of −u ∼ 0, which corresponds to −t > 4 GeV2 .

The separated sigma_T result suggest a flat ∼ 1/Q^1.33±1.21 dependence, whereas sigma_L seems to hold a stronger 1/Q^9.43±6.28 dependence. The sigma_L/sigma_T ratio indicate sigma_T dominance at Q2 = 2.45 GeV2 at the ∼90% confidence level.

After translating the results into the −t space of the published CLAS data, our data show evidence of a backward-angle omega electroproduction peak at both Q2 settings. Previously, this phenomenon showing both forward and backward-angle peaks was only observed in the meson photoproduction data.

Through comparison of our sigma_T data with the prediction of the Transition Distribution Amplitude (TDA) model, and signs of sigma_T dominance, promising indications of the applicability of the TDA factorization are demonstrated at a much lower Q2 value than its preferred range of Q2 > 10 GeV2.

These studies have opened a new means to study the transition of the nucleon wavefunction through backward-angle experimental observables.

A Thesis Submitted to the Faculty of Graduate Studies and Research In Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy in Physics, University of Regina. xii, 216 p.