DOE OSTI2017
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 ω cross sections through the exclusive 1 H(e, e'p)ω 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 (Fπ-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 π in the forward-angle, where the backward-angle omega events were fortuitously detected. The experiment has central Q 2 values of 1.60 and 2.45 GeV 2 , at W = 2.21 GeV. There was significant coverage in phi and epsilon, which allowed separation of σ T,L,LT,TT . The data set has a unique u coverage of -u ~ 0, which corresponds to -t > 4 GeV 2 . The separated σ 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 σL/σ T ratio indicate σ T dominance at Q 2 = 2.45 GeV 2 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 Q 2 settings. Previously, this phenomenon showing both forward and backward-angle peaks was only observed in the meson photoproduction data. Through comparison of our σ T data with the prediction of the Transition Distribution Amplitude (TDA) model, and signs of σ T dominance, promising indications of the applicability of the TDA factorization are demonstrated at a much lower Q 2 value than its preferred range of Q 2 > 10 GeV 2 . These studies have opened a new means to study the transition of the nucleon wavefunction through backward-angle experimental observables.
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗