## Dissertations, Theses, and Masters Projects

1998

Dissertation

#### Degree Name

Doctor of Philosophy (Ph.D.)

#### Department

Physics

John Michael Finn

#### Abstract

The amplitude of the $S\sb{11}$(1535) resonance at values of four-momentum transfer $Q\sp2 > 0$ is not explained by the nonrelativistic constituent quark model, and is currently a topic of theoretical interest; in addition, the large branching fraction of this resonance to the proton-$\eta$ decay channel is not well understood. There is also controversy concerning the $Q\sp2$ regime in which perturbative QCD becomes important in describing nucleon resonances such as the $S\sb{11}$. The $p(e, e\sp\prime p)\eta$ reaction is an excellent system in which to study this resonance and address the issue of possible perturbative effects: the $p\eta$ channel is not accessible to the $\Delta$ (isospin-$3\over2$) resonances and couples only weakly to N* (isospin-$1\over2$) resonances other than the $S\sb{11}$(1535). The differential cross section for the process $p(e, e\sp\prime p)\eta$ was measured in Hall C of the Thomas Jefferson National Accelerator Facility (TJNAF) in experiment E94-14 in December 1996. The angular distribution at center-of-momentum energies near the $S\sb{11}$(1535) was measured for $Q\sp2\approx$ 2.4 and 3.6 GeV$\sp2/c\sp2$ (the latter being the highest-$Q\sp2$ exclusive measurement of this process to date). The Short Orbit Spectrometer (SOS) was used to detect recoil electrons and the High Momentum Spectrometer (HMS) was used to detect outgoing protons, with the $\eta$ identified via missing mass. Results of fits to the differential cross section and an extraction of the helicity amplitude $A\sbsp{1/2}{p}$ are presented. The cross section obtained from these new data is about 30% lower than that of the only other high-$Q\sp2$ exclusive measurement of this process. Comparison with a recent analysis of inclusive $(e, e\sp\prime)$ data provides a lower bound on the $S\sb{11}(1535)\to p\eta$ branching fraction of $b\sb{\eta}$ = 0.45.

#### DOI

https://dx.doi.org/doi:10.21220/s2-45ag-7c85