<p> The electric (<i>G<sub>E</sub></i>) and magnetic (<i> G<sub>M</sub></i>) form factors of the proton are fundamental observables which characterize its charge and magnetization distributions. There are two methods to measure the proton form factors: the Rosenbluth separation method and the polarization transfer technique. However, the ratio of the electric and magnetic form factors measured by those methods significantly disagree at momentum transfer <i>Q</i><sup>2</sup> > 1 GeV<sup>2</sup>. The most likely explanation of this discrepancy is the inclusion of two-photon exchange (TPE) amplitude contributions to the elastic electron-proton cross section which significantly changes the extraction of <i>G<sub>E</sub></i> from the Rosenbluth separation measurement. The Jefferson Lab CLAS TPE experiment determined the TPE contribution by measuring the ratio of positron-proton to electron-proton elastic scattering cross sections. The primary electron beam was used to create an intense bremsstrahlung photon beam. Some of the photons were then converted to a mixed <i>e</i><sup>+</sup>/<i> e</i><sup>-</sup> beam which then interacted with a liquid hydrogen target. The <i>e</i><sup>+</sup><i>p</i> and <i> e</i><sup>-</sup><i>p</i> events were detected by the CLAS (CEBAF Large Acceptance Spectrometer). The elastic cross section ratios ((σ(<i> e</i><sup>+</sup><i>p</i>)/(σ(<i>e</i><sup> -</sup><i>p</i>)) were measured over a wide range of virtual photon polarization <i>ϵ</i> and <i>Q</i><sup>2</sup>. The cross section ratios displayed a strong <i>ϵ</i> dependence at <i>Q</i><sup>2</sup> = 1.45 GeV<sup>2</sup>. There is no significant <i>Q</i><sup>2</sup> dependence observed at <i> ϵ</i> = 0.45. The results are consistent with a recent measurement at the VEPP-3 lepton storage ring in Novosibirsk and with the hadronic calculation by Blunders, Melnitchouk and Tjon. The hadronic calculation resolves the disagreement between the Rosenbluth separation and polarization transfer extractions of <i> G<sub>E</sub>/G<sub>M</sub></i> at <i>Q</i><sup>2</sup> up to 2 – 3 GeV<sup>2</sup>. Applying the GLAS TPE correction to the Rosenbluth cross section measurements significantly decreases the extracted value of <i>G<sub>E</sub></i> and brings it into good agreement with the polarization transfer measurement at <i>Q</i><sup>2</sup>∼1.75 GeV<sup>2</sup>. Thus, these measurements appear to resolve the proton electric form factor discrepancy for <i>Q</i><sup>2</sup> < 2 GeV<sup>2</sup>.</p>
Identifer | oai:union.ndltd.org:PROQUEST/oai:pqdtoai.proquest.com:3580489 |
Date | 18 June 2014 |
Creators | Adikaram, Dasuni Kalhari |
Publisher | Old Dominion University |
Source Sets | ProQuest.com |
Language | English |
Detected Language | English |
Type | thesis |
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