Direct measurements of two photon exchange on lepton-proton elastic scattering using simultaneous electron-positron beams in CLAS

Description

<p> The electric (<i>G_E</i>) and magnetic (<i> G_M</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>² > 1 GeV². 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_E</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>⁺/<i> e</i>^- beam which then interacted with a liquid hydrogen target. The <i>e</i>⁺<i>p</i> and <i> e</i>^-<i>p</i> events were detected by the CLAS (CEBAF Large Acceptance Spectrometer). The elastic cross section ratios ((&sigma;(<i> e</i>⁺<i>p</i>)/(&sigma;(<i>e</i>^-<i>p</i>)) were measured over a wide range of virtual photon polarization <i>&epsiv;</i> and <i>Q</i>². The cross section ratios displayed a strong <i>&epsiv;</i> dependence at <i>Q</i>² = 1.45 GeV². There is no significant <i>Q</i>² dependence observed at <i> &epsiv;</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_E/G_M</i> at <i>Q</i>² up to 2 &ndash; 3 GeV². Applying the GLAS TPE correction to the Rosenbluth cross section measurements significantly decreases the extracted value of <i>G_E</i> and brings it into good agreement with the polarization transfer measurement at <i>Q</i>²&sim;1.75 GeV². Thus, these measurements appear to resolve the proton electric form factor discrepancy for <i>Q</i>² &lt; 2 GeV².</p>

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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