Current Experiments Archive
E04-116: Beyond the Born Approximation: A Precise Comparison of Positron-Proton and Electron-Proton Elastic Scattering in CLAS
In this experiment, researchers will measure the difference between how an electron and its anti-particle (a positron) interact with the proton in order to better understand the structure of the proton.
In order to measure this difference as precisely as possible, a simultaneous mixed identical beam of electrons and positrons will be produced and passed through a proton target. To produce the mixed beam, a high-energy (5.5 GeV) electron beam will be passed through a thin tungsten sheet. A few percent of the electrons will emit high-energy photons. The photons will then pass through another thin tungsten sheet. A few percent of the photons will convert into a high-energy electron (e-) and a high-energy positron (e+).
A series of magnets will focus the electron/positron beam (the matter/antimatter beam) into a liquid hydrogen target at the center of the CLAS detector. The CLAS detector will be used to measure and identify the electrons or positrons that interact with protons in the target, as well as the knocked-out protons.
Other experiments have measured the charge distribution of the proton using two methods that should give the same answer. However, these methods disagree by a factor of three at large momenta. The preferred explanation of this discrepancy predicts a several percent difference between the cross sections (probability) of electron and positron scattering from the proton. Therefore, a precise measurement of this difference is needed to fully understand the charge distribution of the proton, one of the fundamental quantities in nuclear physics.