Resolving the Proton Form Factor Problem with Positron-Proton Scattering

Conference · Mon Oct 01 00:00:00 EDT 2012

DOI:https://doi.org/10.2172/1993491· OSTI ID:1993491

Weinstein, Lawrence

The proton electromagnetic form factors are essential pieces of our knowledge of nucleon structure. However, Rosenbluth separation measurements of the proton electric form factor, $$G_E(Q^2)$$, differ from polarization transfer measurements by a factor of three at $Q^2 = 5.6$ (GeV/$$c$$)$^2$. This discrepancy must be resolved. One possible resolution is to include the contribution of hard two-photon exchange (TPE) contributions. These contributions are very difficult to calculate. However, we can directly determine the TPE effect by measuring the ratio of the positron-proton to electron-proton elastic scattering cross sections, $$R=\sigma(e^+p)/\sigma(e^-p)$$, because the TPE amplitude has the same sign as the $e^+p$ born amplitude and the opposite sign as the $e^-p$ born amplitude. We have measured $$R$$ over a wide range of momentum transfer, $$0.2 \le Q^2 \le 2$$ GeV$^2$, and virtual photon polarization, $$0.1 \le \epsilon \le 0.9$$, using a mixed identical beam of electrons and positrons in Hall B at Jefferson Lab. This talk will describe the experimental techniques used to produce this beam, the analysis techniques to identify elastic scattering events, and some preliminary results.

• Research Organization: Thomas Jefferson National Accelerator Facility (TJNAF), Newport News, VA (United States)
• Sponsoring Organization: USDOE Office of Science (SC), Nuclear Physics (NP)
• DOE Contract Number: AC05-06OR23177
• OSTI ID: 1993491
• Report Number(s): JLAB-PHY-12-1729; DOE/OR/23177-6847
• Resource Relation: Conference: 2012 APS DNP Fall Meeting, Newport Beach, CA, October 24, 2012
• Country of Publication: United States
• Language: English