Measurement of the Born-Forbidden Recoil Proton Normal Polarization in Electron-Proton Elastic Scattering December 2 , 2003

A “new” technique involving a solenoidal recoil polarimeter is proposed to measure the small, beam helicity-independent induced normal proton polarization (PN ) in e+ p elastic scattering. This observable is proportional to the imaginary part of the two-photon exchange amplitude, so the interpretation of the observed asymmetry in terms of two-photon exchange does not require a delicate kinematic separation. The combination of a magnetic spectrometer for electron detection with a recoil polarimeter in the scattering plane provides turn-key isolation of the elastic reaction, a high figure of merit, complete control of the phase of the physics signal to cancel instrumental asymmetries, and the potential for extension to neutron measurements. If current models are correct and PN is of order 1%, non-zero measurements with significance on the order of 10σ are anticipated. We thus expect to provide very tight constraints on model-dependent calculations of two-photon exchange in the most important energy region for precision electroweak measurements on the nucleon.