A beam-normal single-spin asymmetry is generated by the scattering of transversely polarized electrons from unpolarized targets. This parity-conserving observable provides direct access to the imaginary part of the two-photon exchange amplitude. Because it can be calculated from the sum of the amplitudes for all on-shell intermediate states, a measurement provides an integral test of our understanding of the fundamental (γ + nucleon -> X) processes. In addition, this observable is also of special interest to experiments making precision parity-violating asymmetry measurements because small amounts of residual transverse polarization in the beam can lead to percent-level corrections. However, this asymmetry is highly suppressed and hence is only accessible with an apparatus designed to measure asymmetries at the few percent level. I will present the theoretical and experimental status of beam normal single spin asymmetries and their physics interests.
Argonne Physics Division Seminar Schedule