The measurement of parity violation in weak interactions is a well established experimental technique in atomic, particle and nuclear physics. The interpretation of future measurements requires theoretical predictions with uncertainties below those of the experiments. To that end it is mandatory to go beyond the one-photon exchange approximation and include higher-order corrections in the calculations.
The measurement of observables sensitive to two-photon exchange processes is essential to benchmark such higher-order calculations. For this purpose the beam-normal single spin asymmetry (the so-called transverse asymmetry) An in polarized electron-nucleus scattering is an ideal candidate. Since An is a parity conserving asymmetry, arising from the interference of one- and two (or more)-photon exchange amplitudes, it gives direct access to the imaginary part of the two-photon exchange process.
The spectrometer setup of the A1 collaboration is well established for high-resolution electron scattering experiments. In order to study the transverse asymmetry An, the A1 setup was slightly modified by inserting additional fused-silica Cherenkov detectors in the focal plane of the two high-resolution spectrometers A and B. These detectors are capable of handling high data rates and they allow to separate elastic from inelastic events.
