Measurement of the axial and the strangeness magnetic form factor of the proton with a P2 backward angle setup

The P2 experiment at the future accelerator facility MESA in Mainz aims for a precise determination of the weak charge of the proton at low momentum transfer. The experimental method is a measurement of the parity violating asymmetry in elastic electron-proton scattering at forward angle. This asymmetry is dominated by the weak charge, but also the proton structure plays a role. Here we consider a back angle measurement, which is more sensitive to the proton structure, and present its possible implications on the main P2 measurement.