  This article describes the future P2 parity-violating electron scattering
facility at the upcoming MESA accelerator in Mainz. The physics program of the
facility comprises indirect, high precision search for physics beyond the
Standard Model, measurement of the neutron distribution in nuclear physics,
single-spin asymmetries stemming from two-photon exchange and a possible future
extension to the measurement of hadronic parity violation. The first
measurement of the P2 experiment aims for a high precision determination of the
weak mixing angle to a precision of 0.14% at a four-momentum transfer of Q^2 =
4.5 10^{-3} GeV^2. The accuracy is comparable to existing measurements at the Z
pole. It comprises a sensitive test of the standard model up to a mass scale of
50 TeV, extendable to 70 TeV. This requires a measurement of the parity
violating cross section asymmetry -39.94 10^{-9} in the elastic electron-proton
scattering with a total accuracy of 0.56 10^-9 (1.4 %) in 10,000 h of 150
\micro A polarized electron beam impinging on a 60 cm liquid H_2 target
allowing for an extraction of the weak charge of the proton which is directly
connected to the weak mixing angle. Contributions from gamma Z-box graphs
become small at the small beam energy of 155 MeV. The size of the asymmetry is
the smallest asymmetry ever measured in electron scattering with an
unprecedented goal for the accuracy. We report here on the conceptual design of
the P2 spectrometer, its Cherenkov detectors, the integrating read-out
electronics as well as the ultra-thin, fast tracking detectors. There has been
substantial theory work done in preparation of the determination of the weak
mixing angle. The further physics program in particle and nuclear physics is
described as well.
