The P2 experiment: A future high-precision measurement of the weak mixing angle at low momentum transfer

Here we describe the research and development work for the P₂ experiment which aims for a high precision determination of the weak mixing angle sin²θW to a precision of 0.15% at a four-momentum transfer of 4.5×10–³ GeV². This accuracy, comparable to existing measurements at the Z pole, allows for a sensitive test of the Standard Model up to a mass scale of 50 TeV, extendable to 60 TeV. The weak mixing angle is connected to the weak charge of the proton which will be extracted from a measurement of the parity violating cross section asymmetry –39.94×10–⁹ in elastic electron-proton scattering. A total accuracy of 0.57×10–⁹ is achievable in a measurement time of 11000 h using a 150μA polarized electron beam impinging on a 60 cm liquid hydrogen target. The P₂ asymmetry is smaller than any asymmetry measured so far in electron scattering with an unprecedented goal for the accuracy. The use of a solenoid spectrometer with 100% Φ-acceptance as well as an atomic hydrogen trap polarimeter are new features, which have never before been used in parity-violation experiments. In order to collect the enormous statistics required for this measurement, the new Mainz Energy-Recovering Superconducting Accelerator (MESA) is under construction. Plans for the associated beam control system and the polarimetry are described in this article as well. A liquid hydrogen high-power target with an extremely low noise level of 10 ppm needs to be designed and constructed. We report in addition 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. The physics program of the MESA facility comprises indirect, high precision search for physics beyond the Standard Model, measurement of the neutron distribution in nuclear physics, single-spin asymmetries, and a possible future extension to the measurement of hadronic parity violation.