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

We describe the research and development work for the P2 experiment which aims for a high precision determination of the weak mixing angle \(\sin^{2}\theta_{W}\) to a precision of 0.15% at a four-momentumtransfer of \(4.5\times 10^{-3}\) 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 \(-39.94 \times 10^{-9}\) in elastic electron-proton scattering. A total accuracy of \(0.57 \times 10^{-9}\) is achievable in a measurement time of 11000 h using a 150μA polarized electron beam impinging on a 60 cm liquid hydrogentarget. 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% \(\phi\)-acceptance as well as an atomic hydrogen trap polarimeterare 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 polarimetryare described in this article as well. A liquid hydrogenhigh-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 trackingdetectors. The physics program of the MESA facility comprises indirect, high precision search for physics beyondthe 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.