Tracking detector and Møller Polarimeter for the P2 experiment

Abstract

The primary aim of the P2 experiment is to determine the weak mixing angle sin^2(θ_w) precisely. The experiment will take place at the Mainz Energy Recovery Superconducting Accelerator (MESA), which will provide a beam of electrons with alternating longitudinal polarization and energy of 155 MeV and a current of 150 μA. These conditions enable a targeted relative uncertainty of 0.14 % on sin^2(θ_w) at low four-momentum transfer of Q^2 = 4.5 × 10^−3 GeV^2. This high precision allows for a sensitive search for physics beyond the Standard Model of Elementary Particle Physics.

In the experiment, the parity-violating asymmetry will be measured by integrating Cherenkov detectors. Additionally, a tracking detector will determine the four-momentum transfer Q^2 of electrons scattered in the liquid hydrogen target and reconstruct individual electron tracks for systematic studies. The tracking detector will employ High Voltage Monolithic Active Pixel Sensors (HV-MAPS), a novel technology designed to minimize the material budget and thus reduce multiple scattering. A major challenge is represented by the high electron scattering rate into the tracking detector acceptance, reaching approximately 100 GHz. This demands additional requirements on the data acquisition system and the radiation hardness of all used materials and components.

The value of the electroweak mixing angle will be extracted from the parity-violating rate asymmetry measured in the experiment. The measured asymmetry is directly proportional to the beam polarization. Long-term scattering asymmetry measurements at the Mainz Microtron MAMI have shown that beam polarization can fluctuate by up to 10 % during a typical run. To ensure the required accuracy in the asymmetry measurements, beam polarization must be monitored regularly, ideally continuously, with a precision of ≤0.5 %. Møller polarimetry using a low-density gaseous atomic hydrogen target is the only suitable technique to meet these stringent requirements. A concept for this type of polarimeter, known as the Hydro-Møller polarimeter, was originally proposed by V. Luppov and E. Chudakov. This setup allows for online, non-destructive beam monitoring. However, the gaseous target introduces significant technological challenges that must be addressed. As an interim solution, a Møller polarimeter, which will initially use a conventional solid iron target and operate in discontinuous mode until the hydrogen target is ready, is currently under consideration.