Building a Tracking Detector for the P2 Experiment

The P2 Experiment aims to measure the weak mixing angle at low Q${}^2$ via the parity violating asymmetry in elastic electron-proton scattering. It will be located at the new Mainz Energy Recovery Superconducting Accelerator (MESA), which will provide a 150 $\mu$A beam of alternatingly polarized 145 MeV electrons. While the main asymmetry measurement is performed with integrating Cherenkov detectors, the tracking system is developed in order to determine the momentum transfer in the target and for systematic studies. The high signal and background particle rates are the main challenge, in particular because of bremsstrahlung photons produced in the liquid hydrogen target. The particle rates are analysed in a Geant4-based Monte Carlo simulation. Measurements of the sensor response to photons were performed and compared to the simulation. The detector will be built using High Voltage Monolithic Active Pixel Sensors (HV-MAPS) made of silicon thinned to 50 $\mu$m. Four tracking planes will be arranged as two double layers. For each double layer, four modules are built to cover together about one sixth of azimuthal space. Every module consists of about 600 sensors, of which each has 2$\times$2 cm${}^2$ area and an expected power consumption of about 1W. The main mechanical setup of one module will be shown together with simulations of the gaseous helium cooling system. The high signal and background particle rates are the main challenge for the tracking system. The expected rate of electrons that are scattered in the liquid hydrogen target and hit the tracking system is of the order 10${}^5$ mm${}^{-2}$s${}^{-1}$ and is overwhelmed by partly more than 10${}^7$ mm${}^{-2}$s${}^{-1}$ bremsstrahlung photons. Background particle rates of photons and other secondary particles are analysed in a Geant4-based Monte-Carlo simulation. In addition, measurements of the sensor response to photons are presented and compared to the simulation.