The P2 Experiment will make use of the new MESA accelerator in Mainz to measure the weak mixing angle in parity-violating electron-proton scattering at low momentum transfer with unprecedented precision.
The polarised electrons from MESA scatter off a 60 cm long high-power liquid hydrogen target. A 0.6 Tesla solenoid is employed to focus sacttered electrons of interest onto a ring of integrating Chernkov detectors. They are complemented by tracking detectors at backward angles for form-factor measurements and – the contribution of our group – fast thin trackers in forward direction for determining momentum transfer. We also contribute to the polarimetry chain of MESA by designing the spectrometer and detector package of the Moller polarimeter.
The P2 setup will also be used to measure the parity-violating asymmetry on carbon, opening new windows to constrain new physics and on lead, pinning down the nuclear equation of state, relevant for e.g. the size of neutron stars.
For assembling the individual strips of the tracking detectors, we have developed a CNC pick & place and glue dispensing machine , the gluebot. It is capable of placing both drops of glue and pixel chips with micrometer precision and is also used by other groups assembling silicon detectors.
The P2 tracking detector can produce 100s of GBits of data per second. This has to be transported out of the detector volume using radiation-hard electronics and then has to be reduced using advanced algorithms on FPGAs to a level that can be stored and analyzed.
The fast response and high efficiency of the HV-MAPS comes at the cost of a power consumption of about 300 mW/cm2. This sums up to a few kW per sensor module, requiring a custom power solution using low-impedance cables and a low mass cooling system based on high flows of gaseous helium.
Our group is building a tracking detector to measure the momentum transfer of the scattered electrons in P2. We have performed detailed simulations in order to estimate the expected rates and to find the best possible geometry for the tracker allowing for reliable track reconstruction at high rates as well as good momentum resolution. In the process we have developed track finding and fitting algorithms tailored to the geometry and the non-constant magnetic field.
The P2 tracking detector needs to be positioned with high accuracy using lightweight mechanics allowing for rotation in azimuth. The mechanics have to include the cooling channels and incorporate also the PCBs for powering and data readout. The modular design allows for easy maintenance.
High-Voltage Monolithic Active Pixel sensors are a novel type of silicon pixel sensors, allowing for very thin tracking layers and a fast response. Within the Mu3e collaboration we have developed these sensors to the point where they can be integrated in experimental set-ups and are currently working towards a P2-specific ASIC.
The beam polarisation of the MESA accelerator is a key ingredient into to parity violation measurements at P2. A whole chain of polarimeters is foreseen to eventually push the precision this measurement to below 1%. The last step in this chain, placed just before the P2 setup, is a polarimeter using Moller scattering on iron and later atomic hydrogen, for which we developed the detector package.