P2: The weak charge of the proton

Research areas

  • Experimental particle and hadron physics
  • Theoretical particle and hadron physics
  • Accelerator physics

Principal investigators


The main objective of project P2 is the extraction of a precise value of the weak mixing angle \(sin^2(\theta_w)\) from a measurement of the parity-violating (PV) asymmetry in elastic electron proton scattering, with an accuracy comparable to that of the existing collider measurements of \(\Delta  sin^2(\theta_w)\) = 0.0003 (i.e., 0.13 % relative uncertainty). Due to the fact that the momentum transfer in project P2 is lower by many orders of magnitude \(Q^2\) ≈ 0.005 (GeV/c\(^2\)) the information obtained in such an experiment is complementary to past high energy – and upcoming LHC-measurements. P2 is closely connected to the subprojects S1 and P1, due to the interplay with the electromagnetic form factors and quantum corrections involving hadronic structure. In addition, there is a very close connection to the subproject N where parity violation experiments are envisaged in order to constrain neutron skins of heavy nuclei. The experiment will be performed at the upcoming MESA accelerator which is funded through the Mainz cluster of excellence PRISMA. The proposed measurements are complementary to the future experimental program foreseen at JLAB (such as MOLLER and SOLID) aiming for a similar precision in \(sin^2(\theta_w)\). Compared to the goal of the Qweak experiment at JLAB, where data collection has been completed and analysis is ongoing, we are aiming for a factor of three improvement in overall uncertainty of the same quantity, namely the weak charge of the proton. The considerably lower beam energy of 155 MeV at the MESA accelerator results in much smaller systematic uncertainties related to the \(\gamma Z\)-box diagrams. At present, this effect is a source of significant uncertainty for the interpretation of the Qweak experiment. We aim for the start of commissioning of the experiment at the end of the second funding period and envision data taking during the third funding period.

The major goals for the second funding period are:

  • Design and construction of a spectrometer/integrating detector system for the measurement of the parity-violating asymmetry in elastic electron-proton scattering based on measuring the integral flux of scattered electrons with quartz Cherenkov detectors ("flux integration").
  • Design and construction of a tracking system for the measurement of the average momentum transfer \(<Q^2>\) based on thin silicon pixel detectors, see fig below.
  • Development of high precision polarimetry for electron beams.
  • Calculation of electroweak radiative corrections for the measurement of the weak charge at MESA.