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).

Scale dependence of the weak mixing angle, \(\sin^2 \theta_W(\mu)\) compared with existing and forthcoming measurements (Jens Erler, Rodolfo Ferro-Hernández, JHEP 1803 (2018) 196)

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 result of the Qweak experiment at JLAB (Nature 557 (2018) 207), 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 are aiming for the start of the commissioning of the experiment in 2021.
The details of the P2 experiment and its physics program are described in arXiv:1802.04759.

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 the following figure.
  • Development of high precision polarimetry for electron beams.
  • Calculation of electroweak radiative corrections for the measurement of the weak charge at MESA.