P2: The weak charge of the proton

Research areas

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

Project leaders


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 LEP measurements of \( \Delta \sin^2 \theta_W = 0.00037\) corresponding to a 0.16% 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)

The momentum transfer at P2 is lower by many orders of magnitude (\(Q^2 \approx\) 0.05 (GeV/c)\(^2\)) as compared with high-energy experiments. Thus the information obtained in the P2 experiment is complementary to the LEP -- and upcoming LHC -- measurements. P2 is closely connected to the subprojects S1 and P1 since electromagnetic form factors and quantum corrections involving the hadronic structure will be needed in the analysis of the P2 measurement. There is, in addition, a very close connection with the parity violation experiment planned within subproject N. The experiment will be performed at the upcoming MESA accelerator which is funded within the Mainz cluster of excellence PRISMA\(^+\) and will be installed in the new research building (Forschungsbau nach GG §93b). The proposed measurements are also complementary to the future experimental program foreseen at JLab (MOLLER experiment), which aims for a similar precision in a determination of \(\sin^2 \theta_W\). Compared to the goal of the Qweak experiment at JLab, which has recently published the result of their full statistics data, we aim for a factor 3.5 improved precision.

A conceptual design report of the P2 experiment has been
published in Eur. Phys. J. A (2018) 54: 208.

The major highlights for the third funding period are:

  1. Design and construction of an integrating spectrometer for the measurement of the parity violating asymmetry in elastic electron-proton scattering based on integrating readout of quartz Cherenkov detectors, including a tracking system for the measurement of the average momentum transfer \(Q^2\)based on thin silicon pixel detectors, see the figure on the right.
  2. Realisation of high precision polarimetry for electron beams.
  3. Calculation of electroweak radiative corrections and nuclear form factor effects for the measurement of the weak charge of the proton.
  4. Commissioning of the P2 spectrometer, including the MESA control systems for parity violating electron scattering.