Left Versus Right: A High Precision Determination of the Weak Mixing Angle at the P2 Experiment at MESA

The theory of elementary particle physics, the Standard Model (SM), provides a successful description of the basic constituents of matter and the forces acting between them. However, it explains only about 15% of the total mass in the universe, not accounting for the dark matter postulated in the face of a growing number of astrophysical and cosmological observations. The study of the universe at large has shown that our theory of the smallest entities of Nature must be extended.

In the absence of a direct observation of new particles it has become increasingly important to determine the parameters of the SM with the highest possible precision, as new particles and forces would modify their values through quantum corrections.

The existence of the W and Z bosons, and later the top quark, the tau neutrino, and the Higgs boson - the ultimate discovery of the SM - could all be inferred from precision measurements before their direct observations. A cornerstone parameter of the SM is the so-called weak mixing angle, which relates different sectors of the theory in a non-trivial manner and is particularly sensitive to new physics.

The P2 experiment is one of the two flagship experiments at the upcoming MESA accelerator in Mainz.  The goal of this project is to greatly improve its determination at low energy. Only the combination of a low energy measurement with the measurements at the Z-pole will test the fundamental energy dependence of the weak mixing angle, precisely. The interpretation of the weak mixing angle determinations at low energy will test the SM, and probe new physics with sensitivity to mass scales ranging from 70 MeV up to 50 TeV. Further measurements like the neutron distribution in the lead nucleus are directly connected astrophysical observables the size of a neutron star.