Zeptometry · New physics in parity violation. From the Thomson limit to the energy frontier

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 astrophysical and cosmological data. The study of the universe at large shows that our theory of the smallest entities of Nature must be extended.In the absence of a direct observation of new particles it becomes 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 effects. 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 - were all 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 and is particularly sensitive to new physics. The objective of this project is to greatly improve its determination, at energy scales spanning four orders of magnitude, combining information from the LHC with low-energy data from the MESA accelerator. Detector techniques developed for the LHC will be used to optimise the measurements at low energy. The combination of all measurements will test the energy dependence of the weak mixing angle, below the Z peak, on the resonance, and for the first time above the Z, towards the weak scale.Reaching these objectives requires improving theoretical predictions in the SM beyond the current state of the art, reducing the associated uncertainties. The simultaneous interpretation of the weak mixing angle determinations at all energies will test the SM, and probe new physics with sensitivity to mass scales ranging from 70 MeV up to the order of 100 TeV, corresponding to length scales of a zeptometer.