The weak mixing angle at low and high scales : BASKET-P2 – BASKET_P2

The present proposal aims at precise measurements of the electroweak mixing angle with the P2 experiment at
MESA. In connection with measurements from the LEP and LHC colliders, a detailed study and interpretation of the evolution of this fundamental parameter as a function of momentum transfer will be performed for the first time.

Exploiting all available LHC data and the upgraded capabilities of the detector, measurements with ATLAS will probe the Z boson
scale with high precision, and for the first time the evolution of the mixing angle above the Z resonance towards the TeV scale.

The P2 experiment will provide a competitive measurement of this parameter at very low momentum transfer,
via the polarization asymmetry of the electron-proton elastic scattering cross section. Measurements performed at P2 will also provide a deeply improved understanding of the
proton structure, with relevance for astrophysics and neutrino physics.

The main objective of this proposal is the construction of a tracking detector for the measurement of scattered electrons in the backward direction. This measurement is crucial for the success of the P2 experiment as a whole. The proposed detector can measure the diffusion angle with a precision of about 1 degree, the scattered electron momentum with a precision of 2%, and above all guarantee a recording frequency of 100 MHz, which makes this device one of the fastest tracking detectors conceived to date. The final sample is foreseen to be collected in about 1000 hours, and represents about 10^14 events.

Simultaneous measurements at such different scales constitutes a fundamental test of the SM, probing new physics at all scales. A global analysis of the results is proposed.