Studies for New Experiments at the CERN M2 Beamline within "Physics Beyond Colliders": AMBER/COMPASS++, NA64$\mu$, MuonE

The “Physics Beyond Colliders (PBC)” study explores fundamental physics opportunities at the CERN accelerator complex complementary to collider experiments. Three new collaborations aim to exploit the M2 beamline in the North Area with existing high-intensity muon and hadron beams, but also aspire to go beyond the current M2 capabilities with a RF-separated, high-intensity hadron beam, under study. The AMBER/COMPASS++ collaboration proposes an ambitious program with a measurement of the proton radius with muon beams, as well as QCD-related studies from pion PDFs / Drell-Yan to cross section measurements for dark sector searches. Assuming feasibility of the RF-separated beam, the spectrum of strange mesons would enter a high precision era while kaon PDFs as well as nucleon TMDs would be accessible via Drell-Yan reactions. The NA64µ collaboration proposes to search for dark sector mediators such as a dark scalar A′ or a hypothetical Zµ using the M2 muon beam and complementing their on-going A′ searches with electron beams. The MuonE collaboration intends to assess the hadronic component of the vacuum polarization via elastic µ − e scattering, the dominant uncertainty in th