Studies for New Experiments at the CERN M2 Beamline within "Physics Beyond Colliders": AMBER/COMPASS++, NA64mu, MuonE

TL;DR

This paper discusses new experimental proposals at CERN's M2 beamline within the 'Physics Beyond Colliders' initiative, focusing on muon and hadron beam experiments to explore fundamental physics, dark sector searches, and precision measurements.

Contribution

It introduces three novel experimental programs leveraging the M2 beamline, including proposals for high-precision measurements and dark sector searches, with feasibility studies for advanced beam capabilities.

Findings

Proposed measurements of the proton radius using muon beams.

Search strategies for dark sector mediators like A' and Z_mu.

Assessment of the hadronic vacuum polarization via mu-e scattering.

Abstract

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 NA64mu collaboration proposes to search for dark sector mediators such as a dark scalar A' or a hypothetical Z_mu 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 mu-e scattering, the dominant uncertainty in the determination of (g-2)_mu. An overview of the three new experimental programs will be presented together with implications for the M2 beamline and the experimental area EHN2, based on the studies of the PBC "Conventional Beams" Working Group.