A New QCD facility at the M2 beam line of the CERN SPS: COMPASS++/AMBER

In this Letter of Intent, we propose a broad experimental programme for the ``New QCD facility at the M2 beam line of the CERN SPS''. This unrivalled installation will provide the site for a great variety of measurements to address fundamental issues of Quantum Chromodynamics, which are expected to lead to significant improvements in the understanding of QCD as our present theory of strong interactions. The proposed measurements cover the range from lowest-$Q^2$ physics as the determination of the proton radius by elastic muon-proton scattering, over average-$Q^2$-reactions to study hadron spectroscopy, to high-$Q^2$ hadron-structure investigations using the Drell-Yan process and Deeply Virtual Compton Scattering