At MESA, we explore the physics opportunities offered by using the recently established Energy-Recovery-Linac (ERL) accelerator technology, which enables very high electron-beam luminosities on internal targets at low energies. MESA will be the first accelerator to investigate multiturn energy recovery in a superconducting environment. The machine will therefore serve as a test-bed for world-scale facilities such as the LHeC .
The MESA accelerator and the experiments will extend over several underground structures, including the new experimental hall of the CFP research building. Full beam operation and data taking is foreseen to start in 2023.
MESA not only is a pioneering project for ERL applications, it will also pave the route for
a new era of accelerator-based precision physics. Several key experiments are currently designed, two of them, P2 and Magix, are already in the advanced development phase.
The P2 experiment, which will be operated in the external beam mode of MESA, will set a new precision standard for the measurement of the electroweak mixing angle at low energies and will enable a new generation of parity-violation experiments that are required for the determination of the neutron skin in heavy nuclei. At Magix, for the first time a high intensity ERL electron beam will be operated in conjunction with a windowless gas jet target. The Magix dual-arm spectrometer setup will allow for a more accurate extraction for the proton charge radius via electron-proton scattering, as well as searches for messenger particles to the dark sector. A direct search for dark matter particles will be pursued in the beam-dump experiment BDX@MESA and will benefit from the extraordinary high number of electrons accelerated at MESA.
 J.L. Abelleira Fernandez et al., A Large Hadron Electron Collider at CERN: Report on the Physics and Design Concepts for Machine and Detector, J. Phys. G39 (2012) 075001.
 H. Merkel et al., Search for Light Gauge Bosons of the Dark Sector at the Mainz Microtron, Phys. Rev. Lett. 106, 251802 (2011).
 H. Merkel et al., Search at the Mainz Microtron for Light Massive Gauge Bosons Relevant for the Muon g-2 Anomaly, Phys. Rev. Lett. 112, 221802 (2014)