A primary electron beam facility at CERN —eSPS: Conceptual design report

The design of a primary electron beam facility at CERN is described. The study has beencarried out within the framework of the wider Physics Beyond Colliders study. It re-enablesthe Super Proton Synchrotron (SPS) as an electron accelerator, and leverages the devel-opment invested in Compact Linear Collider (CLIC) technology for its injector and as anaccelerator research and development infrastructure. The facility would be relevant for sev-eral of the key priorities in the 2020 update of the European Strategy for Particle Phys-ics, such as an electron-positron Higgs factory, accelerator R&D, dark sector physics, andneutrino physics. In addition, it could serve experiments in nuclear physics. The electronbeam delivered by this facility would provide access to light dark matter production sig-nificantly beyond the targets predicted by a thermal dark matter origin, and for natures ofdark matter particles that are not accessible by direct detection experiments. It would alsoenable electro-nuclear measurements crucial for precise modelling the energy dependenceof neutrino-nucleus interactions, which is needed to precisely measure neutrino oscillationsas a function of energy. The implementation of the facility is the natural next step in thedevelopment of X-band high-gradient acceleration technology, a key technology for com-pact and cost-effective electron/positron linacs. It would also become the only facility withmulti-GeV drive bunches and truly independent electron witness bunches for plasma wake-field acceleration. A second phase capable to deliver positron witness bunches would makeit a complete facility for plasma wakefield collider studies. The facility would be used forthe development and studies of a large number of components and phenomena for a futureelectron-positron Higgs and electroweak factory as the first stage of a next circular colliderat CERN, and its cavities in the SPS would be the same type as foreseen for such a futurecollider. The operation of the SPS with electrons would train a new generation of CERNstaff on circular electron accelerators. The facility could start operation in about five years,and would operate in parallel and without interference with Run 4 of the LHC.