CERN’s Super Proton Synchrotron (SPS) could be upgraded so that not only protons have the possibility to be accelerated, but also electrons. A 173-page conceptual design report posted on arXiv on 15 September describes the installation of a high-energy electron accelerator that could have the potential to be used for accelerator R&D, dark-sector physics, and for electro-nuclear measurements crucial for future neutrino experiments. The “eSPS”, proposed in 2018 by Torsten Åkesson of Lund University and colleagues at CERN, would marry technology developed for the Compact Linear Collider (CLIC) and the Future Circular Collider (FCC), and could also provide a step towards a potential electron-positron Higgs factory. The facility could be made operational in about five years and would operate in parallel and without interference with the next run of the LHC, Run 4, write the authors.
The SPS is one of CERN’s longest running accelerators, commissioned in June 1976 at an energy of 400 GeV and serving numerous fixed-target experiments ever since. It was later converted into a proton-antiproton collider which was used to discover the W and Z bosons in 1983. Then, in addition to its fixed-target programme, the SPS became part of the injection chain for LEP, and most recently, has been used to accelerate protons for the LHC.
The changeover time for using the SPS as a proton accelerator to an electron accelerator is estimated to be around ten minutes
Electrons would be injected into the SPS at an energy of 3.5 GeV by a new compact high-gradient linac based on CLIC’s X-band radio-frequency (RF) cavity technology, which would fill the circular machine with 200 ns-duration pulses at a rate of 100 Hz. An additional 800 MHz superconducting RF system, similar to what is needed for FCC-ee, would then accelerate the electron beam from 3.5 GeV to an extraction energy up to 18 GeV. The changeover time for using the SPS as a proton accelerator to an electron accelerator is estimated to be around ten minutes.
The requirements of the primary electron beam to be delivered by the eSPS were determined by the needs of the proposed Light Dark Matter eXperiment (LDMX), which would use missing-momentum techniques to explore potential couplings between hidden-sector particles and electrons in uncharted regions. The experiment could be housed in a new experimental area (see figure). The beam directly from the linac could also serve two experimental areas for a broad range of accelerator R&D; for example, it could provide multi-GeV drive beam bunches and electron witness bunches for plasma wakefield acceleration.
In a second phase, the facility could be geared to deliver positron witness bunches, which would make it a “complete facility” for plasma wakefield collider studies. Such a programme would naturally build on the work done by the AWAKE collaboration, which uses protons as a drive beam, and significantly broaden plasma wakefield R&D at CERN in line with priorities set out by the recent update of the European strategy for particle physics. Positron production would be a crucial element for any future Higgs-factory, while it would also allow studies of the Low EMittance Muon Accelerator (LEMMA) – a novel scheme for obtaining a low-emittance muon beam for a muon collider, by colliding a high-energy positron beam with electrons in a fixed target configuration at the centre of mass energy required to create muon pairs.
The eSPS proposal came about as a result of work in CERN’s Physics Beyond Colliders study group, and an Expression of Interest that was submitted to the SPS Committee in September 2018.