Electron-Hadron Colliders: EIC, LHeC and FCC-eh

Abstract

Electron-hadron colliders are the ultimate tool for high-precision quantum chromodynamics studies and provide the ultimate microscope for probing the internal structure of hadrons. The electron is an ideal probe of the proton structure because it provides the unmatched precision of the electromagnetic interaction, as the virtual photon or vector bosons probe the proton structure in a clean environment, the kinematics of which is uniquely determined by the electron beam and the scattered lepton, or the hadronic final state accounting appropriately for radiation. The Hadron Electron Ring Accelerator HERA (DESY, Hamburg, Germany) was the only electron-hadron collider ever operated (1991–2007) and advanced the knowledge of quantum chromodynamics and the proton structure, with implications for the physics studied in RHIC (BNL, Upton, NY) and the LHC (CERN, Geneva, Switzerland). Recent technological advances in the field of particle accelerators pave the way to realize next-generation electron-hadron colliders that deliver higher luminosity and enable collisions in a much broader range of energies and beam types than HERA. Electron-hadron colliders combine challenges from both electron and hadron machines besides facing their own distinct challenges derived from their intrinsic asymmetry. This review paper will discuss the major features and milestones of HERA and will examine the electron-hadron collider designs of the Electron-Ion Collider (EIC) currently under construction at BNL, the CERN’s Large Hadron electron Collider (LHeC), at an advanced stage of design and awaiting approval, and the Future Circular lepton-hadron Collider (FCC-eh).

The Large Hadron–Electron Collider at the HL-LHC

“…In 2018 a detailed cost estimate was carried out [34], following the guidance and practice of CERN accelerator studies. The assumptions were also compared with the cost of the European XFEL at DESY.…”

“…The HERA electron beam energy was 27 GeV. The main results, taken from [34], are reproduced in table 1.…”

“…Summary of cost estimates, in millions of Swiss francs (MCHF). Reproduced with permission from [34]. The 60 GeV configuration is built with a 9 km triple racetrack configuration, as described in the CDR [1].…”

“…the 30 GeV option is assumed to be the first stage of the LHeC and upgradeable to 50 GeV ERL. Whenever a choice was to be made on estimates, the conservative number was chosen in [34]. still very powerful, but requiring substantial losses from its Higgs, top, and beyond the standard model (BSM) programmes.…”

“…This can be achieved with an ERL circumference of 1/3 of that of the LHC. Recently, the cost was estimated in quite some detail [34] and also compared with the costs of other accelerator projects. Aiming at cost optimisation and providing an option for staged installation, the cost estimate led to the definition of a new default configuration of E e = 50 GeV, with the option of starting in an initial phase with a beam energy of E e = 30 GeV and a circumference of 5.4 km, which is 1/5 of the LHC length.…”