A Large Hadron Electron Collider at CERN
Foreword and Preface to the Conceptual Design Report
Submitted to Journal of Physics G on June 13, 2012
The traditions of CERN on deep inelastic lepton-hadron scattering date back to the discovery of weak neutral currents by the Gargamelle collaboration and, subsequently, the exploration of the valence and sea-quark contents of the nucleon, tests of Quantum Chromodynamics and electroweak phenomena and the observation of unexpected effects in the behaviour of quarks in protons and nuclei, made in a series of neutrino and muon scattering experiments. Following HERA, the first electron-proton collider built at DESY, with the LHeC there is an opportunity for energy frontier deep inelastic scattering to return to CERN in order to enrich the physics which has been made accessible by the Large Hadron Collider. Using a novel high energy electron beam scattered o LHC protons and also ions, the LHeC would represent the cleanest high resolution microscope in the world, based on new principles which deserve to be developed. The design report, available herewith, covering concepts of the accelerator and detector, together with an evaluation of the physics potential, had been initiated by the CERN Science Policy Committee and been worked out by an international study group, supported by CERN, the European Committee for Future Accelerators, ECFA, and the Nuclear Physics European Collaboration Committee, NuPECC.
The report describes a challenging new opportunity for European and global particle physics. Looking forward to the further development of the LHeC project, CERN with international partners is now evaluating ways of cooperation towards technical designs of the highest energy electron linac, with power recovery, and of a new detector which would enable ultra-precise, large acceptance deep inelastic scattering measurements. By the time the LHC will provide its first luminous results at the design beam energy, in around 2015, a possible upgrade of the LHC as is proposed here may advance. For now, CERN has to thank the scientists and engineers involved, the members of the Scientific Advisory Committee, of ECFA and NuPECC and especially the many expert referees which in the final phase of this study helped in scrutinising the LHeC design.
Sergio Bertolucci (Director of Research and Computing of CERN)
Preparations for new, big machines take time. The idea of an electron-proton (ep) collider in the LEP-LHC tunnel was discussed as early as 1984 , at the first LHC workshop at Lausanne. This was the time when the first ever built ep collider, HERA, was approved by the German government. HERA was a machine of about 30 GeV electron beam energy and nearly 1 TeV proton beam energy, a combination of a warm dipole electron ring with a superconducting dipole proton ring, in a 6 km circumference tunnel. The machine started operation 8 years after its approval. It reached luminosities of 1031 cm-2s-1 in first phase of operation which were increased by about a factor of 4 in the subsequent, upgraded configuration. HERA never attempted to collide electrons with deuterons nor with ions.
The realisation of HERA at DESY had followed a number of attempts to realise ep interactions in collider mode, mainly driven by the unforgettable Bjoern Wiik: since the late 1960s, he and his colleagues had considered such machines and proposed to probe the proton's structure more deeply with an ep collider at DORIS , later at PETRA (PROPER)  and subsequently at the SPS at CERN (CHEEP) . Further ep collider studies were made for PEP , TRISTAN  and also the Tevatron (CHEER) .
In 1990, at a workshop at Aachen, the combination of LEP with the LHC was discussed, with studies [9-11] on the luminosity, interaction region, a detector and the physics as seen with the knowledge of that time, before HERA. Following a request of the CERN Science Policy Committee (SPC), a brief study of the ring-ring ep collider in the LEP tunnel was performed  leading to an estimated luminosity of about 1032 cm-2s-1 .
At the end of the eighties it had been anticipated that there was a possible end to the increase of the energy of ep colliders in the ring-ring configuration, because of the synchrotron radiation losses of an electron ring accelerator. The classic SLAC fixed target ep experiment had already used a 2 mile linac. For ep linac-ring collider configurations, two design sketches considering electron beam energies up to a few hundred GeV were published, in 1988  and in 1990 . As part of the TESLA linear collider proposal, an option (THERA) was studied  to collide electrons of a few hundred GeV energy with protons and ions from HERA. Later, in 2003, the possibility was evaluated to combine LHC protons with CLIC electrons . It was yet realised, that the bunch structures of the LHC and CLIC were not compliant with the need for high luminosities.
In September 2007, the SPC again asked whether one could realise an ep collider at CERN. Some of us had written a paper  in the year before, that had shown in detail, for the first time, that a luminosity of 1033 cm-2s-1 was achievable. This appeared possible in a ring-ring configuration based on the "ultimate" LHC beam, with 1.7 x 1011 protons in bunches 25 ns apart. Thanks to the small beam-beam tune-shift, it was found to be feasible to simultaneously operate pp in the LHC and ep in the new machine, which in 2005 was termed the Large Hadron Electron Collider (LHeC) . Thus it appeared possible to realise an ep collider that was complementary to the LHC, just as HERA was to the Tevatron. The integrated luminosity was projected to be O(100) fb-1, a factor of a hundred more than HERA had collected over its lifetime of 15 years.
It was clear that with a centre-of-mass energy of about √s ≈ 1.5 TeV an exciting programme of deep inelastic scattering (DIS) measurements at the energy-frontier was in reach. This would comprise searches and analyses for physics beyond the Standard Model, novel measurements in QCD and electroweak physics to unprecedented precision, as well as DIS physics at such low Bjorken x, that all the known laws of parton and gluon interactions would have to be modified to account for non-linear parton interaction effects. It had also been realised that the kinematic region, in terms of negative four-momentum-transfer squared, Q2, and 1/x, accessed in lepton-nucleus interactions could be extended by 4 orders of magnitude using the ion beams of the LHC. A salient theme of the LHeC therefore is the precise mapping of the gluon field, over six orders of magnitude in Bjorken x, in protons, neutrons and nuclei, with unprecedented sensitivity.
In the autumn of 2007, (r)ECFA and CERN invited us to work out the LHeC concept to a degree, which would allow one to understand its physics programme, evaluate the accelerator options and their technical realisation. The detector design should be affordable and capable of realising a high precision, large acceptance experimental programme of deep inelastic scattering at the energy frontier. The electron beam energy range was set to be between about 50 - 150 GeV. The wall plug power consumed for the electron beam was limited to 100 MW.
For the installation of the LHC it had been decided to remove LEP from the tunnel and to re-use the injector chain. To realise an ep collider based on the LHC, a new electron accelerator has to be built. The following report details two solutions for the chosen default electron beam energy of Ee = 60 GeV. One option is to build and install a new ring, with modern magnet technology, on top of the LHC, using a new 10 GeV injector. Alternatively, one can build a "linac", actually two 10 GeV superconducting linacs in a racetrack configuration. By employing energy recovery techniques, this configuration could provide the equivalent of about 1 GW available power and reach 1033 cm-2s-1 luminosity. The LHeC linac would be of about the same length as the one used for the discovery of quarks at SLAC [19, 20], but capable of probing parton interactions with a Q2 exceeding that of the 1969 machine by a factor of nearly 105.
It was agreed early on to devote a few years to the report, also because none of the people involved could work anything near to full time for this endeavour. Three workshops were held in 2008-2010, that annually assembled about a hundred experts on theory, experiment and accelerator to develop the LHeC design concepts. The project was presented annually to ECFA and in 2008 to ICFA, see . In view of the unique electron-ion scattering programme of the LHeC, the design e ort became also supported by NuPECC, and the LHeC is now part of the NuPECC roadmap for European nuclear physics as released in 2010 . Following an intermediate report to the Science Policy Committee of CERN, in July 2010, the SPC considered the LHeC "an option for a future project at CERN".
In August 2011, a first complete draft of this conceptual design report was handed to more than twenty experts on various aspects of the physics and technology of the LHeC, which CERN had invited to referee the project and scrutinise its motivation and its design. The report has been completed following often close interactions with the referees and due consideration of their observations.
The LHeC by its nature is an upgrade of the LHC. It substantially enriches the physics harvest related to the gigantic investment in the LHC. Whatever the outcome of the searches at the LHC for physics beyond the Standard Model turns out to be, an ep collider operating at the energy frontier is guaranteed to deepen the understanding of TeV scale physics and thus will support the development of the theory of elementary particles and their interactions.
The LHeC needs the LHC proton and ion beams to be operational and so the design is made for synchronous pp and ep operation, as well as AA and eA, including deuterons. Should the LHC eventually be upgraded to even higher beam energy, beyond 7 TeV per beam , or a new proton collider be built, it would open an even higher energy reach for ep also. There certainly is a future for deep inelastic scattering at the energy frontier. It is herewith envisaged to begin with the LS3 shutdown of the LHC, in the early twenties, likely leading into further decades. As Frank Wilczek put it, "one of the joys of our subject is the continuing of our culture that bridges continents and generations" .
Our science is driven by curiosity, by theoretical expectations, sometimes too great, but also by experiment and technology, and the authors of this study therefore hope that the LHeC may be given the chance to contribute to the common e orts of our community for a deeper understanding of nature.
Max Klein (Chair of the LHeC Steering Committee)
A PS on May 10, 2014: The CDR was submitted for publication to Journal of Physics G prior to the discovery of the Higgs boson announced on July 4, actually on June 13, the day before the 2012 LHeC workshop began. The Higgs boson is produced dominantly in charged current scattering, radiated thus from the W (or Z in neutral currents). Its properties can be very well measured in ep. This was much studied in the design report but it is interesting that it did not make it to the prefaces of the report which are reproduced here as documents describing the past and a future of energy frontier deep inelastic scattering.