The Large Hadron electron Collider as a bridge project for CERN

TL;DR

The paper presents the Large Hadron electron Collider (LHeC) as a bridge project that could operate between HL-LHC runs to deliver high-luminosity electron-proton collisions with $E_ ext{cm}\approx 1~\text{TeV}$ and $\,\mathcal{L}\sim 10^{34}~\text{cm}^{-2}\text{s}^{-1}$ using an Energy Recovery Linac. It outlines a comprehensive physics program spanning QCD, EW, Higgs, top, and BSM, along with high-precision SM parameter measurements and gamma-gamma processes, while also highlighting the strong synergy with HL-LHC and future colliders like FCC-eh and FCC-ee. The document details detector and accelerator feasibility, the role of PERLE as a demonstrator for high-power ERLs, and an implementation plan that contemplates post-2041 installation, energy/cost optimizations, and detector/accelerator technology cross-fertilization. It argues that LHeC provides essential proton-structure and coupling inputs that will sharpen hadron-collider physics and act as a cost-efficient, technology-forward stepping-stone toward a Higgs factory and the broader FCC program. Overall, the LHeC is presented as a scientifically transformative, technically feasible, and strategically valuable project that can extend CERN’s physics reach while advancing key accelerator and detector technologies.

Abstract

The LHeC is the project for delivering electron-nucleon collisions at CERN using the HL-LHC beams. An Energy Recovery Linac in racetrack configuration will provide 50 GeV electrons to achieve centre-of-mass energies around 1 TeV/nucleon and instantaneous luminosities around $10^{34}$ cm$^{-2}$s$^{-1}$. The LHeC program elaborated in the CDR of 2021 included a phase with concurrent operation of electron-hadron and hadron-hadron collisions, followed by a standalone phase of electron-hadron collisions only. In view of the current HL-LHC schedule, in this paper we have examined the possibilities of a program after the regular HL-LHC program with only electron-proton operation. In this operation mode, the LHeC would serve as an impactful bridge project between major colliders at CERN. The standalone physics program comprises electroweak, Higgs, top-quark, BSM and strong-interaction physics. In addition, it empowers the physics analyses at the HL-LHC by retrofitting measurements and searches with significantly more precise knowledge of the proton structure and $α_s$. The accelerator technology deployed in the Energy Recovery Linac for the LHeC is a major stepping-stone for the performance, cost reduction and training for future colliders. The capital investments in the LHeC electron accelerator can be reused in a cost-efficient way as the injector for the FCC-ee. Finally, data from the LHeC are essential to enable the physics potential of any new high-energy hadron collider. The operational plan of 6 years easily fits in the period between two major colliders at CERN. Similar to the LHeC empowering the HL-LHC physics program, the FCC-eh would be an impactful addition to the FCC physics program.

Figures (23)

• Figure 1: The LHeC project as a bridge between current and future major colliders at CERN.
• Figure 2: Coverage of the kinematic plane in deep inelastic lepton-proton scattering by some initial fixed target experiments, with electrons (SLAC) and muons (NMS, BCDMS), and by the $ep$ colliders: the EIC (green), HERA (yellow), the LHeC (blue) and the FCC-eh (brown). $x$ is the momentum fraction of the proton or nucleus taken by the partons involved in the cross section for a given observable, and $Q^2$ the hard scale of the process squared. The physics topics that the LHeC can cover are illustrated in the kinematic plane. Figure modified from LHeC:2020van.
• Figure 3: Selected inclusive production cross sections for an electron--proton collider at the HL-LHC as a function of the electron-beam energy $E_e$. The proton-beam energy is set by the HL-LHC to 7 TeV, so that the $ep$ centre-of-mass energy and the cross sections depend only on $E_e$ of the to-be-build electron accelerator. Displayed are inclusive production cross sections for prompt $Z$ and $W^\pm$ bosons, the Higgs boson ($H$), single top-quark production ($t$) and top-quark pair production ($t\bar{t}$), each for neutral-current (NC) and charged-current (CC) induced channels, respectively. Furthermore, the cross sections for $HZ$ and $HW$ production are displayed, which are mainly induced by vector-boson-fusion channels. Charged-current mediated cross sections scale as  $(1+P_e)$,  with $P_e$ being the longitudinal electron beam polarization which is set to $P_e=80\%$. For $E_e=50$ GeV, the total inclusive NC and CC DIS production cross sections, $6.5\cdot10^8$ fb and $3.0\cdot10^5$ fb, respectively, are not displayed, nor more specialized processes like $b\bar{b}$, $c\bar{c}$, jets, prompt photons, and di-boson production ($WW$, $ZZ$), or others like photoproduction or $\gamma\gamma$ induced ones. For example, $t\bar{t}$ photoproduction has a cross section of $0.7$ pb at $E_e=60$ GeV Bouzas:2013jha which is more than a factor of 30 larger than NC DIS $t\bar{t}$ production displayed here (violet dashed). The right-sided axis displays the expected number of events for an integrated luminosity of $\mathcal{L}=1\,\text{ab}^{-1}$. The predictions are calculated with Sherpa3 Sherpa:2024mfk at next-to-leading order QCD.
• Figure 4: Expected precision for the determination of parton density functions, expressed as a ratio to that of PDF4LHC21 PDF4LHCWorkingGroup:2022cjn, as a function of $x$ at $Q^2=1.9$ GeV$^2$: $u_v$ (top left), $d_v$ (top right), $g$ (bottom left) and $s$ (bottom right). LHeC results at next-to-next-to-leading order (NNLO) for integrated luminosities of 50 fb$^{-1}$ and 1 ab$^{-1}$ are shown with uncertainty bands together with central values of ABMP16 Alekhin:2017kpj, CT18 Hou:2019efy, MSHT20 Bailey:2020ooq and NNPDF4.0 NNPDF:2021njg. Small irregularities, also present in the parton-parton luminosity plots in Sec. \ref{['sec:luminosities']}, are due to those in the baseline set PDF4LHC21.
• Figure 5: Expected sensitivities on the SM $|V_{tb}|$ ($f^1_L \equiv 1+ \Delta f^1_L$) and on anomalous right-handed vector ($f^1_R$), left-handed tensor ($f^2_L$) and right-handed tensor ($f^2_R$) $Wtb$ couplings at the LHeC Dutta:2013mva (left), and on $|V_{ts}|$ exploring three different signal scenarios (Signal 1: $p e^- \to \nu_e \bar{t} \to \nu_e W^- \bar{b} \to \nu_e \ell^-\nu_\ell \bar{b}$; Signal 2: $p e^- \to \nu_e W^- b \to \nu_e \ell^-\nu_\ell b$; Signal 3: $p e^- \to \nu_e \bar{t} \to \nu_e W^- j \to \nu_e \ell^-\nu_\ell j$) Sun:2018gqo (right), as a function of the integrated luminosity.