LEP3: A High-Luminosity e+e− Higgs & Electroweak Factory in the LHC Tunnel

"A possible back-up to the preferred option (FCC-ee and FCC-hh) for the next accelerator for CERN. The 2025 European Strategy for Particle Physics update calls for the identification of prioritized alternative options should the preferred FCC pathway prove infeasible. In this context, we outline the proposal for LEP3, an electron–positron collider reusing the existing LHC tunnel, as a strategic backup to FCC-ee. LEP3 would exploit much of the research and development already carried out for FCC-ee, enabling high-precision studies of the Z, W, and Higgs bosons below the top–antitop production threshold. Combining strong physics potential with reduced cost, LEP3 provides performance comparable or superior to other fallback options—such as linear colliders—while maintaining the technological continuity essential for a future energy-frontier collider. Conceived as a contingency, LEP3 complements, rather than competes with, the FCC-ee proposal.