Physics Prospects for a near-term Proton-Proton Collider

TL;DR

This paper advocates evaluating a near-term, intermediate-energy hadron collider to follow HL-LHC, exploring energies down to $50$ TeV to balance physics payoff with cost and schedule. It develops a framework linking tunnel size and magnet technology to achievable energy and luminosity, and systematically analyzes Higgs physics (including couplings, self-coupling, and absolute normalization), direct searches for new particles, and EFT interpretations to compare against alternative accelerator concepts. The results indicate that a $50$–$70$ TeV hadron collider would offer substantial Higgs precision, enable di-Higgs studies, and provide meaningful direct-production reach (e.g., stops up to $8$–$10$ TeV, $Z'$ up to $33$–$46$ TeV) along with EFT sensitivity, all while offering cost and timeline advantages through staged magnet installation. The paper emphasizes community impact, arguing that such a program could sustain leadership, engage a broad scientific base, and preserve accelerator expertise, provided a decision framework evaluates direct-to-hadron pathways and energy-staged options.

Abstract

Hadron colliders at the energy frontier offer significant discovery potential through precise measurements of Standard Model processes and direct searches for new particles and interactions. A future hadron collider would enhance the exploration of particle physics at the electroweak scale and beyond, potentially uniting the community around a common project. The LHC has already demonstrated precision measurement and new physics search capabilities well beyond its original design goals and the HL-LHC will continue to usher in new advancements. This document highlights the physics potential of an FCC-hh machine to directly follow the HL-LHC. In order to reduce the timeline and costs, the physics impact of lower collider energies, down to $\sim 50$~TeV, is evaluated. Lower centre-of-mass energy could leverage advanced magnet technology to reduce both the cost and time to the next hadron collider. Such a machine offers a breadth of physics potential and would make key advancements in Higgs measurements, direct particle production searches, and high-energy tests of Standard Model processes. Most projected results from such a hadron-hadron collider are superior to or competitive with other proposed accelerator projects and this option offers unparalleled physics breadth. The FCC program should lay out a decision-making process that evaluates in detail options for proceeding directly to a hadron collider, including the possibility of reducing energy targets and staging the magnet installation to spread out the cost profile.

Figures (6)

• Figure 1: Left: Higgs-boson production cross sections as a function of centre-of-mass energies from Ref. HiggsEuropeanStrategy. Right: Annual production of Higgs bosons per collider option per experiment.
• Figure 2: Left: Di-Higgs production cross-section as a function of centre-of-mass energy. The values of 50 and 70 TeV are extrapolated using a quadratic fit. The blue points are from Ref. LHC_Higgs_WG4_YR4. Right: Expected Higgs self-coupling sensitivity at future colliders, based on Ref. Muhlleitner_DESYFutureCollPhys_2024 with updated numbers. The FCC-hh 50 and 70 TeV points are extrapolated.
• Figure 3: Left: Estimated stop exclusion reaches for various colliders and search methods, from Ref. Bose:2022obr. The limits are categorized based on the mass difference into two-body, three-body and four-body decays. The bars represent the maximum excluded stop mass ($m(\tilde{t}_1)$) in each region. Precision Higgs constraints are derived from deviations in Higgs production rates under the assumption that stops are the only source of BSM effects. Right: Summary of the $5\sigma$ discovery reach as a function of the resonance mass for different luminosity scenarios of FCC-hh and HE-LHC From Ref. Helsens:2019brx .
• Figure 4: Comparison of system mass reach calculated with the Collider Reach Tool collidertool (see text) for the collider scenarios shown in Table \ref{['tab:Zimmermann']}. For a given expected mass reach (either 95% CL or $5\sigma$ discovery) with respect to a nominal 84 TeV 920 ab$^{-1}$/year machine on the $x$-axis, the plot gives the fractional shift in expected mass reach compared to that nominal machine on the $y$-axis for other collider scenarios. This shift depends weakly on what type of partons initiate the signal process: gluon-gluon $gg$, quark-antiquark $q\bar{q}$, or quark-gluon $gq$, and flavour-independent quark-quark $qq$.
• Figure 5: Comparison of sensitivities for $\delta k_\gamma$ vs $\delta g_{1z}$ from FCC-hh at 100 TeV Bishara:2022vsc with the FCC-ee sensitivityFCC:2018byv overlaid in purple.