Weak boson probes of Higgs unitarity restoration at 10 TeV parton colliders

TL;DR

The work tackles whether Higgs coupling deviations away from the SM can signal unitarity-restoring resonances at multi-TeV scales and how to directly probe them with a $100~\text{TeV}$ FCC-hh or a $10~\text{TeV}$ muon collider. It models unitarity-restoring scenarios with a minimal spectrum (scalar $H'$ or vector $W'/Z'$) parameterized by $\mu_H = \kappa_H^2$ and resonance masses $m_{H'}, m_{V'}$, then simulates weak-boson-fusion processes and performs bump-hunt analyses using $m_{VV}$ and $m_T$ discriminants. Key findings show that both collider concepts can discover narrow resonances up to about $m_{H'}\sim 6~\text{TeV}$, with sensitivity for vector resonances depending on $\mu_H$ and fermion couplings (which can suppress certain decay channels like $Z'\to t\bar t$). The results support a modern no-lose theorem linking precision Higgs measurements with high-energy resonance searches, complemented by an FCC-ee program to corroborate deviations.

Abstract

Higgs coupling deviations, at levels accessible to the high-luminosity LHC, can imply a phenomenological no-lose theorem for the next generation of collider facilities. Correlating Higgs coupling deviations from the SM expectation in the gauge boson sector with high-scale unitarity requirements, we estimate and compare the sensitivity that can be expected at a future hadron collider (operating at 100 TeV centre-of-mass energy) and a 10 TeV muon collider. Both muon and hadron colliders offer discovery potential for mass scales up to ${\cal{O}}(6~\text{TeV})$ where unitarity violation induced by (sub)percent Higgs coupling modifications is mended. We comment on how an intermediate precision FCC-ee programme can corroborate such deviations.

Figures (4)

• Figure 1: Feynman diagram contributions to the $s$ channel of $WW\to WW$ scattering (suppressing $t$ topologies). Similar diagrams exist for $WZ$ scattering with $W$ resonances appearing as $s$ and $t$ channel contributions.
• Figure 2: Sensitivity of resonance searches in the $WW$ and $ZZ$ weak boson fusion production modes when considering scalar unitarity restoration for $\mu_H<1$. We show two scenarios for comparison related to the width of the (assumed narrow) resonance (a) $\Gamma_{H'}/m_{H'}=10\%$ and (b) $\Gamma_{H'}/m_{H'}=2.5\%$; overlaid on the significance plot is the signal strength that such parameter choices will probe. The magenta lines include the expected improvements from accessing semi-leptonic channels (FCC-hh efficiencies are provided at the end of Sec. \ref{['sec:ana']}, we comment on more conservative current LHC efficiencies in the text).
• Figure 3: Sensitivity of a WBF search of unitarity-restoring vector resonances. Shown are (a) the $Z'$ search and (b) the $W'$ search for two choices of Higgs signal strengths $\mu_H=0.95,0.99$. A significant coupling modification leads to a large signal at an FCC-hh, which decreases more quickly as it approaches the SM, since the kinematic endpoint of the FCC-hh is probed with lower statistical yield compared to the muon collider. The magenta lines include the expected improvements from accessing semi-leptonic channels.
• Figure 4: Sensitivity for wide fermion-aware resonances. The magenta line includes semi-leptonic channels using the FCC-hh extrapolation of efficiencies given in the text.