Multiboson production at multi-TeV lepton colliders as complementary probes to rare Higgs decays

Abstract

We study the potential of very high-energy lepton colliders to probe effective operators affecting the loop-induced Higgs boson decays $h\to γγ$ and $h\to Zγ$, providing constraints complementary to Higgs coupling measurements at the Large Hadron Collider. In particular, we consider processes with two and three bosons in the final state and analyze their individual and combined impact on the effective field theory parameter space. We demonstrate that these processes at a multi-TeV lepton collider can improve the bounds from rare Higgs decays and electroweak precision observables, surpassing the projections for High-Luminosity LHC and FCC-ee. This is especially true for triboson processes, whose sensitivity is enhanced by the growth with energy of the new physics cross-sections, making them the dominant constraint at very high energies, potentially competitive with FCC-hh.