Collider searches for scalar singlets across lifetimes

Abstract

Spin-0 singlets arise in well-motivated extensions of the Standard Model. Their lifetime determines the best search strategies at hadron and lepton colliders. To cover a large range of singlet decay lengths, we investigate bounds from Higgs decays into a pair of singlets, considering signatures of invisible decays, displaced and delayed jets, and coupling fits of untagged decays. We examine the generic scalar singlet and the relaxion, and derive a matching as well as qualitative differences between them. For each model, we discuss its natural parameter space and the searches probing it.

New physics at the FCC-ee: indirect discovery potential

“…(Top-Right) From Ref. [23], constraints on the singlet extension of the SM in the λ hφ -sin θ plane. The portal coupling, λ hφ only impacts the H → φφ decays, while sin θ controls the φ lifetime.…”

“…Displace Vertices (DV) are also shown. See [23] for details. (Bottom) FCC-ee sensitivity to the mass, M, of sterile neutrinos and their mixing, φ with the SM ones.…”

“…The H → φφ decays could manifest as invisible decays, displaced jets or untagged exotic decays, depending on the φ lifetime. With the FCC-ee precision, models leading to invisible (exotic) decays contributing with more than 0.2% (1%) to the total Higgs width would be ruled out at 95% probability [3,23]. 4 The top-right panel in Figure 2, from the recent study in [23], summarizes the different types of constraints that would be possible with the FCC-ee measurements for a benchmark scenario with m φ = 5 GeV.…”

“…With the FCC-ee precision, models leading to invisible (exotic) decays contributing with more than 0.2% (1%) to the total Higgs width would be ruled out at 95% probability [3,23]. 4 The top-right panel in Figure 2, from the recent study in [23], summarizes the different types of constraints that would be possible with the FCC-ee measurements for a benchmark scenario with m φ = 5 GeV. As can be seen from the figure, a combination of indirect and direct searches at the FCC-ee would easily surpass the HL-LHC reach in the whole parameter space.…”

Direct discovery of new light states at the FCCee

“…Models of this kind predict a new, light scalar which mixes with the Higgs, the so-called relaxion. A terra-Z machine like the FCCee would be able to discover or exclude the relaxion for masses above ∼ 5 GeV [42].…”