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.
Highlights
Far beyond Higgs-related observables, as the singlet inherits the couplings of the Higgs to the SM particles, suppressed by the mixing angle
The backreaction potential is responsible for the interactions between the Higgs and the relaxion which are relevant for the collider phenomenology, realizing the Higgs portal structure to the generic singlet extension discussed above [19, 20]
The high luminosity (HL)-Large Hadron Collider (LHC) appears to perform better than the FCCee. Since it is the hadronic environment at the HL-LHC that necessitates this restrictive cut on ∆t, the FCCee can allow for a looser cut, and the limit presented here based on the HL-LHC cut is conservative
Summary
In the following we present the phenomenological features of the real scalar singlet extension of the SM Higgs sector. If the mixing angle or μ2 (φ0) are non-zero, the Higgs couples to a pair of singlets. We denote this coupling of the mass eigenstates by chφφ, which receives the following contributions chφφ = 3cθs2θvλh +. We choose φ0 = 0, which can always be obtained by a shift of the φ field This implies t = −ahφv from the minimum condition of the scalar potential Vs. This implies t = −ahφv from the minimum condition of the scalar potential Vs In this model the mixing angle is sin θ = √1 2. The approximation in the last step holds for λhφv2 + m20 2λhv and ahφ vλh This corresponds to a large mass splitting between the singlet and the Higgs and a small mixing angle. As we will see in the specific case of the relaxion, such naturalness bounds may be violated by orders of magnitude as a consequence of the cosmological evolution of the fields
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