Higgs Inflation with Vector-Like Quark Stabilisation and the ACT spectral index

Recently, the Atacama Cosmology Telescope (ACT) collaboration has reported a scalar spectral index nₛ~=~0.9743~±~0.0034. This is substantially larger than the classical prediction of Higgs Inflation, nₛ ≈ 0.965, which is 2.74$σ$ below the ACT mean value. We show that when an otherwise metastable Standard Model Higgs Inflation potential is stabilised by the addition of vector-like quark pairs and the potential is renormalised in the Jordan frame, the value of nₛ is generally larger than 0.965 and can explain the ACT observation. As an example, assuming the 2022 PDG direct measurement central value for the t quark mass, mₜ = 172.69 GeV, and central values for the SM inputs to the renormalisation group equations, we obtain nₛ = 0.9792 - 0.9844 for the case of three isosinglet vector-like B quarks with mass mQ in the range 1-3 TeV, with the lowest value of the nₛ range being 1.44$σ$ above the ACT mean value. The model predicts primordial gravitational wave with tensor-to-scalar ratio r = 7.87 × 10⁻³ - 1.21 × 10⁻² for mQ = 1-3 TeV, which will be easily observable in forthcoming CMB experiments. Observation of vector-like quarks of mass close to 1 TeV mass combined with a large tensor-to-scalar ratio r ~ 0.01 would support the model.