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

Abstract Recently, the Atacama Cosmology Telescope (ACT) collaboration has reported a scalar spectral index n s = 0.9743 ± 0.0034. This is substantially larger than the classical prediction of Higgs Inflation, n s ≈ 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 n s 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, m t = 172.69 GeV, and central values for the SM inputs to the renormalisation group equations, we obtain n s = 0.9792 - 0.9844 for the case of three isosinglet vector-like B quarks with mass m Q in the range 1–3 TeV, with the lowest value of the n s 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 m Q = 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.