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

TL;DR

The paper addresses the tension between ACT's measured scalar spectral index $n_s$ and the canonical Higgs Inflation prediction $n_s \approx 0.965$, noting the metastability of the SM Higgs potential. It proposes stabilising the potential by adding three vector-like B quarks and performing Jordan-frame renormalisation (Prescription II), which raises $n_s$ to the $0.979$–$0.990$ range and yields $r \sim 0.01$, compatible with ACT within $2\sigma$. The analysis uses 3-loop SM RG equations with Higgs propagator suppression $s(t)$ and VLQ corrections, assuming instantaneous reheating with $N_* \approx 57$. The results imply detectable primordial gravitational waves and TeV-scale vector-like quarks, providing a concrete path to test Higgs Inflation extensions via future CMB observations and collider experiments.

Abstract

Recently, the Atacama Cosmology Telescope (ACT) collaboration has reported a scalar spectral index $n_s~=~0.9743~\pm~0.0034$. This is substantially larger than the classical prediction of Higgs Inflation, $n_s \approx 0.965$, 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 $r = 7.87 \times 10^{-3} - 1.21 \times 10^{-2}$ 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 $r \sim 0.01$ would support the model.

Figures (1)

• Figure 1: Prescription II values of $r$ and $n_{s}$ as $m_{Q}$ varies from 1 TeV to 18 TeV, for the case of three vector-like $B$ quarks. The observational upper bound on $r$ imposes an upper bound on $m_{Q}$ of around 10 TeV. The range of values of $n_{s}$ is 0.979 to 0.989 as $m_{Q}$ increases from 1 TeV to 10 TeV. For $m_{Q} = 1$ TeV, the value of $n_{s}$ (0.9792) is 1.44$\sigma$ above the ACT mean value, with the corresponding value of $r$ being 0.0079.