Vector-like quark stabilised Higgs inflation: implications for particle phenomenology, primordial gravitational waves and the Hubble tension

Abstract

The Standard Model (SM) Higgs potential is likely to be metastable, in which case Higgs Inflation requires an extension of the SM to sufficiently stabilise the Higgs potential. Here we consider stabilisation by adding nQ ≤ 3 Vector-Like Quarks (VLQs) of mass mQ. We consider isosinglet T vector quarks transforming under the SM gauge group as (3, 1, 2/3) and B vector quarks transforming as (3, 1, -1/3). Requiring stability of the finite temperature effective potential after instant reheating, and assuming that the t-quark mass mt equals the mean value of its experimental range, we find that the upper bounds on mQ for T quarks are 5.8 TeV (for nQ = 2) and 55 TeV (for nQ = 3). The corresponding absolute stability upper bounds are 4.4 TeV and 29 TeV. For nQ = 1 there is stability only for mt at its -2-σ value, in which case mQ ≤ 1.6 TeV for one T quark. The upper bounds are generally smaller for B vector quarks, with finite temperature stability for mQ less than 2.8 TeV (for nQ = 2), 18 TeV (for nQ = 3) and 1.0 TeV (for nQ = 1). The upper bounds on mQ are sensitive to the t-quark mass, becoming smaller as mt increases. The inflation predictions depend upon the conformal frame in which the model is renormalised. For renormalisation in the Einstein frame (Prescription I) the predictions are almost indistinguishable from the classical values: ns = 0.966 and r = 3.3 × 10-3. In this case the stability upper bounds on mQ apply. Renormalisation in the Jordan frame (Prescription II) predicts larger values of ns and r, with ns generally in the range 0.980 to 0.990 and r of the order of 0.01. The predicted range of ns is consistent with the CMB range obtained in Hubble tension solutions which modify the sound horizon at decoupling, whilst the predicted values of r will be easily observable by forthcoming CMB experiments. The observational upper bound on r generally imposes a stronger constraint on mQ in Prescription II than the requirement of stability, with the T quark upper bound equal to 2.4 TeV for nQ = 2 and 13 TeV for nQ = 3, assuming mt equals its mean value. nQ = 1 is generally ruled out by the large value of r. The mQ upper bounds rapidly decrease with decreasing r. We conclude that VLQ-stabilised Higgs Inflation with Prescription II renormalisation favours 1-10 TeV vector-like quarks that will be accessible to future colliders, and predicts a tensor-to-scalar ratio that will be observable in forthcoming CMB experiments and values of ns that favour an early-time solution to the Hubble tension.