Low-energy probes of the small cosmic microwave background amplitude in models of the radiative Higgs mechanism

Abstract

Abstract The small cosmic microwave background (CMB) amplitude As ≃ 10−9 (or small temperature fluctuation δT/T ≃ 10−5) typically requires an unnaturally small effective coupling of an inflaton λϕ ∼ 10−14. In models with non-minimal coupling ξ, extra suppression of the amplitude, e.g. by the inflaton’s large field values, usually allows λϕ to be much larger, but at the price of ξ ≫ 1. Although the difficulties have not been strictly quantified, models with λϕ ≪ 1 or ξ ≫ 1 are harder to build. We show that the absence of new physics signals at TeV scale can suggest a relatively small $\xi \lesssim {\cal O}(1\mbox{--}100)$ with $\lambda _\phi \lesssim {\cal O}(10^{-4}\mbox{--}10^{-8})$, while constraining larger ξ with larger λϕ more strongly. Above all, this is possible by a connection between low- and high-energy physics that can be made in scenarios where the U(1)X Higgs is an inflaton at a high scale while its renormalization running also induces the Coleman–Weinberg mechanism for the electroweak symmetry breaking at a low scale. The best TeV-scale signals are Z′ resonances and Higgs signal strengths. We further find the connection particularly useful since the Z′ mass is upper bounded in order to produce the correct As and the weak scale simultaenously. Utilizing the intriguing upper bounds, we work out the prospects for LHC 13 and 100 TeV pp colliders probing the parameter space of the small CMB amplitude in such a model.