Low-energy probes of small CMB amplitude in models of radiative Higgs mechanism

Abstract

The small CMB amplitude $A_s \simeq 10^{-9}$ (or, small temperature fluctuation $\delta T/T \simeq 10^{-5}$) typically requires an unnaturally small effective coupling of an inflaton $\lambda_\phi \sim 10^{-14}$. In successful models, there usually is extra suppression of the amplitude, e.g. by large-field inflaton with non-minimal coupling $\xi$, so that $\lambda_\phi$ can be much larger. But $\lambda_\phi$ and $\xi$ cannot be $\sim {\cal O}(1)$ simultaneously; the naturalness burden is shared between them. We show that the absence of new physics signals at TeV scale may prefer a more natural size of $\xi \lesssim {\cal O}(1-100)$ with $\lambda_\phi \lesssim {\cal O}(10^{-4}-10^{-8})$, constraining larger $\xi$ with larger $\lambda_\phi$ more strongly. This intriguing connection between low- and high-energy physics is made in the scenarios with $U(1)_X$ where inflaton's renormalization running also induces Coleman-Weinberg mechanism for the electroweak symmetry breaking. We particularly work out the prospects of LHC 13 and 100 TeV $pp$ colliders for probing the parameter space of the small CMB amplitude.