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

TL;DR

This paper explores how low-energy collider experiments can provide insights into models of the radiative Higgs mechanism that explain the small amplitude of the cosmic microwave background, linking high-energy physics with cosmological observations.

Contribution

It introduces a framework connecting collider constraints with the naturalness of parameters in radiative Higgs models explaining small CMB fluctuations.

Findings

Collider experiments can probe the parameter space of these models.

Naturalness favors moderate non-minimal coupling $\xi$ and small inflaton coupling $\lambda_\phi$.

Prospects for LHC 13 and 100 TeV colliders to test these models are analyzed.

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.