Improved Predictions on Higgs-Starobinsky Inflation and Reheating with ACT DR6 and Primordial Gravitational Waves

TL;DR

This study combines recent CMB data and primordial gravitational wave constraints to refine the parameter space of Higgs-Starobinsky inflation, significantly limiting viable models and reheating scenarios.

Contribution

It provides the first comprehensive analysis of Higgs-Starobinsky inflation incorporating both direct CMB observations and indirect PGW bounds from $ abla N_{eff}$, tightening model viability.

Findings

Inflationary e-folds are restricted to 57.9-62.2 at 2σ confidence.

Reheating temperature constrained between BBN scale and 10^{12} GeV.

No viable parameter space at 1σ confidence once PGW bounds are included.

Abstract

We investigate the implications of recent CMB observations for Higgs-Starobinsky inflationary models and their associated reheating dynamics, utilizing data from ACT DR6, Planck 2018, BICEP/Keck 2018, and DESI, collectively referred to as P-ACT-LB-BK18. In addition to direct CMB constraints, we incorporate indirect bounds arising from the potential overproduction of primordial gravitational waves (PGWs), particularly through limits on the effective number of relativistic species, $\Delta N_{\rm eff}$, during Big Bang Nucleosynthesis (BBN). These constraints become especially relevant in scenarios featuring a stiff post-inflationary equation of state $w_{\rm RH}\geq 0.58$. Our analysis shows that, when both P-ACT-LB-BK18 data and $\Delta N_{\rm eff}$ bounds are considered, the viable number of inflationary e-folds is restricted to the range ($57.9$-$62.2$) at the $2\sigma$ confidence level (C.L.). Correspondingly, the reheating temperature is constrained to lie between the BBN energy scale and $10^{12}$ GeV, with the post-inflationary equation-of-state parameter satisfying $w_{\rm RH} > 0.41$. However, no parameter space remains viable at the $1\sigma$ C.L. once $\Delta N_{\rm eff}$ constraints from PGWs are included, rendering the Higgs-Starobinsky model highly restricted.