ACT-Planck data and phase transitions from a viable no-scale Standard Model completion

Abstract

Classically scale-invariant (and perturbative) theories provide a way to understand large hierarchies, as scales are generated through dimensional transmutation. They always lead to first-order phase transitions, since symmetries are radiatively broken, and they generically feature quasi-flat potentials, which are suitable for inflation. We construct a simple but fully realistic model of this kind that accounts for all observational evidence of new physics and is remarkably compatible with the most recent constraints on inflationary observables from both the Planck/BICEP/Keck and the Atacama Cosmology Telescope (ACT) collaborations. This model illustrates how classical scale invariance generically leads to a non-standard cosmology in which inflation occurs in two stages: a slow-roll stage and a thermal stage, separated by a radiation-dominated era.

Figures (6)

• Figure 1: Einstein-frame potential. The stars correspond to the field values $N$ e-folds before the end of inflation and the following benchmark points are considered:
• Figure 2: Predictions of the model for $n_s$ and $r$ together with the bounds from Ade:2015lrjBICEP:2021xfz (in gray) and the more recent ones from ACT:2025fjuACT:2025tim. The dot corresponds to the benchmark point given in the caption of Fig. \ref{['potential']} with the same color. The other points on the line are obtained by taking different values of $\bar{\beta}$ (and thus of $\xi$) as indicated in the inset, but keeping the same value of $\chi_0$. Here $N=50$.
• Figure 3: The same as in Fig. \ref{['nsr']} but for $N=60$.
• Figure 4: Values of $\lambda_{ah}$, $\chi_0$ and $m_\chi$ as functions of $m_{Z'}$ and $g_1'$. The dots represent the benchmark points in the caption of Fig. \ref{['potential']} and in Table \ref{['table']}.
• Figure 5: Running of the couplings. The initial conditions correspond to the blue benchmark point. All Yukawa couplings different from $y_t$ are so small to be invisible in this plot for our setup.