Reconciling Nonminimally Coupled Higgs Inflation with ACT DR6 Observations through Reheating

The Higgs inflation model with nonminimal coupling, while disfavored by the 1$\sigma$ region of the latest Atacama Cosmology Telescope Data Release 6 (ACT DR6) observational data, can be reconciled with the ACT DR6 data by incorporating the effects of reheating. In this paper, we consider reheating with a constant equation of state $w_{re}$. For the strong coupling case $\xi=100$, we find that reconciling the model with both the ACT DR6 constraints and the minimum reheating temperature required for successful Big Bang Nucleosynthesis (BBN) demands $w_{re} \geq 0.92$. Specifically, the reheating $e$-folding number must be $N_{ re}= 28.3$ for $w_{ re} = 0.92$, and within $24.9 \leq N_{ re} \leq 27.2$ for $w_{ re} = 1$. In the more general case without assuming the strong coupling limit, consistency with both ACT and BBN requires the nonminimal coupling to satisfy $\xi \geq 0.05$. Our findings suggest that by considering reheating, a wide range of inflationary models, such as $R^2$ inflation, hilltop inflation, E-model inflation, and T-model inflation, can also be made consistent with the ACT DR6 observational data.