Low-reheating scenario in dark Higgs inflation and its impact on dark photon dark matter production

TL;DR

The paper develops a minimal dark $U(1)_D$ extension in which the dark photon provides DPDM and the dark Higgs acts as the inflaton with a nonminimal gravity coupling. It analyzes a low-reheating scenario in which DPDM production occurs during reheating and is diluted by entropy, allowing both WIMP and FIMP regimes to be viable and potentially detectable. RG-improved inflationary dynamics yield $n_s$ and $r$ compatible with Planck, BICEP/Keck, and ACT, while the reheating temperature can be dramatically lowered (down to GeV or even MeV scales) via inflaton decays or scatterings, depending on the Higgs portal coupling. The work highlights that this framework naturally links DM phenomenology with inflationary cosmology and opens sizable, testable parameter spaces for future direct and indirect detection experiments and CMB observations.

Abstract

We investigate dark matter (DM) phenomenology and cosmic inflation within a unified framework based on a dark $U(1)_D$ gauge extension of the Standard Model (SM). The associated dark gauge boson, namely the dark photon, serves as a viable DM candidate, which we call dark photon dark matter (DPDM), whilst the dark Higgs field drives inflation. We explore a low-reheating scenario where DM production occurs during reheating, resulting in significant entropy dilution of the DPDM abundance. Both weakly interacting massive particle (WIMP) and feebly interacting massive particle (FIMP) DM scenarios are explored, depending on the dark gauge coupling strength. For FIMP-type DM, the entropy dilution allows for stronger couplings whilst maintaining the correct relic abundance, potentially bringing these candidates within the reach of current and near-future detection experiments. Similarly, WIMP-type DM can be realised with weaker couplings. We perform a comprehensive parameter scan incorporating constraints from collider data, DM direct and indirect detection experiments, and cosmological observations. Taking quantum corrections and running of the couplings into account, we demonstrate that dark Higgs inflation yields predictions for the spectral index $n_s$ and the tensor-to-scalar ratio $r$ that are consistent with the Planck, BICEP/Keck, and ACT data. The nonminimal coupling of the dark Higgs inflaton field to gravity is shown to be much smaller than in the case of the SM Higgs inflation scenario, avoiding unitarity concerns. We show that reheating temperatures as low as 1 GeV and 1 MeV can be achieved through the decay and scattering processes of the inflaton, respectively, with the latter allowing for larger Higgs mixing angles and enhanced detection prospects. Our results establish that this minimal extension successfully unifies DM physics with inflationary cosmology.

Figures (15)

• Figure 1: Evolution of the DM relic density $\Omega_{W_D}h^2$ in terms of the scale factor $a$ for different values of Hubble parameter $H_I$ at the start of reheating. The other parameters are chosen as $g_{D} = 10^{-5}$, $M_{W_D} = 100$ GeV, $M_{h_2} = 900$ GeV, $\sin\alpha = 0.1$, and $\Gamma_{\rm inf} = 4.4 \times 10^{-18}$ GeV. DM is produced by the freeze-in mechanism obeying the condition $|\Delta| > 10^{-2}$; see the text below Eq. \ref{['eqn:WIMP-FIMP-condition']}.
• Figure 2: Evolution of the DM relic density $\Omega_{W_D}h^2$ in term of $z=M_{W_D}/T$ for different values of the inflaton decay width $\Gamma_{\rm inf}$ (left) and the dark gauge coupling $g_D$ (right). The horizontal magenta line indicates $\Omega_{W_D}h^2 = 0.12$. DM is produced by the freeze-in mechanism, and the rest of the parameters are chosen to be the same as those shown in Fig. \ref{['fig:line-plot-1']}.
• Figure 3: Evolution of the DM relic density $\Omega_{W_D}h^2$ in term of $z=M_{W_D}/T$ for different values of the inflaton decay width $\Gamma_{\rm inf}$ (left) and the dark gauge coupling $g_D$ (right) just like in Fig. \ref{['fig:line-plot-2']}. Unlike Fig. \ref{['fig:line-plot-2']}, however, the DM is produced by the freeze-out mechanism this time, so we have WIMP-type DM here. The other parameters are chosen as $\sin\alpha = 0.1$, $M_{h_2} = 300$ GeV, $M_{W_D} = 500$ GeV, $g_{D} = 0.02$ and $\Gamma_{\rm inf} = 1.05\times 10^{-17}$ GeV. The horizontal magenta line indicates $\Omega_{W_D}h^2 = 0.12$.
• Figure 4: Evolution of the DM relic density $\Omega_{W_D}h^2$ in term of $z=M_{W_D}/T$ for different values of the dark Higgs mass $M_{h_2}$ (left) and the DPDM mass $M_{W_D}$ (right). The horizontal magenta line indicates $\Omega_{W_D}h^2 = 0.12$. The rest of the parameters are chosen to be the same as those shown in Fig. \ref{['fig:line-plot-1']}.
• Figure 5: Left panel: Evolution of the DM relic density $\Omega_{W_D}h^2$ in terms of the dark gauge coupling $g_D$ for three different values of the inflaton decay width $\Gamma_{\rm inf}$. Right panel: Evolution of the DM relic density $\Omega_{W_D}h^2$ in terms of the inflaton decay width $\Gamma_{\rm inf}$ for three different values of the dark gauge coupling $g_D$. The horizontal magenta line indicates $\Omega_{W_D}h^2 = 0.12$. The solid and dashed lines represent the DM production by the freeze-in and freeze-out mechanisms, respectively. The rest of the parameters are chosen to be the same as those shown in Fig. \ref{['fig:line-plot-1']}.