Kination and the Inert Doublet Model

TL;DR

This work shows that the inert doublet model, which standard cosmology deems underabundant in the 120–500 GeV DM-mass window, can yield the correct relic density if freeze-out occurs during a stiff era with $w>1/3$ such as kination. By analyzing standard radiation domination and non-standard histories, the authors identify a re-opened 225–550 GeV window for $m_{H^0}$ that depends on the reheating temperature $T_{rh}$ and the equation-of-state parameter $w$, with $w=1$ allowing $T_{rh}$ between ~100 MeV and ~100 GeV for viable relic abundance. The study uses micrOMEGAs to solve the Boltzmann equations with evolving background and imposes direct and indirect constraints, showing compatibility for small Higgs-portal coupling lambda_L and small mass splittings Delta m. The findings underscore the importance of early-Universe cosmology in DM phenomenology and motivate future probes such as CMB, gravitational waves, and next-generation DM experiments to test such non-standard histories.

Abstract

The inert doublet model is a two-Higgs-doublet extension of the standard model that provides a minimal and versatile framework for frozen-out dark matter. Assuming standard cosmology, if the dark matter mass ranges between approximately 120 GeV and 500 GeV then it turns out to be underabundant, as gauge interactions render its annihilation too efficient. In this work, we show that this mass window becomes allowed in cosmological scenarios where dark matter freeze-out occurs during a period with a stiff equation of state, $w > 1/3$, such as kination. This predictive setup satisfies all current experimental constraints while remaining within the reach of upcoming detection efforts.

Figures (5)

• Figure 1: Left: Maximum value of $|\lambda_L|$ allowed by DM direct detection. Right: Maximum mass splitting $\Delta m \equiv m_{A^0} - m_{H^0}$ (with $m_{A^0} = m_{H^\pm}$) allowed by DM indirect detection, for the maximal and minimal value of $\lambda_L$ (cf. left panel), and $\lambda_L = 0$.
• Figure 2: DM relic abundance for $\lambda_L = 0$ and different values of the mass splitting $\Delta m$. The two black lines bracket the region in blue that can be achieved in the standard cosmological scenario. The red region is in conflict with Fermi-LAT data.
• Figure 3: Values of $T_\text{rh}$ required to explain the entire observed DM abundance in the kination scenario, for $\lambda_L = 0$ and different values of the mass splitting $\Delta m$. The two black lines bracket the region in blue corresponding to the different mass splittings. The red region is in conflict with Fermi-LAT data. The left panel corresponds to kination ($w=1$) while the right panel to $w=2$.
• Figure 4: Same as Figs. \ref{['fig:RD']} and \ref{['fig:kination']} but for $\lambda_L > 0$ (left) or $\lambda_L < 0$ (right). Note that in the latter case, the requirement of vacuum stability essentially excludes the blue-shaded region for masses $m_{H^0} \gtrsim 600$ GeV.
• Figure 5: Maximum value of $T_\text{max}$ allowed by the inflationary scale $H_I$ as a function of $T_\text{rh}$, for different values of $w$.