Reviving $Z$ and Higgs Mediated Dark Matter Models in Matter Dominated Freeze-out

TL;DR

This work investigates the possibility that dark matter decoupled during an early matter-dominated epoch, which alters the Hubble expansion rate and the resulting relic abundance. It first outlines a model-independent framework for matter-dominated freeze-out, including entropy dilution during the transition to radiation domination, and then applies this to Higgs-portal (scalar and fermion) and Z-portal (vector-like and axial) dark matter. The authors show that, due to entropy dilution and modified expansion, classic Higgs and Z portal scenarios can be viable again, evading stringent radiation-dominated bounds while remaining consistent with BBN and cosmological constraints. The results highlight that upcoming experiments probing near the neutrino floor could test these revived minimal portals, offering a compelling link between nonstandard cosmology and detectable dark matter signals.

Abstract

It is quite conceivable that dark matter freeze-out occurred during an early period of matter domination, in which case the evolution and relic abundance differ from standard freeze-out calculations which assume a radiation dominated universe. Here we re-examine the classic models in which dark matter interactions with the Standard Model are mediated via either the Higgs or $Z$ boson in the context of matter dominated freeze-out. We highlight that while these classic models are largely excluded by searches in the radiation dominated case, matter dominated freeze-out can relax these limits and thus revive the Higgs and $Z$ portals. Additionally, we discuss the distinctions between matter dominated freeze-out and decoupling during the transition from matter domination to radiation domination, and we comment on the parameter regimes which lead to non-negligible dark matter production during this transition.

Figures (10)

• Figure 1: Left: Evolution of the fraction of energy density in matter (blue) and radiation (red), for $T_{\rm RH}=10$ MeV, $T_\star=10^7$ GeV and $r=0.99$. The $w$ curve (dashed) parameterizes the change from radiation domination ($w=1/3$) to matter domination ($w=0$). The first intersection of blue and red line signifies the onset of matter domination and the vertical black line shows the point $T=T_{\rm EV}$ at which entropy production in the bath is non-negligible. Right: Orderings of events in time parameter $\tau$ as a function of DM mass for $T_\star=10^7$ GeV, and $r=0.99$ with $T_{\rm RH}$ fixed to reproduce observed DM relic density. The black line indicates the time of freeze-out, the radiation domination era shaded in red, the period in which entropy violation is considerable is shaded in yellow, and matter domination freeze-out occurs in the white region.
• Figure 2: Left: The total Higgs Portal annihilation cross-section for the complex scalar DM in units of $\langle\sigma v\rangle_0=3\times10^{-26}$cm$^3/$s, for a fixed value of the coupling $\kappa=0.01$ of the mixed quartic $|H|^2|\chi|^2$. Right: The fraction of the total annihilation cross-section at freeze-out going to various Standard Model final states.
• Figure 3: The above plots examine the case of complex scalar DM annihilating through the Higgs portal for the parameter values $r = 0.99$ and $T_{\star} = 10^{5}$ GeV (left) and $10^{7}$ GeV (right). The solid black curves show contours of the reheat temperature ($T_{\rm RH}$) which give the observed DM relic density. The dashed black line shows the analogous case of radiation dominated DM freeze-out (without an entropy injection). The shaded regions signify parameter values for which the consistency conditions fail: $\phi$ decay before freeze out $T_{f}<T_{\Gamma}$ (green), decays of $\phi$ cannot be neglected $T_f\ll T_{\rm EV}, T_{\rm RH}$ (red, orange), and DM decouples relativistically with $x_f<3$ (blue). Observe that viable parameter space remains with the observed DM relic density.
• Figure 4: Experimental constraints on the complex scalar DM annihilating through Higgs portal from XENON1T Aprile:2018dbl (dashed red), LUX Akerib:2016vxi (dashed orange), Fermi-LAT Ackermann:2015zua (dashed purple), invisible Higgs width Escudero:2016gzxKhachatryan:2016whc (dashed brown). Also shown is the neutrino floor (dashed green). The grey region shows the combined consistency restrictions from Figure \ref{['fig:kContour-ScalarDM']}.
• Figure 5: Similar to Figure \ref{['fig:HiggsExpConScalar']} but for fermion DM annihilating through the EFT Higgs portal for the parameter values $r = 0.99$ and $T_{\star} = 10^{5}$ GeV (left) and $10^{7}$ GeV (right). The solid black curves show contours of the reheat temperature ($T_{\rm RH}$) which give the observed DM relic density. The dashed black line shows the analogous case of radiation dominated DM freeze-out (without an entropy injection). The shaded regions signify parameter values for which the consistency conditions fail: $\phi$ decay before freeze out $T_{f}<T_{\Gamma}$ (green), decays of $\phi$ cannot be neglected $T_f\ll T_{\rm EV}, T_{\rm RH}$ (red, orange), DM decouples relativistically with $x_f<3$ (blue), and $T_{{\rm RH}} > 10$ MeV required for successful BBN (yellow). Observe that viable parameter space remains with the observed DM relic density for reheat temperatures less than 1 TeV.