Maximizing Direct Detection with Highly Interactive Particle Relic Dark Matter

We estimate the maximum direct detection cross section for sub-GeV dark matter (DM) scattering off nucleons. For DM masses in the range 10 keV–100 MeV, cross sections greater than ${10}^{-36}-{10}^{-30}{\mathrm{cm}}^2$ seem implausible. We present a DM candidate which realizes this maximum cross section: highly interactive particle relics (HYPERs). After HYPERs freeze-in, a dark sector phase transition decreases the mediator’s mass. This increases the HYPER’s direct detection cross section without impacting its abundance or measurements of big bang nucleosynthesis and the cosmic microwave background.

Figure 1

Constraints in the mediator mass-nucleon coupling plane from cooling of HB stars [25] and SN 1987A [12], as well as rare kaon decays [26] (gray shading). Also shown are values for $(m_{\varphi }^{\min },y_n^{\max })$.

Figure 2

${\sigma }_{\chi n}^{\max }$ for the $(m_{\varphi }^{\min },y_n^{\max })$ values from Fig. 1, as well as hadrophilic HYPER space for the $T_{\mathrm{PT}}=1\mathrm{MeV}$ benchmark. The current constraint from CDEX [29] is shaded gray, while future projected sensitivities are shown with dashed gray lines [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 30].