Super-cool Dark Matter

TL;DR

This work investigates a novel dark matter production mechanism in dimensionless, scale-invariant theories in which a period of thermal inflation (super-cooling) dilutes the relic density. The end of super-cooling, potentially triggered by the QCD phase transition, links the dark matter mass to the weak scale and allows TeV-scale DM to arise without standard freeze-out; reheating then determines whether a sub-thermal population persists. To generate the observed baryon asymmetry, the authors explore post-super-cooling baryogenesis, focusing on leptogenesis facilitated by additional neutrino sectors in two concrete models: SU(2)$_X$ and U(1)$_{B-L}$. The results show that super-cool DM can reproduce the correct relic density for a wide range of DM masses (up to PeV) with small couplings, and they outline distinct experimental signatures in direct detection and collider searches that can probe these scenarios.

Abstract

In dimension-less theories of dynamical generation of the weak scale, the Universe can undergo a period of low-scale inflation during which all particles are massless and super-cool. This leads to a new mechanism of generation of the cosmological Dark Matter (DM) relic density: super-cooling can easily suppress the amount of DM to the desired level. This is achieved for TeV-scale DM, if super-cooling ends when quark condensates form at the QCD phase transition. Along this scenario, the baryon asymmetry can be generated either at the phase transition or through leptogenesis. We show that the above mechanism takes place in old and new dimension-less models.

Figures (5)

• Figure 1: Left: The nucleation temperature given by vacuum decay, ignoring the QCD phase transition. Right: 3-loop RGE running in the massless SM.
• Figure 2: Left: number of $e$-folds of thermal inflation, $N = \ln T_{\rm infl}/T_{\rm end}$ for $\langle h \rangle_{\rm QCD} = \unit[100]{MeV}$. Right: reheating temperature in $\,{\rm GeV}$ (solid red curves) and $M_s/\,{\rm GeV}$ (diagonal dashed lines).
• Figure 3: Left: The observed DM abundance is reproduced along the solid curves, computed for different values of the uncertain QCD factor $\langle h \rangle_{\rm QCD}$. The region shaded in orange (blue) is excluded by direct DM searches (collider searches for the singlet $s$). Dashed curves show future detection prospects. Right: Sample evolution of the DM density, of entropy, of the scalar energy.
• Figure 4: Leptonic CP asymmetry $\epsilon_{\rm CP}$ needed to obtain successful leptogenesis, assuming right-handed neutrinos in thermal equilibrium at the temperature $T_{\rm RH}$. For $M_N \gtrsim M_h$ ($M_N \,\hbox{$<$$\sim$}\, M_h$) the asymmetry comes from decays of right-handed neutrinos (of the Higgs), and the needed $\epsilon_{\rm CP}$ is obtained from right-handed neutrinos degenerate at the $\Delta M_N/M_N \approx 10^{-7}$ level (at the $10^{-5}$ level). We fixed the Yukawa couplings to $|Y_N|^2 v^2/M_N = \unit[10^{-11}]{eV}$.
• Figure 5: The observed DM abundance is reproduced along the solid curves, computed for different values of the uncertain QCD factor $\langle h \rangle_{\rm QCD}$. The region shaded in orange (blue) is excluded by direct DM searches (collider searches for the singlet $s$). The region shaded in yellow is excluded by precision data. Dashed curves indicate future detection prospects.