Two coincidences are a clue: Probing a GeV-scale dark QCD sector

Abstract

The similarity between the dark matter and baryon energy densities suggests an existence of a dark sector analogous to QCD. In addition, small-scale structure anomalies can be addressed by dark matter self-interactions with cross sections comparable to those of QCD. Both observations point toward a GeV-scale dark QCD sector. Motivated by these two coincidences, we investigate the parameter space of a distinctive chiral dark QCD model featuring a MeV-scale dark photon with axial-vector couplings. We also discuss a possible third coincidence associated with the latest measurement of $N_{\rm eff}$. Current constraints leave a finite and testable region of parameter space that can be probed by future experiments such as the Gamma Factory.

Figures (3)

• Figure 1: The favored parameter space (shaded band) for the chiral dark QCD model to explain the two coincidences. Left: On the $(m_D,f)$ plane, the blue band is determined by the $(\sigma_D/m_D)_{\rm low}$ with values shown in the legend. The three black lines represent different values of Yukawa coupling $Y_D$ of the dark matter. Center: The ratio between $(\sigma_D/m_D)_{\rm high}$ and $(\sigma_D/m_D)_{\rm low}$ determine the required value of $m_{\gamma'}/m_D$ as shown on the $(m_{\gamma'},m_D)$ plane. Right: The favored parameter space on the $(m_{\gamma'},f)$ plane with black lines representing different values of gauge coupling $g_D$.
• Figure 2: Constraints on the $(m_D,\sigma m_{\gamma'}^4)$ plane for the chiral dark QCD model. The green shaded region is excluded by the strongest direct detection bound from PandaX-4T PandaX:2023xgl. The brown shaded region is excluded by the $N_{\rm eff}$ bound on dark photon considering the relation in Eq. \ref{['sigmam4']}.
• Figure 3: Constraints on the dark photon of the chiral dark QCD model with kinetic mixing. The brown shaded region is excluded by the $N_{\rm eff}<2.67$ with a black dashed line corresponding to the lifetime $\tau_{\gamma'}=0.1$ s. The brown line represents the favored values with $N_{\rm eff}=2.89$. The pink shaded region is excluded by the E137 experiment Bjorken:1988as. The red dashed line is the projected bound from the Gamma Factory with the energy of $200$ MeV Chakraborti:2021hfm. The green shaded region is excluded by the direct detection following the relation in Eq. \ref{['sigmam4']}.