Axion-Like Electrophilic Portal for Pion Dark Matter

TL;DR

This work investigates a SIMP dark-matter framework with a confining dark sector of dark pions connected to the Standard Model via an electrophilic ALP portal that couples exclusively to electrons. The authors analyze how kinetic and chemical equilibrium between the dark sector and the SM can be maintained through ALP-mediated interactions, deriving constraints from CMB, indirect detection, and laboratory experiments. They show that ALP masses as low as $\mathcal{O}(10)\, \text{MeV}$ can serve as viable portals, including a potential link to the $17\,\text{MeV}$ PADME hint, and further demonstrate that introducing a nonzero dark-sector $\theta$ angle opens up the possibility of heavier ALPs ($m_a>m_\pi$) by enabling new thermalization channels. The results broaden the landscape of ALP-SIMP portal scenarios and motivate continued model-building and experimental exploration, with intriguing connections to light-boson anomalies such as the $X_{17}$ resonance.

Abstract

We investigate a scenario where Strongly Interacting Massive Particle (SIMP) dark matter interacts with an axion-like particle (ALP) that couples exclusively to electrons. This minimal setup provides interactions which enforce thermal equilibrium between dark matter and the SM in the early Universe. We analyze the cosmological evolution of the dark sector and the constraints arising from dark matter annihilations, ALP laboratory searches and astrophysical observations. Our results show that the allowed parameter space is wider than previous studies and an ALP with mass $m_a \sim {\cal O}(10)~\text{MeV}$ can act as a viable portal between the visible and dark sectors. Interestingly, this mass range overlaps with the parameter space suggested by the reported $X_{17}$ anomaly. Furthermore, the introduction of non-vanishing $θ$ angle in the dark sector of the model opens up the parameter space to heavy ALP masses.

Figures (4)

• Figure 1: Summary of the constraints for different choices of dark matter mass. Left: thermalization between the ALPs and the SM is efficient above the solid lines. The maximal value of $m_a$ allowed from CMB and indirect detection constraints is shown as a dashed line. The gray region is excluded from laboratory searches and SN1987A. The available parameter space corresponds to the white region above the solid line and at the left of the dashed line, as shown by the arrows. Right: the available parameter space corresponds to the colored regions. These are bounded from above by CMB and indirect detection constraints and from below by the ALP-dark matter thermalization condition. The red dotted line corresponds to an ALP of mass $m_a=17$ MeV.
• Figure 2: We show for the pseudo-Goldstone interpretation of the ALP, $g_{ae}/m_e\equiv 1/f_a$, the combination of all the constraints of Fig. \ref{['fig:alp1']}. The red (green) [blue] area is allowed for $m_\pi=0.1(0.5)[1]$ GeV. The gray area is excluded by laboratory and SN constraints, while the white region is excluded by CMB/indirect detection and failure to achieve an efficient ALP-DM thermalization, see the caption of Fig. \ref{['fig:alp1']} and the main text for details. The value of $g_{ae}$ is always sufficiently large to achieve an efficient ALP-SM thermalization. The red dotted line corresponds to $m_a=17$ MeV.
• Figure 3: Summary of the constraints in the case of a non vanishing $\theta$ angle and $m_a>m_\pi$. The value of $g_{ae}$ is chosen so that all constraints from ALP laboratory searches are satisfied. In the gray region the dark matter does not thermalize with the SM efficiently, while the orange region is excluded by CMB and indirect detection constraints. The available parameter space corresponds to the white region. See the main text for details.
• Figure 4: The value of branching ratio for the process $a \xrightarrow{} e^+ e^-$ is shown for two representative benchmark points.