A taste of dark matter: Flavour constraints on pseudoscalar mediators

TL;DR

This work systematically constrains a light pseudoscalar mediator bridging dark matter and the Standard Model by analyzing loop-induced flavour-changing processes in rare meson decays and fixed-target experiments. It derives the effective FCNC couplings and decay properties of the mediator across several coupling structures, then maps these bounds onto DM relic density, thermal history, and self-interaction requirements. The results show that flavour constraints are typically stronger than direct or indirect detection bounds, and that explaining the DAMA modulation or the Galactic Centre gamma-ray excess with such a mediator is highly constrained, especially under thermal relic assumptions. The study emphasizes the strong interplay between flavour physics and dark-sector phenomenology and motivates future searches for displaced vertices and refined UV completions.

Abstract

Dark matter interacting via the exchange of a light pseudoscalar can induce observable signals in indirect detection experiments and experience large self-interactions while evading the strong bounds from direct dark matter searches. The pseudoscalar mediator will however induce flavour-changing interactions in the Standard Model, providing a promising alternative way to test these models. We investigate in detail the constraints arising from rare meson decays and fixed target experiments for different coupling structures between the pseudoscalar and Standard Model fermions. The resulting bounds are highly complementary to the information inferred from the dark matter relic density and the constraints from primordial nucleosynthesis. We discuss the implications of our findings for the dark matter self-interaction cross section and the prospects of probing dark matter coupled to a light pseudoscalar with direct or indirect detection experiments. In particular, we find that a pseudoscalar mediator can only explain the Galactic Centre excess if its mass is above that of the B mesons, and that it is impossible to obtain a sufficiently large direct detection cross section to account for the DAMA modulation

Figures (12)

• Figure 1: Flavour-changing transitions such as $b \to s A$ (and also $s\to d A$ after relabelling the external lines) are generated by diagrams with heavy quarks and $W^{\pm}$-bosons.
• Figure 2: Left: Branching ratios of the pseudoscalar for Yukawa-like couplings according to the perturbative spectator model (see Appendix \ref{['sec:decays']} for further details). Right: Total width in units of the pseudoscalar mass for Yukawa-like couplings to all fermions (with $g_Y = 10$), Yukawa-like couplings only to quarks (with $g_{Yq} = 10$) and universal quark couplings (with $g_q = 0.1$). For the latter case, we do not attempt to estimate the total width once hadronic decay channels open up (when $m_A>3 m_{\pi}$). The narrow width approximation is valid for all parameter values that we consider.
• Figure 3: Excluded parameter regions for a pseudoscalar $A$ with Yukawa-like couplings to all fermions (left) and Yukawa-like couplings only to quarks (right); the coupling $g_Y$ was defined in Eq. \ref{['eq:LYukawa']}.
• Figure 4: Excluded parameter regions for a pseudoscalar $A$ with universal couplings to quarks; the coupling $g_q$ was defined in Eq. \ref{['eq:ASM']}. The left panel shows the constraints when $A$ couples to all quarks, while the right panel shows the constraints when $A$ couples only to third generation quarks $Q=\{b,t\}$.
• Figure 5: Excluded parameter regions for an invisibly decaying pseudoscalar $A$. The left panel assumes Yukawa-like couplings to quarks, while the right panel considers universal quark couplings.