Charting the Flavour Structure of Dark Matter

Abstract

What flavour structure of $t$-channel thermal dark matter remains compatible with current flavour physics and direct detection bounds? We broadly chart the space of hypotheses using the framework of flavour symmetries and their breaking patterns. We then focus on scenarios in which the fermionic dark matter and its scalar mediator are flavour singlets, falling into the class of rank-1 flavour violation. For two representative benchmarks, quarkphilic ($q_L$) and leptophilic ($e_R$), we perform a comprehensive phenomenological analysis, fitting the relic abundance and examining the interplay among flavour observables, direct detection, and collider searches. Our results quantify the allowed deviations from flavour-symmetric limits and assess the discovery prospects in future flavour and direct detection experiments.

Figures (11)

• Figure 1: Schematic illustration of the space of flavour-protected $t$-channel DM scenarios in different flavour symmetry frameworks.
• Figure 2: Visual representation of the parametrization on the octant of the sphere in terms of the polar angle $\theta_\tau$ and azimuthal angle $\phi_{e\mu}$, and corresponding symmetric limits.
• Figure 3: Leptophilic DM. Bounds on the mediator mass $M_\Phi$ from lepton flavour-violating decays, with the dependence on the couplings $y_i$ factored out. On the left-hand side of the plot are the three-body decays, and on the right are the radiative decays. The different shades of the colour correspond to different values of $\delta_\text{DM}= (M_\Phi-M_\chi)/M_\chi$, while dashed (solid) lines denote future projections for $\delta_{\rm{DM}} = 0.05$ ($\delta_{\rm{DM}}\gg 1$). The Dirac and Majorana cases are shown separately on the left. Note that $\delta_\text{DM} \gg 1$ corresponds to the limit $M_\chi/M_\Phi \ll 1$ while keeping $M_\Phi$ fixed.
• Figure 4: Leptophilic Majorana DM. Left panel: Constraints in the $(M_\chi,\delta_{\rm DM})$ plane, with $\delta_{\rm DM}\equiv M_\Phi/M_\chi-1$. Shown are 90% CL bounds from collider searches (green, purple, and blue, corresponding to couplings to first-, second-, and third-generation leptons, respectively), DD (pink, with darker shades indicating couplings to heavier lepton generations; only first- and second-generation bounds are visible), and relic abundance considerations; representative contours for $y=1$ and $y=2.2$ are shown. The DD limits are evaluated along relic-density contours with varying couplings. Gray regions indicate parameter space where the coupling required to reproduce the observed relic abundance is non-perturbative, or where the correct abundance cannot be achieved via thermal freeze-out due to overdepletion. Dashed lines denote projected sensitivities from DARWIN. Right panel: Constraints from flavour physics and DD in the $(\phi_{e\mu},\,\theta_\tau)$ plane, evaluated at the benchmark point indicated by the black star in the left panel ($M_\chi \simeq 170 \text{ GeV},\delta_\text{DM} \simeq 0.14, y \simeq 0.98$). Dashed lines denote future projected sensitivities (see \ref{['sec:lep_flavour']}). The boxes in the upper-left and upper-right corners show zoomed-in views of the parameter space, illustrating that flavour constraints become ineffective in these regions and that, at the benchmark point, DD excludes the $U(1)_\mu$ limit but not the $U(1)_e$ limit. This region of parameter space is best visualised in \ref{['fig:basis_reparametrization']}. See \ref{['sec:interplay_leptophilic']} for details.
• Figure 5: Leptophilic Dirac DM. Left panel: Constraints in the $(M_\chi,\delta_{\rm DM})$ plane, with $\delta_{\rm DM}\equiv M_\Phi/M_\chi-1$. Shown are 90% CL bounds from collider searches (green, purple, and blue, corresponding to couplings to first-, second-, and third-generation leptons, respectively), DD (pink, with darker shades indicating couplings to heavier lepton generations; here the bound is essentially the same for all generations), and relic abundance considerations; representative contours for $y=1$ and $y=2.2$ are shown. The DD limits are evaluated along relic-density contours with varying couplings. Gray regions indicate parameter space where the coupling required to reproduce the observed relic abundance is non-perturbative, or where the correct abundance cannot be achieved via thermal freeze-out due to overdepletion. Right panel: Constraints from flavour physics and DD in the $(\phi_{e\mu},\,\theta_\tau)$ plane, evaluated at the benchmark point indicated by the black star in the left panel ($M_\chi \simeq 3.5 \text{ TeV},\delta_\text{DM} \simeq 1, y \simeq 3.35$). Dashed lines denote future projected sensitivities discussed in \ref{['sec:lep_flavour']}. See \ref{['sec:interplay_leptophilic']} for details.