The Dark Side of a Tera-Z Factory

TL;DR

The paper assesses how a Tera-Z factory can indirectly probe dark matter models in which DM is a SM singlet coupled to the visible sector via t-channel mediators. By performing a one-loop SMEFT matching and a global EWPO fit, complemented by relic abundance, direct detection, and indirect detection analyses, it shows that precision Z-pole observables—especially R_b and A_FB^b—can reveal or constrain a wide class of t-channel portals, with significant two-loop running effects enhancing sensitivity in several cases. The work demonstrates that, in certain parameter regions, Tera-Z measurements can be competitive with or complementary to direct-detection and gamma-ray searches (DARWIN/CTAO), and that the pattern of EWPO deviations provides discriminating power between different portals. It highlights the need for improved SM calculations and targeted EFT studies to maximize the discovery potential of future high-precision lepton colliders for dark sectors.

Abstract

The future circular $e^+e^-$ collider (FCC-ee or CEPC) will provide unprecedented sensitivity to indirect new physics signals emerging as small deviations from the Standard Model predictions in electroweak precision tests. Assuming new physics scenarios containing a dark matter candidate and a $t$-channel mediator, we analyse the synergy and interplay of future Tera-$Z$ factories and non-collider tests conducted through direct and indirect searches of dark matter. Our results highlight the excellent prospect for a Tera-$Z$ run to indirectly probe the presence and nature of dark matter.

Figures (17)

• Figure 1: $\chi \Phi q_L$ case: Parameter space with fixed mediator mass $M_\Phi = 2$ TeV (upper panel) and fixed portal coupling $y_Q=1.5$ (lower panel). The solid green line indicates the parameter values yielding the observed dark matter relic abundance. The surrounding green region corresponds to scenarios where $\chi$ is a subcomponent of dark matter and represents the target parameter space. The blue region is excluded by the projected Tera-$Z$ sensitivity at 95% CL for $\kappa_{1,2}=0$ while current direct detection bounds exclude the red region. The DARWIN projections exclude the parameter space below the dash-dotted red line.
• Figure 2: $\chi \Phi q_L$ case: The variation of the most influential electroweak observables, $R_b$ and $A_{\rm FB}^b$, as a function of the portal coupling. The green bands denote the Tera-$Z$ experimental sensitivity at $1\sigma$, while the blue bands reflect the variation of the dark matter mass $M_\chi \in [0.5, 2]$ TeV. The mediator mass is fixed to $M_\Phi = 2$ TeV. The dashed lines show the effect of non-vanishing $\kappa_1$ and $\kappa_2$, where the main effect comes from $\kappa_2$, see the main text.
• Figure 3: Feynman diagrams showing the leading-order contributions to the electroweak precision observables in the $\chi \Phi q_L$ scenario (left) and the $\chi \Phi u_R$ scenario (right).
• Figure 4: Feynman diagram showing the next-to-leading-order contribution to the electroweak precision observables in the $\chi \Phi q_L$ case.
• Figure 5: $\chi\Phi q_L$ case: Parameter space with fixed DM mass $M_\chi=1.5$ TeV and $y_Q=0$. The green lines indicate the values reproducing the observed relic density for different values of $\kappa_1$. The blue lines represent the exclusion limits by the projected Tera-$Z$ sensitivity at 95% CL.