Resurrecting the Fraternal Twin WIMP Miracle

TL;DR

This paper modifies the Fraternal Twin Higgs model by introducing hypercharged scalars to reduce twin tau's direct detection cross section, making it a viable dark matter candidate detectable in future experiments.

Contribution

The paper proposes the $ ext{Z}_2$FTH model with hypercharged scalars that break twin electromagnetism, enabling the twin tau to be a consistent dark matter candidate with testable predictions.

Findings

Twin tau can have acceptable relic abundance and evade current direct detection bounds.

Large parts of the parameter space are accessible to upcoming direct detection experiments.

The model predicts observable signals in gamma rays, cosmic rays, and CMB measurements.

Abstract

In Twin Higgs models which contain the minimal particle content required to address the little hierarchy problem (i.e. fraternal models), the twin tau has been identified as a promising candidate for dark matter. In this class of scenarios, however, the elastic scattering cross section of the twin tau with nuclei exceeds the bounds from XENON1T and other recent direct detection experiments. In this paper, we propose a modification to the Fraternal Twin Higgs scenario that we call $\mathbb{Z}_2$FTH, incorporating visible and twin hypercharged scalars (with $Y = 2$) which break twin electromagnetism. This leads to new mass terms for the twin tau that are unrelated to its Yukawa coupling, as well as additional annihilation channels via the massive twin photon. We show that these features make it possible for the right-handed twin tau to freeze out with an acceptable thermal relic abundance while scattering with nuclei at a rate that is well below existing constraints. Nonetheless, large portions of the currently viable parameter space in this model are within the reach of planned direct detection experiments. The prospects for indirect detection using gamma rays and cosmic-ray antiprotons are also promising in this model. Furthermore, if the twin neutrino is light, the predicted deviation of $\Delta N_\mathrm{eff} \approx 0.1$ would be within reach of Stage 4 CMB experiments. Finally, the high luminosity LHC should be able to probe the entire parameter space of the $\mathbb{Z}_2$FTH model through charged scalar searches. We also discuss how searches for long-lived particles are starting to constrain Fraternal Twin Higgs models.