Abstract
In twin Higgs models that 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 ${\mathbb{Z}}_{2}\mathrm{FTH}$, 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 $\mathrm{\ensuremath{\Delta}}{N}_{\mathrm{eff}}\ensuremath{\approx}0.1$ would be within reach of Stage 4 cosmic microwave background experiments. Finally, the high luminosity LHC should be able to probe the entire parameter space of the ${\mathbb{Z}}_{2}\mathrm{FTH}$ model through charged scalar searches. We also discuss how searches for long-lived particles are starting to constrain fraternal twin Higgs models.
Highlights
The hierarchy problem and the unknown nature of dark matter have long motivated searches for new physics at colliders and direct detection experiments
We have presented a Z2 symmetric version of the fraternal twin Higgs model, which contains a viable dark matter candidate in the form of the dominantly righthanded twin tau, τ01 ≈ τ0R
This Z2-symmetric fraternal twin Higgs scenario (Z2FTH) model extends the standard fraternal twin Higgs scenario by adding twinhypercharge-breaking scalars with Y 1⁄4 2 in order to supply the necessary Z2 symmetry breaking [61]. This addition allows us to rescue the fraternal twin weakly interacting massive particle (WIMP) miracle [44,47] by decoupling the twin tau mass from its Yukawa coupling to the twin Higgs, and by adding annihilation channels which proceed through twin photon exchange
Summary
The hierarchy problem and the unknown nature of dark matter have long motivated searches for new physics at colliders and direct detection experiments. Any physics beyond the SM that is capable of solving both of these mysteries would be well motivated This has long bolstered interest in supersymmetry [2], which requires each fermion to be accompanied by a bosonic partner (and vice versa) with identical gauge charges and couplings, thereby canceling quadratically divergent contributions to the Higgs mass. Unlike dark matter in the conventional FTH model, our scenario is consistent with existing direct detection constraints This is made possible due to the new τ0 Majorana mass terms that are unrelated to the elastic scattering cross section, as well as new annihilation processes mediated by the twin photon. The Z2FTH scenario leads to a variety of potentially observable signals, including those at future direct detection experiments, exotic Higgs decay searches, CMB measurements, and indirect dark matter searches
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