Alleviating Cosmological Tensions with the Hadrosymmetric Twin Higgs

TL;DR

The paper investigates the cosmological implications of the Hadrosymmetric Twin Higgs (HTH) model, which embeds all three generations of twin quarks while omitting light twin states to keep $ΔN_{\rm eff}\approx 0$ and evade collider constraints. By implementing the HTH sector in the Boltzmann code CLASS and performing MCMC analyses with Planck 2018 data (and, in some cases, local $H_0$ priors), the authors model the twin sector as an effective decaying-dark-matter component that transfers energy to visible photons. They find that HTH can reduce the Hubble tension from $>4σ$ to ~2.5σ and alleviate the $σ_8$ discrepancy, with the variant including a local $H_0$ prior (HTH*) shifting toward higher $H_0$ values without favoring very low $H_0$. An extended case with light twin leptons (HTH+$ΔN_{\rm twin}$) discussed in the Appendix suggests the possibility of fully resolving the $H_0$ tension, illustrating a meaningful connection between naturalness-inspired new physics and precision cosmology.

Abstract

The Hadrosymmetric Twin Higgs (HTH) model provides a natural solution to the little hierarchy problem by incorporating all three generations of quarks in a twin sector. Unlike other Twin Higgs scenarios, such as the Mirror Twin Higgs (MTH), the HTH framework avoids introducing additional light states or radiation and thus remains consistent with stringent bounds on the effective number of relativistic species, $ΔN_{\rm eff}$. Its particle content and interactions also make it difficult to probe at colliders, highlighting the importance of cosmological tests. In this work, we study the cosmological implications of the HTH model, focusing on the persistent tensions in the Hubble constant ($H_0$) and the matter clustering amplitude ($σ_8$). Implementing the HTH sector in a Boltzmann code and confronting it with cosmic microwave background (CMB) data and local $H_0$ measurements, we find that the scenario reduces the Hubble tension from more than $4σ$ to about $2.5σ$, while also alleviating the $σ_8$ discrepancy. These results demonstrate that the HTH framework not only addresses naturalness in particle physics but also offers a viable route to mitigating current cosmological tensions, thereby strengthening the link between fundamental theory and precision cosmology.

Figures (6)

• Figure 1: Likelihood contours for the $\Lambda$CDM (blue) and HTH (grey) models obtained using only the CMB dataset. The HTH contours are broader due to the additional free parameters. Compared to $\Lambda$CDM, the Hubble tension is reduced, and the $\sigma_8$ discrepancy is milder. The red contours labeled HTH* include both CMB and local $H_0$ measurements. In this case, lower $H_0$ values are excluded, and the $\sigma_8$ tension is effectively resolved.
• Figure 2: Preferred ranges of $S_8 = \sigma_8\sqrt{\Omega_m/0.3}$ and $H_0$ for the HTH and $\Lambda$CDM models using the Planck 2018 dataset. The HTH* contours include both Planck 2018 data and the local $H_0$ measurement ($73.04 \pm 1.04$ km/s/Mpc) Riess:2021jrx. For comparison, the SH0ES measurement of $H_0$ (green) and the KV450 measurement of $S_8 = 0.737^{+0.040}_{-0.036}$ (violet) are also shown Hildebrandt:2018yau. Grey, red, and blue contours correspond to HTH, HTH*, and $\Lambda$CDM, respectively.
• Figure 3: Posterior distributions for all free parameters of the $\Lambda$CDM (blue), HTH (grey), and HTH* (red) models.
• Figure 4: Likelihood distributions for the HTH+$\Delta N_{\rm twin}$ (red) model and $\Lambda$CDM (blue) scenario. The addition of ultra-relativistic species clearly alleviates the Hubble tension.
• Figure 5: Preferred ranges of $S_8 = \sigma_8 \sqrt{\Omega_m / 0.3}$ and $H_0$ for the HTH+$\Delta N_{\rm twin}$ and $\Lambda$CDM models. Red and blue contours correspond to HTH+$\Delta N_{\rm twin}$ and $\Lambda$CDM, respectively.The local Hubble measurement ($H_0 = 73.04 \pm 1.04~\rm km/s/Mpc$) and KV450 $S_8$ value are shown for reference Riess:2021jrxHildebrandt:2018yau.