1. Introduction
2. Thermal history of mirror twin Higgs models
• | The simplest solution is removing the degrees of freedom of twin neutrinos, such as making the twin quarks vector-like [70], or the fraternal twin Higgs setup [25] in which the twin sector only contains one generation of twin fermion and a single twin neutrino with ΔNeff ≈ 0.075 is consistent with the current observation. In addition, the twin photon can also be made massive to further reduce the dark radiation [69, 71, 72]. |
• | Another simple solution is to raise the mass of the three twin left-handed neutrinos, such that their contribution to ΔNeff can be removed. This can be done by lifting the twin sector Yukawa couplings [26, 27] or by assigning different Majorana masses to the right-handed neutrinos from the SM and the twin sector [28]. |
• | In contrast to lifting the mass of twin neutrinos, raising the mass of twin charged particles allows twin neutrinos to decouple from the thermal bath at a much earlier time, about a few GeVs, and thus the twin neutrinos contribution to ΔNeff can be diluted by other particles leaving the thermal bath afterward [29]. |
• | Alternatively, asymmetric reheating provides another solution to this problem. A late decay of some additional particles dominantly to SM particles after the twin sector decouples from the SM sector may dilute the energy density of the twin sector [35, 36]. |
• | We propose a new model in which a spontaneous ${{\mathbb{Z}}}_{2}$ symmetry breaking lifts the twin neutrino masses above ${ \mathcal O }$(MeV), and thus has no contribution to the dark radiation. |
• | We realize the twin cogenesis in the fraternal twin Higgs setup with only one species of light twin tau neutrino which is consistent with the current limit on ΔNeff. |
• | Introducing a small difference between ${\alpha }_{s}^{{\prime} }({\rm{\Lambda }})$ and αs(Λ) at the scale Λ = 4πf. ${{\rm{\Lambda }}}_{\mathrm{QCD}}^{{\prime} }\approx 5.5\,{{\rm{\Lambda }}}_{\mathrm{QCD}}$ corresponds to a ${ \mathcal O }(10 \% )$ splitting of the QCD and twin QCD coupling constants through RGE running from the Λ [33]. This can be done via dynamics from scales higher than Λ [73, 74]. |
• | Introducing additional ${{\mathbb{Z}}}_{2}$ breaking term to modify the twin ${b}^{{\prime} }$ quark Yukawa coupling, possibly originated from the mixing between the twin ${b}^{{\prime} }$ and heavy composite fermions [75]. It has been shown in [76] that a ${ \mathcal O }(10 \% -20 \% )$ difference of b and ${b}^{{\prime} }$ Yukawa couplings at Λ = 4πf can result in ${{\rm{\Lambda }}}_{\mathrm{QCD}}^{{\prime} }/{{\rm{\Lambda }}}_{\mathrm{QCD}}\sim 5.5$. |
3. Neutrino-philic twin two Higgs doublet model
4. Twin cogenesis for mirror twin Higgs models
5. Twin cogenesis for the fraternal twin Higgs model
6. Phenomenology
6.1. Higgs signal strength and Higgs exotic decays
Figure 1. The signal strength in gluon fusion production and subsequent decays to gauge boson pairs, and the Higgs invisible branching ratio as a function of v/f. The bound on the invisible decay branching ratio is Brinv < 0.26 based on the combined ATLAS results. |