Reconciling Large And Small-Scale Structure In Twin Higgs Models

We study extensions of the Twin Higgs model that solve the Hierarchy problem and simultaneously address problems of the large- and small-scale structures of the Universe. Besides naturally providing dark matter (DM) candidates as the lightest charged twin fermions, the twin sector contains a light photon and neutrinos, which can modify structure formation relative to the prediction from the $\Lambda$CDM paradigm. We focus on two scenarios. First, we study a Fraternal Twin Higgs model in which the spin-3/2 baryon $\hat{\Omega}\sim(\hat{b}\hat{b}\hat{b})$ and the lepton twin tau $\hat{\tau}$ contribute to the dominant and subcomponent dark matter densities. A non-decoupled scattering between the twin tau and twin neutrino arising from a gauged twin lepton number symmetry provides a drag force that damps the density inhomogeneity of a dark matter subcomponent. Next, we consider the possibility of having the twin hydrogen atom $\hat{H}$ as the dominant DM component. After recombination, a small fraction of the twin protons and leptons remains ionized during structure formation, and their scattering to twin neutrinos through a gauged U$(1)_{B-L}$ force provides the mechanism that damps the density inhomogeneity. Both scenarios realize the Partially Acoustic dark matter (PAcDM) scenario and explain the $\sigma_8$ discrepancy between the CMB and weak lensing results. Moreover, the self-scattering neutrino behaves as a dark fluid that enhances the size of the Hubble rate $H_0$ to accommodate the local measurement result while satisfying the CMB constraint. For the small-scale structure, the scattering of $\hat{\Omega}$'s and $\hat{H}$'s through the twin photon exchange generates a self-interacting dark matter (SIDM) model that solves the mass deficit problem from dwarf galaxy to galaxy cluster scales.