Dark Matter and Neutrino Masses from a Composite Hidden Sector

Abstract

Despite the remarkable success of the Standard Model in explaining the interactions of the elementary particles, there is now indisputable evidence that it is incomplete. Although the Standard Model predicts that neutrinos are massless, over the last few decades experiments have established that the masses of the neutrinos, although very small, are nonvanishing. Furthermore, cosmological and astrophysical observations have established that about 80% of the matter in the universe is composed of some form of non-luminous dark matter, but there is no particle in the Standard Model that can play this role. Any explanation of the origin of neutrino masses and the nature of dark matter therefore requires physics beyond the SM. In this thesis, we present a novel class of models that can explain both the origin of neutrino masses and the observed abundance of dark matter. In these models, the particle that constitutes dark matter arises as the composite state of a strongly coupled hidden sector that couples to the Standard Model through the neutrino portal. A discrete symmetry ensures that the dark matter particle is stable and does not decay. The hidden sector is in thermal equilibrium with the Standard Model in the early universe. The abundance of dark matter is set by its annihilation into final states containing neutrinos. The neutrino portal coupling also gives rise to small Majorana masses for the neutrinos through the inverse seesaw mechanism, with the role of the singlet neutrinos being played by composite states. The Standard Model neutrinos mix with the singlet neutrinos, and so the Standard Model neutrinos are partially composite in this framework. The dynamics of the hidden sector is taken to be approximately conformal in the ultraviolet, and a relevant deformation leads to breaking of the conformal symmetry in the infrared. Since the hidden sector is uncharged under the Standard Model gauge groups, the compositeness scale can lie below the weak scale, leading to striking experimental signals. We employ the AdS/CFT correspondence to construct a holographic dual of this scenario. This takes the form of a Randall Sundrum model with two branes. Within this framework we explore the signals of these models at various current and future experiments. These include searches for lepton flavor violation in $\mu \rightarrow e \gamma$ and $\mu \rightarrow e$ conversion experiments, direct and indirect detection of dark matter and searches at colliders and beam dumps. We determine the current bounds on this scenario and show that future experiments can significantly expand the reach.