Particle physics probes from cosmology

Abstract

In this dissertation, we explore the cosmological sensitivity of well-motivated extensions of the Standard Model (SM) of particles. We focus on two specific models, the vector portal and the Higgs portal, that can connect the SM to a dark sector of new hidden particles. We find that both portals have sensitivity in the ultra-weak coupling regime, where the relic abundance is set by the freeze-in mechanism. Provided that the mediators of the portal interactions decay into the SM, we derive the constraints on masses and couplings of such states from precision cosmology. As a primary source of constraints, we use Big Bang Nucleosynthesis (BBN), the Cosmic Microwave Background (CMB) and the diffuse X-ray background. For the Higgs portal scalar, we improve the relic abundance calculation in the literature and provide an estimate of thermal corrections to the freeze-in yield. We find that the cosmological bounds are relatively insensitive to improvements in the abundance accuracy, and a full finite-temperature calculation is not needed.

We also investigate the BBN constraints for hypothetical long-lived metastable scalars particles $S$ that can be produced at the Large Hadron Collider from decays of the Higgs boson. We find that for viable branching ratios Br($h \to SS$), the early universe metastable abundance of $S$, regulated by its self-annihilation through the Higgs portal, is so large that the lifetime of $S$ is strongly constrained to $\tau_S < 0.1$~s to maintain the consistency of BBN predictions with observations. This provides a useful upper bound on the lifetimes of $S$ particles that a purposely-built detector, such as the one suggested in the MATHUSLA proposal, seek to discover.

We also investigate the viability and detectability of freeze-in self-interacting fermionic dark matter communicating with the SM via a vector portal. We focus on the parameter where the $\chi \bar{\chi} \to A'A'$ is negligible, as required by a variety of indirect detection constraints. We find that planned upgrades to the direct detection experiments will be able to probe the region of parameter space that can alleviate small scale structure problems of dark matter via self-interactions for a dark fine structure constant as small as $\alpha_d =10^{-4}$. We forecast the sensitivity for Lux-ZEPLIN, XENONnT and PandaX-4T.