Conformal Freeze-In, Composite Dark Photon, and Asymmetric Reheating

TL;DR

This paper proposes a novel dark sector model using conformal field theories to explain asymmetric reheating and dark matter production, predicting a composite dark photon and unique freeze-in mechanisms.

Contribution

It introduces a conformal freeze-in scenario with a dark CFT coupled to the Standard Model, providing a dynamical explanation for asymmetric reheating and dark matter relic abundance.

Findings

Correct dark matter relic abundance achieved with parameter choices.

Dark photon emerges as a composite particle with small kinetic mixing.

COFI production differs significantly from traditional dark photon-mediated freeze-in.

Abstract

Large classes of dark sector models feature mass scales and couplings very different from the ones we observe in the Standard Model (SM). Moreover, in the freeze-in mechanism, often employed by the dark sector models, it is also required that the dark sector cannot be populated during the reheating process like the SM. This is the so called asymmetric reheating. Such disparities in sizes and scales often call for dynamical explanations. In this paper, we explore a scenario in which slow evolving conformal field theories (CFTs) offer such an explanation. Building on the recent work on conformal freeze-in (COFI), we focus on a coupling between the Standard Model Hypercharge gauge boson and an anti-symmetric tensor operator in the dark CFT. We present a scenario which dynamically realizes the asymmetric reheating and COFI production. With a detailed study of dark matter production, and taking into account limits on the dark matter (DM) self-interaction, warm DM bound, and constraints from the stellar evolution, we demonstrate that the correct relic abundance can be obtained with reasonable choices of parameters. The model predicts the existence of a dark photon as an emergent composite particle, with a small kinetic mixing also determined by the CFT dynamics, which correlates it with the generation of the mass scale of the dark sector. At the same time, COFI production of dark matter is very different from those freeze-in mediated by the dark photon. This is an example of the physics in which a realistic dark sector model can often be much richer and with unexpected features.