Dark Matter and Naturalness

TL;DR

This paper explores natural dark matter models based solely on fundamental principles like relativity and quantum mechanics, avoiding additional symmetries or fine-tuning, leading to stable baryon-like candidates with specific cosmological implications.

Contribution

It introduces a systematic framework for constructing natural dark matter models without extra global symmetries, focusing on chiral models with confining gauge groups and analyzing their cosmological viability.

Findings

Stable dark matter candidates analogous to baryons are possible with a confinement scale around 100 TeV.

The minimal model involves a dark sector with $SU(3)\times SU(2)$ gauge groups and one generation of chiral dark quarks and leptons.

Potential impact on BBN due to massless dark leptons and possible solutions include a lower reheat temperature or additional heavy degrees of freedom.

Abstract

The Standard Model of particle physics is governed by Poincar\'e symmetry, while all other symmetries, exact or approximate, are essentially dictated by theoretical consistency with the particle spectrum. On the other hand, many models of dark matter exist that rely upon the addition of new added global symmetries in order to stabilize the dark matter particle and/or achieve the correct abundance. In this work we begin a systematic exploration into truly natural models of dark matter, organized by only relativity and quantum mechanics, without the appeal to any additional global symmetries, no fine-tuning, and no small parameters. We begin by reviewing how singlet dark sectors based on spin 0 or spin ${1\over2}$ should readily decay, while pure strongly coupled spin 1 models have an overabundance problem. This inevitably leads us to construct chiral models with spin ${1\over2}$ particles charged under confining spin 1 particles. This leads to stable dark matter candidates that are analogs of baryons, with a confinement scale that can be naturally $\mathcal{O}(100)$TeV. This leads to the right freeze-out abundance by annihilating into massless unconfined dark fermions. The minimal model involves a dark copy of $SU(3)\times SU(2)$ with 1 generation of chiral dark quarks and leptons. The presence of massless dark leptons can potentially give rise to a somewhat large value of $\Delta N_{\text{eff}}$ during BBN. In order to not upset BBN one may either appeal to a large number of heavy degrees of freedom beyond the Standard Model, or to assume the dark sector has a lower reheat temperature than the visible sector, which is also natural in this framework. This reasoning provides a robust set of dark matter models that are entirely natural. Some are concrete realizations of the nightmare scenario in which dark matter may be very difficult to detect, which may impact future search techniques.