A Closer Look in the Mirror: Reflections on the Matter/Dark Matter Coincidence (2401.12286v2)

Abstract: We argue that the striking similarity between the cosmic abundances of baryons and dark matter, despite their very different astrophysical behavior, strongly motivates the scenario in which dark matter resides within a rich dark sector parallel in structure to that of the standard model. The near cosmic coincidence is then explained by an approximate $\mathbb{Z}_2$ exchange symmetry between the two sectors, where dark matter consists of stable dark neutrons, with matter and dark matter asymmetries arising via parallel WIMP baryogenesis mechanisms. Taking a top-down perspective, we point out that an adequate $\mathbb{Z}_2$ symmetry necessitates solving the electroweak hierarchy problem in each sector, without our committing to a specific implementation. A higher-dimensional realization in the far UV is presented, in which the hierarchical couplings of the two sectors and the requisite $\mathbb{Z}_2$-breaking structure arise naturally from extra-dimensional localization and gauge symmetries. We trace the cosmic history, paying attention to potential pitfalls not fully considered in previous literature. Residual $\mathbb{Z}_2$-breaking can very plausibly give rise to the asymmetric reheating of the two sectors, needed to keep the cosmological abundance of relativistic dark particles below tight bounds. We show that, despite the need to keep inter-sector couplings highly suppressed after asymmetric reheating, there can naturally be order-one couplings mediated by TeV scale particles which can allow experimental probes of the dark sector at high energy colliders. Massive mediators can also induce dark matter direct detection signals, but likely at or below the neutrino floor.

• The paper presents a novel framework linking baryonic matter with a mirror dark sector through a Z symmetry that stabilizes dark neutrons to account for the fivefold matter abundance.
• The analysis employs higher-dimensional theories and asymmetric reheating models to reconcile cosmic observations with precision limits on dark radiation.
• The study proposes experimental tests via high-energy colliders and dark matter detectors to validate the predicted parity between visible and dark sectors.

Reflections on the Matter/Dark Matter Coincidence

The paper, "A Closer Look in the Mirror: Reflections on the Matter/Dark Matter Coincidence," explores the parity between the cosmic abundances of baryonic matter and dark matter, exploring whether a hidden connection exists between these two entities. The authors propose a framework in which dark matter sections align structurally with the Standard Model but operate under distinct but related dynamics, potentially stabilized as dark neutrons within a mirror-like dark sector.

The principal argument centralizes the cosmic coincidence problem—why the density of dark matter ($\rho_{DM}$) is approximately five times that of baryonic matter ($\rho_{b}$). This paper posits that this near equality is not accidental but perhaps indicative of a deeper symmetry. In this model, a hypothetical $Z$ symmetry exchange mechanizes the energy scales between two sectors, with the dark neutrons posited as the stable constituents of dark matter. The symmetry stands as parachronous, where its deconstruction could hint at an alternate path for solving the electroweak hierarchy dilemma in each of the sectors.

To substantiate these claims, the authors introduce a higher-dimensional theory embodying additional symmetries that naturally reconcile late-time behaviors, particularly considering asymmetric reheating scenarios. Their framework allows for substantial initial equilibrium between the visible and dark particles, tempered by modest $Z$-breaking which aids in establishing traceable imbalances observable today. Specifically, the inclusion of a reheaton—a hypothesized scalar that couples weakly across the sectors—redistributes thermal energy preferentially, harmonizing with precision measurement limits on dark radiation.

The paper integrates topological complexities around confinement scale disparities (e.g., differing QCD phenomena in the dark sector) and the freeze-out process of weakly interacting massive particle (WIMP) analogues; these factors help conceive theoretical replicates of Standard Model processes within the dark sector. Importantly, this symmetry bordering facilitates inventive solutions like threshold suppression in recrudescent nucleosynthesis, addressing the non-formation of dark atoms—a significant concern given observational limits on self-interacting dark matter.

Boldly, the researchers speculate on future implications and opportunities for empirical verification. They identify possible detection routes via high-energy colliders that could reveal the heavy mediators bridging the sectors or encounter displaced decay signatures exclusive to baryogenesis models like twin baryogenesis. Additionally, they broach the pursuance of massive $B-L$ gauge bosons that articulate cross-sector dynamics without violating thermal constraints, suggesting that direct dark matter detection may also become feasible, albeit beneath the neutrino floor.

In conclusion, this work extends the discourse by marrying cosmological observations with theoretical physics, leveraging the mathematical symmetries of higher dimensions. The $Z$-related formulation posited could offer an elegant unification, with profound implications on our understanding of cosmic matter's balance, potentially facilitating groundbreaking advancements in dark matter characterizations and prospects for solving enduring theoretical dichotomies within particle physics. This landscape affords fertile ground for experimental validation, promising an exciting future for subatomic and cosmological exploration.