Dark Matter’s Hidden Link to Baryon Asymmetry

Author: Denis Avetisyan

A new model proposes a connection between dark matter, the imbalance between matter and antimatter, and a fundamental ‘neutron portal’ interaction.

The search for weakly interacting massive particles-specifically, those decaying via a hidden sector with a scale <span class="katex-eq" data-katex-display="false">\Lambda_{n}</span>-reveals a parameter space tightly constrained by both cosmological observations and collider experiments; Big Bang nucleosynthesis and cosmic microwave background data exclude regions where particle decay disrupts primordial element abundances, while searches at CHARM, NA62, and SHiP progressively narrow the viable range, permitting only short-lived (<span class="katex-eq" data-katex-display="false">\tau_{\chi} \lesssim 0.1\,{\rm s}</span>) or exceedingly long-lived (<span class="katex-eq" data-katex-display="false">\tau_{\chi} \gtrsim 10^{25}\,{\rm s}</span>) particles, or those with a hidden sector scale between 2 and 100 TeV if decaying before the era of nucleosynthesis. — The search for weakly interacting massive particles-specifically, those decaying via a hidden sector with a scale $\Lambda_{n}$ -reveals a parameter space tightly constrained by both cosmological observations and collider experiments; Big Bang nucleosynthesis and cosmic microwave background data exclude regions where particle decay disrupts primordial element abundances, while searches at CHARM, NA62, and SHiP progressively narrow the viable range, permitting only short-lived ( $\tau_{\chi} \lesssim 0.1\,{\rm s}$ ) or exceedingly long-lived ( $\tau_{\chi} \gtrsim 10^{25}\,{\rm s}$ ) particles, or those with a hidden sector scale between 2 and 100 TeV if decaying before the era of nucleosynthesis.

This review explores a UV completion linking a neutron portal operator to the observed baryon asymmetry and GeV-scale dark matter, potentially resolving the dark matter-baryon coincidence.

The persistent coincidence between the observed baryon and dark matter densities presents a fundamental puzzle in cosmology. This paper, ‘Neutron Portal and Dark Matter-Baryon Coincidence: from UV Completion to Phenomenology’, proposes a dynamical solution rooted in a ‘neutron portal’ operator connecting visible and dark sector asymmetries. We demonstrate that a GeV-scale asymmetric dark matter mass can be naturally correlated with a multi-TeV ultraviolet completion of this portal, dynamically generating a confinement scale within the dark sector. Could this framework not only resolve the coincidence problem but also provide a compelling link to the nano-Hz gravitational wave signals detected by pulsar timing arrays?

The Universe’s Hidden Symmetry: A Coincidence Demanding Explanation

Cosmological observations consistently demonstrate a remarkable parity between the estimated abundance of dark matter and that of baryonic matter – the ‘normal’ matter composing stars, planets, and life. This isn’t a trivial alignment; calculations suggest that, in the early universe, the conditions required for these two forms of matter to arise in roughly equal proportions were exceedingly specific. The prevalence of both is sensitive to particle interactions and decay rates during the universe’s first moments, and the observed near-equality strongly implies a shared genesis. Researchers theorize that dark matter and baryonic matter may have been intrinsically linked in the primordial universe, perhaps originating from a common source or being governed by interconnected physical processes. This ‘Dark Matter Baryon Coincidence’ isn’t merely a numerical quirk; it’s a crucial clue pointing toward a more fundamental understanding of the universe’s composition and evolution, prompting exploration of models beyond the standard cosmological paradigm.

The unexpectedly similar abundance of dark matter and ordinary matter – a phenomenon dubbed the ‘Dark Matter Baryon Coincidence’ – has spurred investigation into asymmetric dark matter. These models posit that, much like the universe exhibits more matter than antimatter, an initial asymmetry existed in the dark matter sector itself, favoring dark matter particles over their antiparticles. This imbalance, crucially, wouldn’t require dark matter to be perfectly stable; annihilation would still occur, but a residual population would survive, accounting for the observed dark matter density. Furthermore, explaining this asymmetry necessitates a connection between the dark sector and the baryon asymmetry – the surplus of matter over antimatter in the visible universe – potentially through interactions in the early universe that converted an initial dark matter asymmetry into the observed baryon asymmetry. This framework offers a compelling avenue for understanding both the nature of dark matter and the origin of matter itself, suggesting a deep interconnectedness between the visible and dark components of the cosmos.

The persistent mystery of dark matter’s abundance is deeply intertwined with the origin of matter itself, specifically the Baryon Asymmetry – the observed imbalance between matter and antimatter in the universe. Current theories suggest that resolving the dark matter puzzle necessitates a connection between the dark sector – encompassing dark matter particles – and the mechanisms responsible for generating this asymmetry. This implies that the initial conditions or interactions within the dark sector somehow influenced, or were influenced by, the processes that favored matter over antimatter in the early universe. Researchers are actively exploring models where dark matter particles decay or interact in ways that directly contribute to the Baryon Asymmetry, or conversely, are produced through the same physical principles. Establishing this link isn’t merely about quantifying dark matter; it’s about uncovering a fundamental principle governing the very existence of everything we observe.

The Neutron Portal: A Bridge Between Worlds

The Neutron Portal Operator posits a mechanism for transferring asymmetry between the dark and visible sectors via interactions involving Standard Model quarks and dark sector particles. Specifically, this operator facilitates interactions involving a bilinear combination of left-handed up and down quarks coupled to a dark sector scalar or fermion. This coupling allows for the creation of a dark sector asymmetry through the mixing of quark flavor states, potentially resolving the observed baryon asymmetry of the universe. The effectiveness of this asymmetry transfer hinges on the specific coupling strength and mass scales associated with the involved particles, making it a testable hypothesis through direct and indirect detection experiments focused on dark matter interactions.

The Neutron Portal Operator, while successfully mediating interactions between dark and visible matter, introduces ultraviolet (UV) divergences at high energies. These divergences necessitate a ‘UV Completion’ – a more fundamental theory that describes physics at energies beyond the current model’s validity – to provide a mathematically consistent and physically realistic description. Without such a completion, calculations involving the operator become unreliable at sufficiently high momentum transfers. The UV completion effectively ‘sums up’ contributions from higher-energy physics, regularizing the divergent integrals and providing a finite, predictive framework. Two prominent approaches to achieve this UV completion involve either introducing new colored scalar particles at the tree level or utilizing loop-level corrections from particles existing in quantum loops.

Current theoretical models attempting to resolve high-energy divergences within the Neutron Portal Operator framework propose two primary Ultraviolet (UV) completions. The first, a Tree-Level UV Completion, introduces new colored scalar particles to the model. Alternatively, a Loop-Level UV Completion utilizes particles existing in quantum loops to achieve the same result. Both completion pathways converge on a predicted dark Quantum Chromodynamics (QCD) confinement scale of approximately 1.1 GeV, suggesting a characteristic energy at which dark sector particles would bind together, forming composite states analogous to hadrons in the Standard Model.

With parameters <span class="katex-eq" data-katex-display="false"> (nΨ,nψ,nσ,nφ)=(2,5,1,14) </span> and <span class="katex-eq" data-katex-display="false"> (n̄ψ,n̄φ)=(4,4) </span>, the running couplings <span class="katex-eq" data-katex-display="false"> g_{s} </span> (orange) and <span class="katex-eq" data-katex-display="false"> g_{d} </span> (red) demonstrate that the dark QCD confinement scale is approximately 1.1 GeV, and <span class="katex-eq" data-katex-display="false"> g_{d}^{*} </span> remains stable above 3 TeV while <span class="katex-eq" data-katex-display="false"> g_{s} </span> decreases. — With parameters $(nΨ,nψ,nσ,nφ)=(2,5,1,14)$ and $(n̄ψ,n̄φ)=(4,4)$ , the running couplings $g_{s}$ (orange) and $g_{d}$ (red) demonstrate that the dark QCD confinement scale is approximately 1.1 GeV, and $g_{d}^{*}$ remains stable above 3 TeV while $g_{s}$ decreases.

A Hidden Mirror: The Architecture of Dark QCD

Dark QCD proposes a parallel strong interaction sector, distinct from the Standard Model’s quantum chromodynamics (QCD), but sharing its fundamental characteristic of being a non-abelian gauge theory. This means the force carriers within the dark sector, analogous to gluons in QCD, themselves carry the force charge and thus interact with each other. Specifically, the theory utilizes an $SU(N_c)$ gauge group, where $N_c$ represents the number of dark color charges, potentially differing from the three colors in our universe. This self-interacting nature of the dark force leads to phenomena such as asymptotic freedom and confinement, mirroring the behavior observed in QCD, but operating on a set of ‘dark quarks’ and ‘dark gluons’ that do not directly interact with Standard Model particles except possibly through very weak interactions, thereby explaining the sector’s ‘darkness’.

Dark baryons are composite particles predicted by the Dark QCD framework, formed through the strong interaction of hypothetical dark quarks. Analogous to the formation of protons and neutrons from up and down quarks in the Standard Model, these dark baryons represent potential dark matter candidates due to their predicted stability and neutral charge. The binding energy resulting from the dark strong force confines these quarks, creating massive, collisionless particles that do not interact with electromagnetic radiation, aligning with observed dark matter characteristics. Their mass is primarily determined by the confinement scale of Dark QCD, and their abundance is dependent on the dynamics of the early universe and the production mechanisms of dark quarks.

The confinement scale in Dark Quantum Chromodynamics (QCD), currently estimated to be approximately 1.1 GeV, is a crucial parameter governing the properties of dark baryons. This scale represents the energy at which the dark strong force becomes strong enough to bind dark quarks into composite particles. Consequently, the masses of the resulting dark baryons are directly proportional to the confinement scale; a higher confinement scale implies heavier dark baryons. Furthermore, the strength of interactions between these dark baryons and Standard Model particles is also influenced by the confinement scale, primarily through potential kinetic or mass mixing effects at the boundary between the dark and visible sectors. Precise determination of this scale is therefore essential for modeling dark matter phenomenology and predicting detectable signals.

Echoes from the Darkness: Signatures and Constraints

Theoretical frameworks extending the Standard Model, specifically those positing a “dark sector” governed by “Dark Quantum Chromodynamics” (Dark QCD), predict the existence of bound states analogous to the pions found in ordinary matter. Spontaneous chiral symmetry breaking within Dark QCD results in the formation of “Dark Pions”-pseudoscalar mesons which, unlike their Standard Model counterparts, interact primarily with dark matter particles. Crucially, these Dark Pions aren’t entirely isolated; they can couple to the familiar world of quarks and leptons through the “Higgs Portal”-a hypothetical interaction that allows them to mix with the Higgs boson. This mixing implies that Dark Pions could, in principle, be produced in Higgs boson decays or contribute to precision measurements of Higgs couplings, offering a tantalizing avenue for indirect detection and providing a bridge between the visible and dark universes. The strength of this interaction, and therefore the observability of Dark Pions, is directly tied to the underlying parameters governing the dynamics of Dark QCD.

The theoretical framework predicting ‘Dark Pions’ – particles arising from a hidden dark sector – faces stringent tests from observations of the early universe. Specifically, the abundance of light elements formed during Big Bang Nucleosynthesis places firm limits on the strength of interactions between dark matter and ordinary matter. Any significant coupling, mediated by a ‘Neutron Portal’ allowing dark pions to influence neutron-proton ratios, would drastically alter the predicted primordial abundances of helium and deuterium. Consequently, detailed calculations reveal that the energy scale at which this neutron portal interaction effectively shuts off – its cut-off scale – must be less than approximately 100 TeV. This constraint significantly narrows the parameter space for models involving dark pions and highlights the power of cosmological observations in probing the fundamental properties of dark matter.

The dynamics of Dark Quantum Chromodynamics (Dark QCD) suggest that phase transitions within the dark sector could have generated a stochastic background of low-frequency gravitational waves. These waves, a relic of the early universe, are now within the sensitivity range of Pulsar Timing Array (PTA) experiments, which monitor subtle variations in the arrival times of radio pulses from millisecond pulsars. The specific characteristics of these gravitational waves-their amplitude and frequency spectrum-are intimately linked to the parameters of the dark sector, notably the mass of dark matter particles. Current theoretical modeling indicates that detection of such waves would strongly suggest a dark matter mass around 5 GeV, providing a crucial link between cosmological observations and the fundamental properties of dark matter, and offering a novel pathway to probe the hidden sector beyond the Standard Model.

The pursuit of a UV completion, as detailed in the paper, isn’t merely a mathematical exercise; it’s a translation of deeply-held assumptions about order into the language of particles and forces. This mirrors a fundamental truth about modeling: the structure isn’t discovered, it’s imposed. As Immanuel Kant observed, “We do not see things as they are; we see them as we are.” The paper’s exploration of a neutron portal, and its attempt to reconcile the dark matter-baryon coincidence, isn’t about finding an objective reality, but about building a coherent narrative – a story that makes sense given the biases and expectations inherent in the theoretical framework. The dark confinement scale, therefore, isn’t a property of the universe itself, but a consequence of the story being told.

Where Do We Go From Here?

This work, linking a neutron portal to the persistent riddle of the baryon asymmetry and the GeV-scale mass of dark matter, is less a solution than a carefully constructed provocation. It highlights, yet again, the human tendency to seek elegance-a ‘coincidence’ resolved-even when the universe rarely operates with such neatness. The proposed UV completion, while internally consistent, relies on the existence of new particles at a confinement scale that, while motivated, remains firmly in the realm of hopeful speculation. Rationality is a rare burst of clarity in an ocean of bias, and the persistence of these theoretical constructs suggests a deeper desire for order than empirical evidence currently supports.

Future explorations must confront the limitations inherent in assuming a simple connection between visible and dark sector physics. The model’s sensitivity to fine-tuning-a common ailment-demands rigorous investigation. Perhaps a more fruitful avenue lies in abandoning the quest for a singular ‘portal’ and instead accepting a multitude of weak interactions, a messy, probabilistic dance between the known and the unknown. The market is just a barometer of collective mood, and the theoretical landscape is no different.

Ultimately, the true test will not be the mathematical beauty of the model, but its ability to withstand the cold scrutiny of experiment. Detection of the predicted particles, or even indirect evidence of their influence, will be required to elevate this intriguing possibility from an elegant conjecture to a compelling piece of the cosmic puzzle.

Original article: https://arxiv.org/pdf/2604.21168.pdf

Contact the author: https://www.linkedin.com/in/avetisyan/

The Universe’s Hidden Symmetry: A Coincidence Demanding Explanation

The Neutron Portal: A Bridge Between Worlds

A Hidden Mirror: The Architecture of Dark QCD

Echoes from the Darkness: Signatures and Constraints

Where Do We Go From Here?

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