Neutron Portal and Dark Matter-Baryon Coincidence: from UV Completion to Phenomenology

arXiv:2604.21168 · hep-ph, astro-ph.CO, hep-ex · Submitted 2026-04-23 · Read on arXiv

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Introduction to the show: ident: Astrophysics Radio. Generated commentary on the latest astrophysics papers.

Vera: Next we'll be talking about the paper "Neutron Portal and Dark Matter-Baryon Coincidence: from UV Completion to Phenomenology".

Jocelyn: The paper was written by the authors from.

Vera: Stay tuned as we take you through the paper and discuss its implications.

Jocelyn: We also have Subrahmanyan with us today — guest researcher.

Vera: Alright, let's get started.

The Core Mechanism: Vera: Now, let's look at the core mechanism proposed in the abstract of "Neutron Portal and Dark Matter-Baryon Coincidence: from UV Completion to Phenomenology." They’re proposing that this framework explains why dark matter and baryons are so closely matched in density.

Jocelyn: The key idea is that this dark sector undergoes a strong, supercooled phase transition—a PT—which naturally generates an energy density at that roughly GeV level. This provides us with a concrete thermal marker for the time our dark matter was created and also accounts for the nano-Hz gravitational wave signal we've been monitoring.

Subrahmanyan: It’s really important to see how they connect this dynamic, GeV-scale dark QCD activity to the "neutron portal" by showing that after integrating out all heavy states required to complete the physics, the system naturally settles into this manageable low scale. It’s a self-consistent physical process that explains why things work so neatly within our models.

Vera: That concept of integrating out heavy particles to establish a lower scale is very intuitive for me; it feels like pruning the complexity down to find the essential physics that matters in our low-energy observations today, which helps us narrow our search parameters.

Jocelyn: And when this dynamic process is tied back to the ADM framework, it means we aren't just looking at a random dark particle; we are seeing its historical equivalent as a "dark baryon," which is much more structured than what we might have assumed previously.

Subrahmanyan: It’s truly important to see that this whole structure is designed to solve the coincidence problem by ensuring the resulting asymmetry shares the same origin as visible baryogenesis, making it a truly unified solution.

Improvements and Methodology: Vera: We've seen how they generate this core mechanism, so let’s talk about how they suggest testing this model in detail through experimentation. The authors are very practical in the paper, outlining specific ways to verify the theory using both collider data and cosmological constraints.

Jocelyn: The most direct avenue they suggest is using beam dump searches and colliders to probe that neutron portal operator, which gives experimentalists a clear target for us, essentially asking us to measure that n cut-off directly in the lab setting.

Subrahmanyan: Beyond just testing the model, they also discuss how this entire framework ties into established theoretical structures like supersymmetric extensions of the Standard Model. This gives the model a solid grounding in existing physics, allowing us to predict where certain particle masses should sit in relation to our observations.

Vera: That connection to established frameworks makes the particles feel less like abstract ghosts and more like components of a complex machine we can actually begin to map out. It gives us clearer tools for analysis when we eventually see them in our data sets.

Jocelyn: And their focus on constraints from BBN and the CMB is critical, acting as a necessary filter for us to ensure that any proposed model doesn't violate what we already know about the early universe.

Subrahmanyan: The refinement here is not just adding checks; it is showing how to calculate the effect of these new states in a way that achieves a targeted, predictable confinement scale. We are essentially engineering the particle content until we hit that precise piece of target physics needed to generate the GeV scale.

The Results and Constraints: Vera: We've looked at the mechanics and methodology, so let’s see what "Neutron Portal and Dark Matter-Baryon Coincidence: from UV Completion to Phenomenology" tells us about our view of dark matter. What is your take on how these findings constrain our interpretation?

Subrahmanyan: The most important finding is that we have a concrete mechanism for solving the DM-baryon coincidence puzzle by linking the dark sector's internal confinement scale—that GeV-scale event—to the multi-TeV neutron portal cut-off. We have a natural explanation for why both components of matter are so numerically matched.

Jocelyn: For us observing with PTA, it’s extremely encouraging to see that this model isn't just theoretical window dressing; it directly addresses the parameters we are seeing in our data, suggesting a shared history for every piece of cosmic matter we observe.

Vera: It feels like a major leap forward for us to have such a clear theoretical handle on the mechanisms behind those anomalies we see in our sky surveys; it gives us real context and direction.

Subrahmanyan: We must emphasize that this framework allows us to make very specific, narrow predictions, which will be essential as we gather more high-precision data from both our telescopes and the next generation of colliders. The results are quite sharp, which is excellent for verification.

Jocelyn: It’s also great that the paper shows how both short-lifetime and long-lived scenarios for the dark particle are constrained, giving us a much clearer picture of what we should be looking for in experimental searches; we're not left with just one possibility.

Conclusion: Vera: As we wrap up our discussion on "Neutron Portal and Dark Matter-Baryon Coincidence: from UV Completion to Phenomenology," it’s clear this paper offers an incredibly elegant way to tie together several seemingly separate cosmic puzzles into one unified narrative.

Jocelyn: And I find it immensely hopeful, Vera; the idea that our current observations of the nano-Hz gravitational wave signal might be directly linked to this dynamic process is truly exciting for anyone working with pulsar timing data. It’s a major validation of our methods.

Subrahmanyan: The theoretical impact, as far as I see it, is that we have provided a concrete physical mechanism—the neutron portal—that explains how the mass scale of dark matter can naturally emerge from the high-energy physics of a confining dark sector. It’s a powerful explanation for existence.

Vera: That natural emergence is what gives us confidence; it feels like the authors have moved beyond just searching for random particles to understanding how their fundamental properties shape our universe as a principle, not just an object.

Jocelyn: I think this gives us a very sharp focus for our future data analysis, as we now have specific decay channels and expected signatures to look for in our long-term sky surveys. The guidance is clear on where to look next.

Subrahmanyan: It’s essential to remember that the model requires testing against both tree-level and loop-level UV completions, making sure the conclusions aren't dependent on only one simple assumption about the particles involved; we need to verify all scenarios thoroughly.

Vera: That comprehensive approach is key; it shows a level of scientific rigor that makes this framework feel very robust and well thought out. It’s thorough science, and it feels like a genuine breakthrough in methodology for us.

Jocelyn: I agree, and seeing how these theoretical constraints map onto our observational data really solidifies the link between discovery and prediction. It's exciting to see if we can confirm the theory with our instruments in the coming years.

Subrahmanyan: It also provides a solid foundation for future work on how these concepts might fit into a grand unified structure, expanding this framework even further toward broader physical laws.

hep-ph, astro-ph.CO, hep-ex

Submitted: 2026-04-23

Updated: 2026-09-04

Comments: 49 pages, 6 figures, v2

Journal ref: JHEP 09 (2026) 035

DOI: 10.1007/JHEP09(2026)035

License: http://creativecommons.org/licenses/by/4.0/

Importance score: 84/100

The gist: The paper addresses the "dark matter-baryon coincidence puzzle"—the observation that dark matter (DM) and visible baryon abundances are not orders of magnitude apart—by proposing a dynamical

Key concepts

DM-Baryon Coincidence: This is a puzzle where the densities of dark matter and visible baryonic matter are closely matched. The paper provides a unified solution, suggesting that both components share the same origin in the universe's history.
Neutron Portal: This mechanism involves integrating out heavy states to establish a lower energy scale. It is a self-consistent physical process that connects dynamic dark QCD activity to observable low-energy physics.
Phase Transition (PT): The dark sector undergoes a strong, supercooled phase transition. This event generates an energy density at the GeV level, providing a concrete thermal marker for when the dark matter was created.

Terminology

Abundance — 21× in this paper · explained in 1 episode
Asymmetric Dark Matter — 9× in this paper
pulsar timing arrays — 6× in this paper · explained in 2 episodes
Core — 5× in this paper · explained in 2 episodes
Reheating — 3× in this paper · explained in 2 episodes
Big Bang Nucleosynthesis — 2× in this paper
D and Standard Model — 2× in this paper
Phase Transitions — 2× in this paper · explained in 2 episodes
BBN and Cosmic Microwave Background — 1× in this paper
Benchmark — 1× in this paper · explained in 2 episodes
Consistency — 1× in this paper · explained in 3 episodes
Grounding — 1× in this paper · explained in 3 episodes
Hubble Tension — 1× in this paper · explained in 9 episodes
Institute for Basic Science — 1× in this paper
Inverse Problem — 1× in this paper · explained in 2 episodes
Pruning — 1× in this paper · explained in 4 episodes
Symmetry Breaking — 1× in this paper · explained in 3 episodes
Verification — 1× in this paper · explained in 2 episodes

Summary

The paper addresses the dark matter-baryon coincidence puzzle—the observation that dark matter (DM) and visible baryon abundances are not orders of magnitude apart—by proposing a dynamical solution within the Asymmetric Dark Matter (ADM) framework. The authors suggest that a strongly supercooled dark confinement phase transition, which can account for the nano-Hz stochastic gravitational wave signal observed by pulsar timing arrays, simultaneously generates both the dark matter and baryon asymmetry in the Universe. This mechanism requires explaining how to naturally link a GeV-scale dark sector (DS) to a multi-TeV cut-off for the neutron portal operator.

The Coincidence Problem and Dark Sector Dynamics

The ADM paradigm is motivated by the fact that DM about 5.4 times b. To explain this, one must explain why the DM mass lies in the GeV range. A hint for a GeV-scale DS emerges from the possibility of explaining the nano-Hz stochastic gravitational waves (GWs) observed by pulsar timing arrays (PTA). The authors consider a DS governed by a nearly conformal dynamics in the ultraviolet (UV) that undergoes a first-order confinement–deconfinement PT. This scenario is appealing because, as it generates both the baryon asymmetry and DM utilizing the PT itself, it naturally links to the ADM framework. The goal is to explain how this emergence of the GeV scale in the DS can be achieved by addressing the coincidence puzzle.

The Neutron Portal Mechanism

A necessary ingredient for ADM is a portal that shares asymmetry between dark and visible sectors. This dark asymmetry, created in the DS (e)g., through an anomaly or heavy particle decay), is reprocessed into the visible sector via a portal operator that violates both U(1) D and Standard Model (SM) baryon number B. Since the relevant temperature of this process is below 130 GeV, effective electroweak sphalerons are frozen out. The lowest dimensional effective operator used for this purpose is the neutron portal operator:

O about (chi c d R)(u R d R)
The asymmetry in chi is communicated to the DM and visible baryons.

UV Completion and Scale Correlation

To understand how the GeV scale originates, the authors present two cases for reproducing the neutron portal operator: tree-level and loop-level processes. The core of their argument is a dynamic correlation between these scales:

The dark sector is governed by an approximate infrared (IR) fixed point.
When new states required to UV complete the portal are integrated out, the dark QCD flows away from the fixed point and confines at dQCD about O(GeV).
This confinement scale is then directly linked to the effective neutron portal scale n.

Phenomenological Constraints and Experimental Probes

The paper analyzes several constraints on this model:

BBN Constraint: The BBN analysis excludes a broad region of parameter space, specifically for A 0 1, restricting the viable lifetime to tau chi 0.1 s.
CMB Constraint: This further constrains long lifetimes, requiring tau chi 10 25 s for a dark-matter-like component.
Beam Dump Searches: The reach of current and future proton beam-dump experiments can probe the neutron portal operator, with the SHiP experiment projected to probe n 5 TeV.
Collider Searches: The model also provides predictions for jet plus missing energy signatures in colliders, providing a lower bound on n 1.5 TeV from ATLAS analysis.

Improvements for AI systems

This analysis assumes the role of a highly specialized AI Research Lead tasked with integrating complex theoretical physics data into a powerful computational model.

The scientific paper provides an extremely rich dataset—a framework connecting high-energy UV completions to low-energy cosmological observables. To improve AI systems, we must move beyond simple data ingestion and create functional modules that automate the complex calculations and correlations presented in the paper.

Here are the specific improvements to the AI system, categorized by function:

This module automates the exploration of parameter space, replacing manual plotting and calculation with a high-dimensional search algorithm.

Improvement: Implement a Constraint Satisfaction Solver (CSS) that maps the entire (n, m chi) parameter space against all derived constraints simultaneously.

What it can do:

Identify Viable Regions: Automatically calculate and delineate the viable regions defined by delta Y p 0.01 (BBN constraint), eta 0.039 (BBN-CMB mismatch constraint), and tau chi 10 25 s (CMB dark matter limit).
Predictive Mapping: For any input set of parameters, the AI can instantly output the predicted decay width chi t and calculate whether the resulting n NF/A 0 ratio falls within the allowed range for t NF.

This module digitizes and operationalizes the complex mathematical derivations from Sections 2.2, Appendix A, and Section 4.1.

This module connects the theoretical predictions to real-world experimental limits.

This module is designed for advanced research synthesis, connecting the disparate concepts of the paper into a unified knowledge structure.

Abstract

We present a dynamical solution to the dark matter-baryon coincidence problem based on the neutron portal operator connecting the visible and dark sector asymmetries. This framework is motivated by the possibility that a strongly supercooled dark confinement phase transition accounts for the nano-Hz stochastic gravitational wave signal observed by pulsar timing arrays, while also generating the dark matter and baryon asymmetry in the Universe. We show that the GeV-scale mass of asymmetric dark matter can be naturally correlated with the (multi-)TeV scale cut-off for the neutron portal through its ultraviolet completion. The dark sector is governed by an approximate fixed point and confines once the heavy portal states are integrated out, dynamically generating a scale of O (GeV). We analyze both tree and loop-level ultraviolet completions and demonstrate how the resulting confinement scale is linked to the effective neutron portal scale. We also discuss cosmological constraints and experimental prospects in beam dump searches and colliders for probing the neutron portal.