Genesis of baryon and dark matter asymmetries through ultraviolet scattering freeze-in

This paper proposes a new mechanism where ultraviolet-dominated freeze-in scatterings involving heavy Majorana neutrinos simultaneously generate baryon and dark matter asymmetries, potentially explaining observed cosmic abundances through processes like dark wash-in and symmetric population depletion.

The Gist

Here is an explanation of the paper "Genesis of Baryon and Dark Matter Asymmetries through Ultraviolet Scattering Freeze-in," translated into everyday language with creative analogies.

The Big Mystery: Why Are We Here?

Imagine the universe as a giant party that started with the Big Bang. At the very beginning, there should have been equal amounts of "matter" (the stuff we are made of) and "antimatter" (its evil twin that destroys matter on contact). If they were perfectly equal, they would have annihilated each other instantly, leaving behind a universe filled only with light and no one to enjoy it.

But here we are. We exist. This means something tipped the scales. There was a tiny bit more matter than antimatter. Scientists call this the Baryon Asymmetry.

At the same time, we know there is a mysterious substance called Dark Matter holding galaxies together. It's invisible, but it's there. The weird coincidence is that the amount of Dark Matter in the universe is about five times the amount of normal matter. It's like finding a stack of five identical boxes of cereal for every single box of cereal you own. Why are they so closely matched?

This paper proposes a new story to explain how both the "extra" matter and the "extra" Dark Matter were created at the same time, using a mechanism called UV Freeze-in.

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The Cast of Characters

To understand the story, let's meet the players:

1. The Visible Sector (The "Main Stage"): This is our familiar universe—protons, electrons, photons, and the Higgs boson.
2. The Dark Sector (The "Backstage"): A hidden world we can't see, containing new particles like a heavy fermion ($\chi$, the Dark Matter candidate) and a light fermion ($\eta$, the "Sink").
3. The Heavy Neutrinos (The "Messengers"): These are super-heavy, invisible particles that act as a bridge between the Main Stage and Backstage. They are too heavy to be created directly after the Big Bang in this model, but they mediate interactions.
4. The "Sink" ($\eta$): A massless particle in the Dark Sector that acts like a vacuum cleaner.

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The Story: How the Party Got Out of Hand

1. The Setup: A Cold Start

Usually, scientists think the universe was hot and everything was mixed together. But in this story, the universe reheated (got hot again after inflation) to a temperature that was too cold to create the Heavy Neutrinos directly.

Think of it like a kitchen where the oven is set to 300°F. You want to bake a cake that requires 500°F. You can't just throw the ingredients in and wait; they won't cook.

2. The Mechanism: "UV Freeze-in" (The High-Energy Spark)

Since the oven isn't hot enough to make the Heavy Neutrinos, how do we get the Dark Matter?
The authors propose that the "Main Stage" particles (like electrons and Higgs bosons) occasionally smash into each other with enough energy to briefly create a virtual Heavy Neutrino. This neutrino instantly decays into Dark Sector particles.

• The Analogy: Imagine two people on the Main Stage throwing a very heavy ball (the Heavy Neutrino) at each other. They don't have enough strength to hold the ball, but if they throw it hard enough (high energy), it bounces off and lands on the Backstage.
• "Freeze-in": Because the interaction is so weak, the Dark Sector never gets hot enough to be in equilibrium with us. It just slowly "freezes in" a small amount of particles over time, like frost forming on a window on a cold night.

3. The Magic Trick: Creating the Imbalance (Asymmetry)

The universe needs more matter than antimatter. How do we get that?
The Heavy Neutrinos have a special property: they violate CP symmetry. In plain English, they treat "matter" and "antimatter" differently. When the collisions happen, they are slightly more likely to produce Dark Matter particles than Dark Antimatter particles.

• The Analogy: Imagine a biased coin flip. Every time the Main Stage particles collide and send something to the Backstage, the coin is slightly weighted. It lands on "Dark Matter" 51% of the time and "Dark Antimatter" 49% of the time. Over billions of collisions, this tiny bias adds up to a huge pile of Dark Matter and a tiny pile of Dark Antimatter.

4. The "Dark Wash-in" (The Secret Transfer)

Here is the clever part. The paper suggests that the imbalance didn't just happen in the Dark Sector; it happened in both sectors simultaneously.
Sometimes, the "wash-out" processes (which usually try to erase the imbalance) actually work in reverse. They take the imbalance created in the Dark Sector and transfer it to our Visible Sector.

• The Analogy: Imagine two buckets connected by a leaky pipe. One bucket (Dark) gets a little more water than the other. The pipe is leaky, but because of the pressure difference, some of that extra water from the Dark bucket actually flows back into the Visible bucket, boosting the water level there too. This is called "Dark Wash-in." It explains why the amount of Dark Matter and Normal Matter are so similar—they are siblings born from the same biased coin flip.

5. The Cleanup Crew: The "Dark Sink"

There's a problem. Even with the bias, the collisions also create a lot of symmetric Dark Matter (equal amounts of matter and antimatter). If this symmetric stuff stayed, it would be too heavy and crush the universe (overclosure).

The solution? The Dark Sink ($\eta$).
The Dark Sector has a massless particle that acts like a vacuum cleaner. The symmetric Dark Matter particles ($\chi$ and $\bar{\chi}$) find each other, annihilate, and turn into these massless sink particles.

• The Analogy: Imagine you have a pile of sand (symmetric Dark Matter) and a pile of gold dust (asymmetric Dark Matter). The "Sink" is a magical vacuum that only sucks up the sand. The gold dust is too heavy to be sucked up, so it stays behind.
• The Result: The symmetric stuff disappears, leaving only the "gold dust" (the asymmetric Dark Matter). This explains why we don't see too much Dark Matter today.

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Why This Matters

This paper is exciting for a few reasons:

1. It Solves Two Puzzles at Once: It explains why we have matter (Baryogenesis) and why we have Dark Matter (Dark Matter Genesis) in a single, connected story.
2. It Breaks the Rules: Previous theories said that if you create Dark Matter this way, you can't create enough asymmetry to match the visible universe. This model breaks that rule by using the "Dark Wash-in" trick and the "Sink" to clean up the mess.
3. It Fits the Data: The math shows that this scenario works with Dark Matter masses ranging from very light (like an electron) to quite heavy (like a proton), covering a huge range of possibilities.

The Bottom Line

The universe is like a complex machine where two different gears (Normal Matter and Dark Matter) are turning together. This paper suggests that a "biased coin flip" happening in the high-energy collisions of the early universe, mediated by heavy messengers, spun both gears at the same time. Then, a "vacuum cleaner" in the dark sector swept away the excess junk, leaving us with the perfect, slightly unbalanced universe we live in today.

It's a story of biased collisions, hidden messengers, and cosmic cleanup crews working together to create the ingredients for our existence.

Technical Summary

Here is a detailed technical summary of the paper "Genesis of Baryon and Dark Matter Asymmetries through Ultraviolet Scattering Freeze-in."

1. Problem Statement

The paper addresses two fundamental puzzles in cosmology and particle physics:

1. The Origin of Dark Matter (DM): The nature of DM and its production mechanism remain unknown.
2. The Baryon Asymmetry: The observed dominance of matter over antimatter in the universe.
3. The Coincidence Problem: The energy densities of baryons ($\rho_B$) and dark matter ($\rho_{DM}$) are remarkably similar ($\rho_{DM} \approx 5\rho_B$), suggesting a common origin (cogenesis).

Existing models often rely on thermal freeze-out (WIMPs) or leptogenesis via heavy neutrino decays. However, connecting these to DM often requires specific mass ranges or fine-tuning. Furthermore, recent theoretical constraints (based on CPT and unitarity) suggest that generating asymmetries via freeze-in scattering between two sectors is highly suppressed if a global charge is conserved, typically leading to an overabundance of symmetric DM that violates observational bounds.

2. Methodology and Model Setup

The authors propose a new mechanism based on Ultraviolet (UV) Freeze-in scattering, mediated by heavy Majorana neutrinos acting as a "neutrino portal."

The Model

• Visible Sector: Standard Model (SM) particles.
• Dark Sector: Contains a Dirac fermion $\chi$ (DM candidate), a complex scalar $\phi$, and a massless Weyl fermion $\eta$ (the "dark sink").
• Portal: Heavy Majorana neutrinos ($N_a$) with masses $M_a$ that couple to both sectors via Yukawa interactions:
  • $y_{ia} H \cdot \ell_i N_a$ (SM coupling)
  • $\lambda_a \phi \chi N_a$ (Dark sector coupling)
• Key Assumptions:
  • The universe reheats to a temperature $T_{RH}$ significantly below the heavy neutrino masses ($T_{RH} \ll M_a$).
  • Heavy neutrinos are never thermally populated; they act only as virtual mediators.
  • The dark sector is initially empty and populated solely via $2 \to 2$ scattering processes from the visible sector (UV freeze-in).

Mechanism of Asymmetry Generation

1. UV Freeze-in: Immediately after reheating, $2 \to 2$scattering processes (e.g.,$\ell H \to \chi \phi$) populate the dark sector. Because$T_{RH} \ll M_a$, these are effective field theory interactions suppressed by$1/M_a$.
2. CP Violation: Asymmetries arise from the interference between tree-level and one-loop diagrams involving the heavy neutrinos. This requires complex phases in the Yukawa couplings ($y$ and $\lambda$).
3. Breaking of Extended Lepton Number: The model explicitly breaks the combined global symmetry $U(1)_{L+L_x}$ (where $L_x$ is a dark lepton number) via the Majorana masses of $N_a$. This explicit breaking is crucial to evade the CPT/unitarity constraints that would otherwise suppress charge transfer between sectors.
4. Dark Wash-In: A novel feature where the "wash-out" processes (which usually deplete asymmetries) in the dark sector act as a source for the visible sector. Asymmetries generated in the dark sector are transferred to the visible sector, reprocessing the primordial dark charge into a SM $B-L$ asymmetry.
5. Dark Sink: The massless fermion $\eta$ allows the DM candidate $\chi$ to annihilate ($\chi \bar{\chi} \to \eta \bar{\eta}$). This efficiently depletes the symmetric component of the DM population while preserving the asymmetric component, leaving a relic density dominated by the asymmetry.

3. Key Contributions

• UV Freeze-in Cogenesis: The paper presents the first viable model where both baryon and dark matter asymmetries are generated simultaneously via UV-dominated $2 \to 2$ scattering, without relying on the decay of long-lived heavy neutrinos.
• Evasion of Unitarity Constraints: By explicitly breaking the shared global charge ($L+L_x$) via Majorana masses, the authors demonstrate that charge transfer between sectors can occur at leading order in the portal coupling, overcoming the suppression predicted by previous literature (e.g., Ref. [69]).
• Dark Wash-In Mechanism: The authors identify a regime where the dominant source of the SM baryon asymmetry is the transfer of asymmetry from the dark sector via wash-out terms, a process they term "dark wash-in."
• Dark Sink Phenomenology: The inclusion of a massless dark fermion allows for the efficient removal of the symmetric DM component, enabling the model to accommodate a wide range of DM masses (from keV to TeV) without overclosing the universe.

4. Results

The authors solve the Boltzmann equations for the evolution of charge densities ($B-L$ and $L_x$) and particle yields.

• Parameter Space:
  • Heavy Neutrino Masses: $M_N \gtrsim 10^{10}$ GeV.
  • Reheating Temperature: $T_{RH} \gtrsim 4 \times 10^8$ GeV (slightly lower than the standard Davidson-Ibarra bound for thermal leptogenesis due to the wash-in effect).
  • Dark Matter Mass:
    • Asymmetric DM: $0.1 \text{ GeV} \lesssim m_\chi \lesssim 10^3 \text{ GeV}$.
    • Symmetric DM: Can be viable down to $\sim \text{keV}$ if the symmetric component is not fully depleted.
• Asymmetry Yields:
  • The model successfully reproduces the observed baryon asymmetry ($Y_B \approx 8.7 \times 10^{-11}$) and the DM-to-baryon density ratio.
  • In the "strong dark wash-out" regime, the dark sector asymmetry is significantly depleted and transferred to the visible sector, creating a hierarchy where the visible asymmetry is much larger than the dark asymmetry.
• Cosmological Constraints:
  • $\Delta N_{eff}$: The temperature ratio between the dark and visible sectors ($\xi = T_x/T$) is calculated to be $\lesssim 0.5$, satisfying current and future constraints on the effective number of neutrino species from the CMB.
  • Direct Detection: The model is essentially invisible to current direct detection experiments due to the heavy mediator mass and the nature of the interactions.

5. Significance

• Theoretical Robustness: This work provides a concrete, UV-complete framework for asymmetric dark matter that does not rely on the thermal equilibrium of the dark sector or the decay of heavy particles, addressing a gap in the literature regarding freeze-in cogenesis.
• New Phenomenology: The concept of "Dark Wash-In" offers a new paradigm for baryogenesis where the visible sector asymmetry is a byproduct of dark sector dynamics, challenging the traditional view that the visible sector must be the primary source of asymmetry.
• Mass Flexibility: By utilizing a "dark sink," the model decouples the DM mass from the asymmetry generation scale, allowing for viable DM candidates across a vast mass range (keV to TeV), including regions previously difficult to explain with asymmetric DM models.
• Experimental Outlook: While direct detection is unlikely, the model predicts specific correlations between neutrino masses, CP phases, and the reheating temperature, offering indirect targets for future neutrino experiments (e.g., Hyper-Kamiokande, DUNE) and cosmological surveys (CMB-S4).

In summary, the paper establishes that UV freeze-in scattering mediated by heavy neutrinos, combined with explicit symmetry breaking and a dark sink, is a robust mechanism for generating the observed baryon and dark matter abundances simultaneously, resolving the coincidence problem without fine-tuning the DM mass.