Searching for UFOs from the early universe: direct detection prospects for relativistically decoupling dark matter

This paper demonstrates that ultrarelativistically frozen-out (UFO) dark matter candidates, particularly those mediated by a $Z'$ portal, represent a viable and detectable alternative to traditional WIMPs, with current experiments like LZ and XENONnT already constraining their parameter space and future detectors such as SuperCDMS SNOLAB poised to explore significant regions in the 0.5–10 GeV mass range.

The Gist

The Big Picture: What is a "UFO" in this context?

First, let's clear up a misunderstanding. In this paper, UFO does not stand for "Unidentified Flying Object" from outer space. It stands for Ultrarelativistic Freeze-Out.

Imagine the early universe as a giant, boiling pot of soup (the "Standard Model bath") filled with all kinds of particles zipping around at incredible speeds. Usually, scientists look for Dark Matter candidates that act like WIMPs (Weakly Interacting Massive Particles). Think of WIMPs as heavy, slow-moving rocks that eventually sink to the bottom of the pot and stop moving around. They "freeze out" when they get too heavy and slow to keep up with the soup.

UFOs are different. Imagine a swarm of hyper-active bees that are moving so fast they are practically vibrating. These particles were once part of the hot soup, but they decided to leave the party while they were still moving at near-light speed. Because they left while they were still "hot" and fast, they are called "Ultrarelativistic."

The Setting: The "Reheating" Phase

The paper focuses on a very specific, chaotic time in the universe's history called Reheating.

• The Analogy: Imagine the universe had just finished a long nap (a period called "Inflation"). When it woke up, it was cold and empty. Then, a giant "heater" (called the Inflaton) turned on, dumping massive amounts of energy into the universe to create the soup of particles we know today.
• The Problem: Most Dark Matter theories assume the universe was already a hot, stable soup when the Dark Matter formed. But this paper asks: What if the Dark Matter formed while the heater was still warming up the room?

The Main Characters: The Portal and the Particles

The authors use a specific model to test their idea:

1. The Dark Matter (χ): The "UFO" particles. They can be heavy fermions (like electrons) or scalars (like Higgs bosons).
2. The Portal (Z'): A heavy messenger particle that acts like a bridge between our visible world and the Dark Matter world.
3. The Interaction: The Dark Matter talks to normal matter through this heavy portal.

The Core Discovery: The "Goldilocks" Zone

The paper runs a massive simulation to see if these UFO particles could explain the Dark Matter we see today. They found a very interesting "Goldilocks" zone where everything works:

1. The Escape: The Dark Matter particles interact with the hot soup, reach a balance (equilibrium), and then escape (freeze out) while the universe is still being heated up.
2. The Dilution: Because they escape before the heating is finished, the universe keeps expanding and creating more normal matter (the soup). This extra soup "dilutes" the Dark Matter, making it less dense. This is crucial because it prevents the universe from being overfilled with Dark Matter.
3. The Result: These particles end up being "Cold Dark Matter" (slow-moving today) even though they left the party while they were "hot." This solves a problem that usually makes fast-moving particles bad candidates for Dark Matter.

The Detective Work: Can We Catch Them?

The most exciting part of the paper is the "Direct Detection" section. This is about trying to catch these invisible particles by seeing if they bump into atoms in giant underground detectors (like LZ, XENONnT, and PandaX).

• The Old News: For years, we thought Dark Matter was too weak to be seen.
• The New News: The authors found that UFOs are actually easier to catch than some other types of Dark Matter (like FIMPs, which are "Feebly Interacting" and practically invisible).
• The Evidence:
  • Already Caught: Current experiments have already ruled out (excluded) a huge chunk of the possible UFO territory. If UFOs existed in those specific mass ranges, we would have seen them by now.
  • Still Hiding: There is still a "safe zone" where UFOs could exist. This zone is for Dark Matter particles with masses between 0.4 GeV and 1 TeV (roughly the mass of a proton up to a heavy atom).
  • The Fog: There is a "Neutrino Fog" (a background noise from solar neutrinos) that makes it hard to see anything below a certain point. However, the authors found that UFOs can exist above this fog, making them detectable.

The Future: SuperCDMS SNOLAB

The paper highlights a specific experiment coming online soon: SuperCDMS SNOLAB (expected to start collecting data in 2026).

• The Metaphor: Think of current experiments as looking for a needle in a haystack with a flashlight. SuperCDMS SNOLAB is like bringing in a super-sensitive metal detector.
• The Prediction: This new machine is expected to scan a massive area of the "UFO parameter space" (specifically for particles between 0.5 and 10 GeV). If UFOs exist in this range, SuperCDMS SNOLAB has a very good chance of finding them.

The "Double Identity" Problem

One of the paper's clever findings is a "degeneracy" or a case of mistaken identity.

• The Analogy: Imagine you see a shadow. It could be a person standing still (WIMP), or it could be a person running away very fast (UFO).
• The Science: For certain settings of the "portal" (the Z' mass), the universe produces the exact same amount of Dark Matter whether the particles freeze out slowly (WIMP style) or quickly while moving fast (UFO style).
• Why it matters: This means that if we find Dark Matter, we have to be very careful to figure out how it was made. It could be the old-fashioned slow way, or this new fast way.

Summary of Claims

1. UFOs are viable: Dark Matter particles that decouple while moving at light speed during the universe's "reheating" phase are a strong candidate for the Dark Matter we see today.
2. They are detectable: Unlike some other exotic theories, these UFOs interact strongly enough that we might be able to see them in current or upcoming experiments.
3. Current limits: Experiments like LZ and XENONnT have already ruled out many possibilities, but a large, viable region remains.
4. Future hope: The upcoming SuperCDMS SNOLAB experiment is perfectly positioned to find these particles if they exist in the 0.5–10 GeV mass range.
5. A new tool: Direct detection experiments aren't just looking for particles; they are effectively acting as "time machines" to tell us about the temperature of the universe when it was just being born (the reheating temperature).

In short, the paper argues that we shouldn't give up on finding Dark Matter just because we haven't found the "classic" WIMPs yet. There is a whole new class of candidates (UFOs) that are hiding in plain sight, and we are about to get the tools to find them.

Technical Summary

Technical Summary: Searching for UFOs from the early universe: direct detection prospects for relativistically decoupling dark matter

Problem Statement
The nature of dark matter (DM) remains unresolved, with the traditional Weakly Interacting Massive Particle (WIMP) paradigm facing increasing pressure due to the lack of detection in experiments such as XENONnT, PandaX, and LUX-ZEPLIN. While alternatives like Feebly Interacting Massive Particles (FIMPs) offer a solution to the non-detection problem by positing particles that never reach thermal equilibrium, they introduce significant theoretical challenges. Specifically, FIMP models often require extremely weak couplings that render them undetectable in current direct detection experiments and suffer from unresolved initial condition problems (e.g., gravitational production) since they lack a thermal history. Conversely, standard WIMPs decouple non-relativistically, but their parameter space is increasingly constrained. This paper investigates a third category: Ultrarelativistic Freeze-Out (UFO) candidates. These are particles that reach thermal equilibrium with the Standard Model (SM) bath but decouple while still relativistic ($T_{FO} \gg m_\chi$). Unlike standard neutrinos, which decouple during radiation domination and act as hot dark matter, UFO candidates decouple during the reheating phase of the early universe. This timing allows for entropy dilution that can render them viable cold dark matter candidates while maintaining couplings strong enough to be potentially observable.

Methodology
The authors analyze the direct detection prospects of UFO dark matter using a $Z'$-portal model as a case study. The study considers both Dirac fermion and complex scalar DM candidates ($\chi$) interacting with SM fermions via a heavy vector mediator ($Z'$) with mass $M_{Z'} \geq 1$ TeV.

1. Theoretical Framework: The authors derive the relevant Lagrangians for vector and axial-vector couplings. They calculate annihilation cross-sections ($\chi\chi \to f\bar{f}$) and scattering cross-sections (spin-independent and spin-dependent) on nucleons.
2. Relic Density Calculation: The relic density is computed by solving the Boltzmann equation during the reheating phase, assuming the universe is dominated by the coherent oscillations of an inflaton field with a quadratic potential ($V(\phi) \propto \phi^2$). The temperature evolution scales as $T \propto a^{-3/8}$, distinct from the adiabatic $T \propto a^{-1}$ scaling. The authors delineate three regimes based on the reheating temperature ($T_{RH}$), DM mass ($m_\chi$), and mediator mass ($M_{Z'}$):
   • Freeze-In (FIMP): No thermal equilibrium is reached.
   • UFO: Thermal equilibrium is reached, but decoupling occurs while $\chi$ is relativistic.
   • WIMP-like Freeze-Out: Decoupling occurs when $\chi$ is non-relativistic.
3. Direct Detection Constraints: The authors map the parameter space ($m_\chi, T_{RH}, M_{Z'}$) required to satisfy the observed relic density ($\Omega_\chi h^2 \approx 0.12$) onto the direct detection cross-section plane ($\sigma_{SI}, \sigma_{SD}$ vs. $m_\chi$). They compare these theoretical contours against current exclusion limits from LZ, XENONnT, PandaX, and DarkSide-50, as well as projected sensitivities for SuperCDMS SNOLAB.

Key Contributions and Results

• Degeneracy between UFO and WIMP: The paper identifies a region of parameter space where a specific DM mass and reheating temperature can yield the correct relic abundance via two distinct mechanisms: non-relativistic WIMP-like freeze-out or relativistic UFO. This degeneracy arises because, for a fixed $m_\chi$ and $T_{RH}$, varying the mediator mass $M_{Z'}$ can shift the system from a regime where annihilations dominate (WIMP) to one where post-freeze-out production dominates (UFO).
• Exclusion of Light WIMP-like Candidates: For the heavy portal model considered ($M_{Z'} \gtrsim 1$ TeV), the parameter space for light DM ($m_\chi \lesssim 10$ GeV) produced via non-relativistic freeze-out during reheating is almost entirely excluded by current direct detection limits.
• Viable UFO Parameter Space: In contrast, a significant portion of the UFO parameter space remains viable and accessible.
  • Mass Range: For fermionic DM, viable regions exist above the "neutrino fog" for $0.4 \text{ GeV} \lesssim m_\chi \lesssim 1 \text{ TeV}$. For scalar DM, the viable mass range extends up to nearly 1 TeV, with cross-sections roughly 4 times larger than the fermionic case in the UFO regime due to differences in the annihilation dynamics.
  • Reheating Temperature: Detectable UFO candidates require relatively low reheating temperatures, specifically $T_{RH} \lesssim 2$ GeV. Higher $T_{RH}$ values necessitate heavier mediators to maintain the correct relic density, which pushes the scattering cross-sections below the neutrino fog.
  • Mediator Mass: The viable UFO regions accessible to direct detection correspond to mediator masses in the range $1 \text{ TeV} \lesssim M_{Z'} \lesssim 300 \text{ TeV}$.
• Spin-Dependent Scattering: The authors note that current experimental constraints on spin-dependent scattering have not yet probed the UFO parameter space, leaving this channel open for future investigation.
• Scalar vs. Fermion Differences: While the scattering cross-sections are similar, the relic density calculations differ. Scalar DM annihilation is p-wave suppressed, leading to different dependencies on $T_{RH}$ and $m_\chi$ compared to fermionic DM. Consequently, scalar UFO candidates can be detected at higher masses and have larger cross-sections for the same relic abundance.

Significance and Claims
The paper argues that UFO dark matter represents a "versatile class of well-motivated" candidates that bridge the gap between the detectability of WIMPs and the theoretical robustness of avoiding initial condition problems. Unlike FIMPs, UFOs thermalize, thereby erasing the dependence on unknown initial conditions (such as gravitational production). Unlike WIMPs, they do not require the specific electroweak-scale couplings that are currently being ruled out.

The authors claim that direct detection experiments are already probing and excluding large portions of the UFO parameter space. Specifically, they highlight that:

1. Ongoing experiments (LZ, XENONnT, etc.) have excluded a large portion of the UFO space for $m_\chi$ between a few GeV and $\sim 200$ GeV.
2. The upcoming SuperCDMS SNOLAB experiment is expected to access a large region of the UFO parameter space in the $0.5-10$ GeV mass range.
3. For heavy portal interactions ($M \gtrsim 1$ TeV), UFOs are generally more accessible to direct detection than FIMP candidates due to their comparatively larger cross-sections.

The study concludes that UFOs are attractive candidates for the "post-WIMP era," transforming direct detection experiments into tools for exploring the physics of the early universe, specifically constraining the reheating temperature and the nature of DM-SM interactions prior to Big Bang Nucleosynthesis.