Composite Ultraheavy Dark Matter

• Composite ultraheavy dark matter is defined as stable, massive bound states from hidden sectors, with masses spanning from GeV to Planck scales and formed via strong dynamics or gravitational binding.
• These models are produced through nonthermal and freeze-in mechanisms, enabling them to evade traditional WIMP unitarity bounds and support extended spatial structures.
• Key experimental signatures include enhanced geometric annihilation cross sections, distinctive gravitational wave bursts, and novel detection prospects via quantum sensor arrays.

Composite ultraheavy dark matter (UHDM) refers to scenarios in which stable, massive dark-sector bound states—often with masses greatly exceeding the weak scale, typically GeV–PeV and up to Planckian values—constitute the dark matter. Unlike WIMPs, these candidates are not point-like fundamental particles but composites, with structure arising from strong dynamics, hidden gauge sectors, or exotic constituent particles. The term encompasses models ranging from multi-TeV atomic bound states, heavy glueballs, supermassive states formed via cosmological phase transitions, to Planck-scale objects with macroscopic radii. These frameworks are motivated by the absence of new particles at collider energies, as well as theoretical considerations—such as perturbative unitarity or black hole formation—that forbid stable, elementary particles with masses above certain critical scales. Composite UHDM may interact through feeble portals (Yukawa, gravitational, higher-dimension operators) and exhibit distinctive experimental, cosmic, and astrophysical signatures.

1. Theoretical Constructions of Composite Ultraheavy Dark Matter

Composite UHDM models are heterogeneous in binding mechanism, constituent content, and energy scale:

• Conformal/Strong Sector Bound States: In models with a confining conformal field theory (CFT) at $\Lambda_{\rm IR} \sim$ TeV–Planck (e.g., composite Higgs frameworks), ultraheavy scalars can arise as elementary singlets stabilized by $Z_2$ symmetry, with masses up to $10^{18}$ GeV. Such states couple feebly to both standard and composite SM fields through higher-dimensional operators suppressed by the cutoff, and can thermally populate the observed relic density solely via freeze-in through CFT interactions (Ismail, 2023).
• Atomic Dark Matter Models: Dark-sector analogues of atoms, composed of heavy Dirac fermions (“dark protons” and “dark electrons”) bound by a dark U(1) gauge interaction, yield neutral stable bound states. The atom–anti-atom annihilation is governed by geometric-scale cross sections (proportional to squared Bohr radius), allowing masses in the $10^6$–$10^{10}$ GeV range and surpassing the unitarity bound for elementary thermal relics (Qiu et al., 2023).
• Glueball and Axion-like Composites: Pure Yang–Mills dark sectors confine at scale $\Lambda_D$, leading to massive glueball states (e.g., pseudoscalar “Glueball ALP” or GALP) with highly suppressed couplings to photons and naturally ultraheavy masses $\sim$MeV–PeV, set by the confinement scale and radiatively induced couplings via heavy fermion loops (Carenza et al., 2024).
• Composite States via Modified Gravity in Extra Dimensions: In the presence of Arkani-Hamed–Dimopoulos–Dvali (ADD) extra dimensions, gravity strengthens as $1/r^{n+1}$ for $r \ll R$ (size of extra dimensions), enabling the binding of standard-model fermions into compact “nuggets” with $M_{\rm DM} \gg \text{TeV}$ and radii set by the interplay of kinetic and gravitational energy (Flambaum, 26 Jan 2025).
• Supercooled Confining Sectors: In scale-invariant or nearly-conformal theories, first-order phase transitions can undergo strong supercooling, injecting large entropy and enabling DM masses up to $10^6$ TeV via out-of-equilibrium processes and subsequent freeze-out below the reheat temperature (Baldes et al., 2021).

2. Relic Formation Mechanisms and Unitarity Considerations

Relic abundance mechanisms in UHDM models differ markedly from those assumed in point-like WIMP scenarios:

• Freeze-In via Higher-Dimensional Operators: For ultraheavy scalars associated with a confining sector, the dominant production is through freeze-in, where initial abundance is negligible and slowly accumulates via extremely suppressed, nonrenormalizable couplings. The yield is set by integrating the Boltzmann equation for the production rate over temperatures above the CFT’s confinement scale. This decouples the final relic density from the precise value of the annihilation cross section and evades the Lee–Weinberg unitarity bound (Ismail, 2023).
• Geometric Cross Section Enhancement (“Annihilation via Rearrangement”): In atomic DM, the dominant process post-formation is rearrangement—atom–anti-atom collisions leading to rapid annihilation. The cross section is set by atomic size, $\sigma_{\rm reac} v \sim \pi a_D^2$, which is parametrically much larger than the perturbative unitarity limit for a point particle and allows for stable relics well above the 100 TeV unitarity ceiling (Qiu et al., 2023).
• Phase-Transition-Driven or Nonthermal Production: Supercooled transitions dilute pre-existing relics and generate heavy DM through string fragmentation or out-of-equilibrium processes at reheating. The dilution and nonthermal yields relax mass bounds and produce relic densities matching observations for $M_{\chi}$ up to $10^6$ TeV (Baldes et al., 2021).
• Glueball Freeze-In/Freeze-Out: For heavy glueballs, the freeze-in yield is set by the temperature at confinement, itself determined by reheating processes via heavy fermion annihilation, radiatively seeding the dark and visible sector with partitioned temperatures. Relic abundance is then controlled by the dark-to-visible temperature ratio and $\Lambda_D$ (Carenza et al., 2024).

A plausible implication is that any phenomenology assuming the standard $⟨σv⟩\sim10^{-26}$ cm$^3$s$^{-1}$ for thermal relics is inapplicable in the composite UHDM regime. Geometric and nonthermal mechanisms change not only the allowed mass scales but also predict different freeze-out/freeze-in "regimes," which in some cases (e.g., rearrangement-dominated atomic DM) lead to required masses $\gg10^5$ GeV.

3. Experimental Signatures and Constraints

Direct, indirect, and cosmological searches for composite UHDM exploit their distinctive production and interaction properties:

• Direct Detection: Sensitivity drops sharply with increasing DM mass due to flux suppression, but composite structure enables new signals. In particular, quantum sensor arrays sensitive to gravitational or Yukawa-mediated impulses from individual (spatially extended) DM clumps offer a path to directly probe masses up to and above $M_{\rm Pl}$. The signature depends sensitively on the object’s density profile (tophat, Gaussian, exponential) and size $R$, with optimal detection at radii similar to the detector’s inter-sensor separation (Amaral et al., 10 Dec 2025).
• Indirect Detection:
  • Ultraheavy Atom/Composite Annihilations: In classical composite regimes, the velocity-dependent geometric cross section enhances the annihilation rate at astrophysically relevant velocities, with observable gamma-ray signals. VERITAS dSph observations have set upper limits on the composite DM radius ($R_\chi < 0.6\,$fm for $M_\chi = 1\,$PeV) (Acharyya et al., 2023).
  • Collision-induced Bursts: Macroscopic UHDM blobs can collide, releasing intense, ultrashort (ns–μs) gamma-ray bursts due to core–core interactions or subsequent fireball formation, yielding distinctive time-domain astrophysical signals. Imaging atmospheric Cherenkov telescopes (IACTs) and wide-field arrays (PANOSETI) have projected sensitivity to these burst-like transients, opening parameter space far above the reach of conventional gamma-ray line searches (Kaplan et al., 2024).
  • Decay Signatures: For glueball ALPs, radiative decay into photons at rates dictated by $g_{a\gamma\gamma}$ is constrained by cosmic and galactic gamma-ray line searches, with Planckian-scale suppression naturally ensuring stability for $m_a\gtrsim 100\,\text{MeV}$ (Carenza et al., 2024).
• Collider and Astrophysical Limits: Stable charged constituents are strongly constrained by colliders, BBN, CMB, and anomalous-isotope searches. For OHe-type models, accelerator searches for stable doubly-charged tracks reach $m_Q \gtrsim 700$ GeV, while indirect detection of ionization or radiative capture signals confronts model-dependent target-nucleus coupling (Khlopov, 2015).

Key experimental observables are summarized as follows:

Observable Type	Probing Scale	Derived Limits
Direct detection	$10^9$–$10^{19}$ GeV	$\sigma_{\phi N}\lesssim10^{-45}$ cm$^2$ typically
Indirect $\gamma$	$1$–$30$ PeV	$R_\chi \lesssim 0.6$ fm @ 1 PeV (VERITAS)
Time-domain bursts	$10^{2}$–$10^{23}$ g	Sensitivity down to few bursts/yr (VERITAS, CTA)

4. Distinctive Phenomenological Consequences

Composite UHDM models share several salient phenomenological features distinct from point-like dark matter:

• Evading the Unitarity Bound: The geometric enhancement of (anti-)atom or baryonic composite cross sections, due to extended spatial structure, enables stable relics with masses orders of magnitude above the point-particle unitarity ceiling ($\lesssim 100$ TeV). This provides an explicit realization of classic WIMP limitations as in Griest–Kamionkowski (Acharyya et al., 2023, Qiu et al., 2023).
• Astrophysical Structure and Self-Interaction: Strong self-interactions (e.g., dark-atom scattering cross-section $\sigma_{\rm SI}\sim\pi a_D^2/m_A$) potentially address certain small-scale structure issues (core–cusp, too-big-to-fail) for parameter regimes where $\sigma/m\gtrsim 1$ cm$^2$/g (Qiu et al., 2023).
• Gravitational Wave Signatures: For scenarios with supercooled phase transitions, bubble nucleation, and vacuum-dominated cosmic histories, stochastic gravitational-wave backgrounds detectable by LISA, B-DECIGO, and ET are produced. These signals directly probe the supercooled UHDM production regime, irreducible from all other known cosmological events (Baldes et al., 2021).
• Suppressed Baryonic/SM Couplings: Many models are automatically invisible to standard direct searches due to their dimensional suppression (e.g., $\sim 1/\Lambda^2$), feeble higher-dimensional portals, or high charge neutrality (e.g., OHe atoms, glueballs), naturally satisfying constraints from BBN, CMB, and laboratory anomalous isotope abundance (Ismail, 2023, Khlopov, 2015, Carenza et al., 2024).
• Experimental Fingerprints of Composite Structure: For quantum sensor arrays, the spatial profile of the UHDM object modulates the impulse distribution across the array. Experimental ability to resolve tophat vs Gaussian vs exponential density profiles can, in principle, distinguish among binding mechanisms (e.g., strong vs gravitational) (Amaral et al., 10 Dec 2025).

5. Viable Parameter Space and Outstanding Challenges

Across different composite UHDM models, the allowed parameter space is set by dynamical, cosmological, and experimental constraints:

• Lower Bounds:
  • Anomalous isotope, BBN, and collider constraints forbid light, stable charged constituents unless efficiently bound and neutralized; thus, only doubly charged or SM-neutral species are viable in “dark atom” scenarios.
  • For strong-interaction models, DM self-interaction bounds set $m_a \gtrsim 120$ MeV (for glueball ALPs), and similarly, $m_{O}\gg1$ TeV in OHe dark atom models (Khlopov, 2015, Carenza et al., 2024).
• Upper Bounds:
  • For Planck-scale or beyond-Planck DM, the requirement that the Compton wavelength exceeds the Schwarzschild radius mandates spatially extended objects, not point-like particles (Amaral et al., 10 Dec 2025).
  • Energetic consistency requires $m_\phi^2\,\Lambda^2\lesssim M_P^4$ in FIMPzilla models, and $R>2GM$ to avoid prompt black-hole collapse in macroscopic composites (Ismail, 2023, Kaplan et al., 2024).
• Parameter Ranges from Key Models:

Model/Class	Mass Range	Scale/Radius	Key Constraints
FIMPzilla (CFT)	$10^{10}$–$10^{18}$ GeV	$\Lambda$ (cutoff)	CMB tensor–scalar, $r$, isocurvature
Atomic DM	$10^6$–$10^{10}$ GeV	$a_D\sim(\alpha_D\mu)^{-1}$	Freeze-out, relic, BBN, CMB
Glueball ALP	$100$ MeV–$10^{10}$ GeV	$R\sim\Lambda_D^{-1}$	$g_{a\gamma\gamma}$, lifetime
Gravitational Nugget	$>1$ TeV up to $M_P$	$10^{-13}$–$10^{-11}$ cm	$\sigma/M \lesssim 0.1$ cm$^2$/g
Macroscopic Blobs	$10^2$–$10^{23}$ g	$>R_S$, up to macroscopic	Microlensing, bursts, Bullet Cluster

No explicit calculation exists for the full cosmological production history for extra-dimensional gravitational nuggets; similarly, the microphysics of glueball formation at high $\Lambda_D$ and the interplay between collider and cosmological signatures in composite dark atoms remain open.

6. Outlook and Future Probes

The detection prospects for composite UHDM are strongly model-dependent but generally require experimental strategies beyond the WIMP paradigm:

• Quantum Sensor Arrays offer sensitivity to impulsive gravitational or Yukawa-force events from spatially extended UHDM, enabling measurement of both mass and structural scale for Planck-scale objects (Amaral et al., 10 Dec 2025).
• IACT and Time-Domain Surveys can access parameter space for rare, ultrashort gamma-ray burst events arising from blob–blob collisions, with the possibility to discriminate against astrophysical backgrounds by timescale and spatial coherence (Kaplan et al., 2024).
• Gravitational-Wave Interferometers will probe the entire parameter region of supercooled composite UHDM scenarios via stochastic backgrounds (Baldes et al., 2021).
• Gamma-ray Telescopes (Fermi, CTA, AMEGO, e-ASTROGAM) and cosmic-ray detectors will test decaying glueball ALP signatures and indirect annihilation signals, examining spectral lines at energies inaccessible by laboratory experiments (Carenza et al., 2024, Acharyya et al., 2023).

A plausible implication is that composite UHDM scenarios demand fundamentally new detection philosophies, utilizing advances in sensor technology, timing precision, and multi-messenger astrophysics to fully exploit their unique phenomenology and testability.