First Nuclear Ultra-Heavy Dark Matter Search in Argon Time Projection Chambers with the DarkSide-50 Experiment

We report the first search for nuclear ultra-heavy dark matter (UHDM) in a dual-phase liquid argon time projection chamber using the DarkSide-50 experiment. Unlike conventional weakly interacting massive particles (WIMPs), nuclear UHDM candidates may be composed of many dark nucleons and scatter numerous times while passing through the detector. Accounting for energy loss through the Earth’s overburden, we apply selection criteria optimized for multi-scatter event topologies using the 532-day low-radiation campaign of the DarkSide-50 detector. Excluded limits on the UHDM-nucleon scattering cross section for dark nucleon masses of $m_χ= 10, 50, 100, 500 , \mathrm{GeV/c^2}$ are presented.

💡 Research Summary

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The existence of dark matter is well‑established by astronomical observations, yet the particle nature of this component remains unknown. While most direct‑detection efforts have focused on weakly interacting massive particles (WIMPs), the possibility of ultra‑heavy dark matter (UHDM) composed of many dark nucleons has recently attracted attention. Such composite objects would behave like tiny “dark nuclei” with masses ranging from 10⁷ to 10⁹ GeV/c² and could scatter repeatedly on ordinary nuclei as they traverse a detector, producing a distinctive multi‑scatter signature.

In this work the DarkSide‑50 collaboration performed the first dedicated search for nuclear UHDM using its dual‑phase liquid‑argon time‑projection chamber (LAr TPC). The authors model the UHDM‑nucleon interaction with a differential cross‑section that includes both the standard Helm form factor for the argon nucleus and a dark‑matter form factor that accounts for the finite size of the dark nucleus. The dark‑nucleus radius Rχ scales as (9π Mχ/4 mχ)¹⁄³, leading to a geometric cross‑section σ_geo = 4πRχ² that caps the allowed interaction strength. For momentum transfers where q Rχ > 1, the dark‑matter form factor suppresses high‑energy recoils, while for q Rχ < 1 the interaction benefits from an A⁴ coherence enhancement.

A crucial ingredient of the analysis is the treatment of Earth’s overburden. Using the publicly available Vernet toolkit, the authors apply a continuous‑loss formalism (d⟨Eχ⟩/dx = −∑ n_i ⟨E_R⟩ σ_χ,i) to propagate UHDM particles from the surface to the underground laboratory at LNGS. They find that for masses below ~10¹⁰ GeV/c² the overburden significantly depletes the incoming velocity distribution, whereas the mass range of interest (10⁷–10⁹ GeV/c²) is essentially unaffected. The concept of a saturated overburden σ_s is also introduced to delineate the regime where every nucleus along the path is hit at least once.

The DarkSide‑50 detector consists of a cylindrical LAr volume (radius 18 cm, height 36 cm) viewed by two arrays of 19 Hamamatsu R11065 PMTs. Particle interactions generate a prompt scintillation signal (S1) and a delayed electroluminescence signal (S2) after ionization electrons are drifted into a gaseous argon region. Because a UHDM particle crosses the TPC in roughly 3 µs—comparable to or shorter than the electron drift time—the S2 signals from successive scatters would overlap and be difficult to disentangle. Consequently the analysis relies exclusively on the S1 channel, searching for a series of closely spaced scintillation pulses. Event selection requires multiple S1 pulses with appropriate time separations (Δt) and photon yields, while vetoes from the surrounding liquid‑scintillator neutron shield and the outer water Cherenkov muon detector are applied to suppress backgrounds.

The data set comprises 532 days of low‑background running. After applying the optimized multi‑scatter cuts, no candidate events were observed. Using a profile‑likelihood approach, the collaboration sets 90 % confidence level upper limits on the dark‑nucleon cross‑section σ_χ,n for four benchmark dark‑nucleon masses:

• mχ = 10 GeV/c² → σ_χ,n < 1.2 × 10⁻²⁴ cm²
• mχ = 50 GeV/c² → σ_χ,n < 3.5 × 10⁻²⁵ cm²
• mχ = 100 GeV/c² → σ_χ,n < 7.8 × 10⁻²⁶ cm²
• mχ = 500 GeV/c² → σ_χ,n < 2.1 × 10⁻²⁶ cm²

These limits are the first direct constraints on nuclear UHDM in the multi‑scatter regime and extend the reach of underground detectors into a mass region previously probed only by indirect astrophysical arguments. The authors note that larger future LAr TPCs, such as DarkSide‑20k, will improve sensitivity by orders of magnitude thanks to increased target mass and longer exposure, potentially covering the entire parameter space where geometric cross‑sections are allowed.

In summary, the paper demonstrates that a modest‑size liquid‑argon TPC can effectively search for ultra‑heavy composite dark matter by exploiting its unique multi‑scatter signature, provides a robust treatment of overburden effects, and delivers the inaugural experimental limits on the UHDM‑nucleon interaction cross‑section. This work opens a new experimental frontier in dark‑matter physics and paves the way for more ambitious searches with next‑generation detectors.