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

Using data from the 532-day DarkSide-50 low-radiation campaign, this paper presents the first search for nuclear ultra-heavy dark matter in a dual-phase liquid argon time projection chamber, establishing new exclusion limits for dark nucleon masses ranging from 10 to 500 $\text{GeV/c}^2$.

The Gist

The Cosmic Bowling Ball: A Search for "Ultra-Heavy" Dark Matter

Imagine you are standing in a dark room, and someone tells you there are invisible bowling balls rolling across the floor. You can’t see them, you can’t hear them, and they pass right through the walls. How would you prove they are there?

You might wait for one to bump into a pile of bells. If you hear a single ding, it might just be a tiny pebble. But if you hear a rapid-fire ding-ding-ding-ding!—a series of rhythmic strikes in a split second—you know you haven’t been hit by a pebble. You’ve been hit by something massive, something moving with incredible momentum, something that hit multiple bells in a single pass.

This is essentially what scientists at the DarkSide-50 experiment just did, but instead of bells, they used liquid argon, and instead of bowling balls, they were looking for Nuclear Ultra-Heavy Dark Matter (UHDM).

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1. The Target: What is UHDM?

For decades, scientists have been looking for "WIMPs" (Weakly Interacting Massive Particles). Think of WIMPs like ghostly ping-pong balls: they are light, they move fast, and they usually only hit one thing before disappearing.

UHDM is different. Instead of a single particle, imagine a "dark nucleus"—a massive, heavy cluster made of many smaller dark particles stuck together. If a WIMP is a ping-pong ball, UHDM is a heavy, invisible wrecking ball. Because it is so massive and "chunky," it doesn't just hit one atom in a detector and stop; it plows through, hitting dozens of atoms in a rapid-fire chain reaction.

2. The Detector: The Liquid Argon "Bell Jar"

To catch these invisible wrecking balls, the DarkSide-50 team used a specialized tank filled with liquid argon.

When a particle hits an argon atom, the atom "glows" (this is called scintillation). The scientists use ultra-sensitive light sensors to watch for these flashes.

• A WIMP would create one single, lonely flash of light.
• UHDM would create a "staccato" of flashes—a rapid sequence of light pulses as the heavy dark matter crashes through the liquid, hitting atom after atom like a machine gun.

3. The Obstacle: The "Earth Filter"

There is one problem: the Earth itself is in the way. Because these dark matter particles are so heavy, they don't just pass through the Earth effortlessly; they actually lose energy as they plow through the rocks and soil to get to the detector.

It’s like trying to throw a bowling ball through a thick forest. By the time the ball reaches the other side, it might be moving much slower than when it started. The scientists had to use complex math (a tool called Verne) to calculate exactly how much "speed" these particles lose so they wouldn't misinterpret a slow-moving wrecking ball as something else.

4. The Results: What did they find?

After analyzing 532 days of data, the scientists did not find any UHDM.

However, in science, not finding something is still a huge win. By not finding it, they were able to draw a "No-Go Zone" on the map of the universe. They can now say with certainty: "If these heavy dark matter wrecking balls exist, they cannot be this heavy, and they cannot be this 'sticky' (interacting this strongly), because if they were, our detector definitely would have seen them."

Why does this matter?

This paper is a "first of its kind." Before this, most dark matter searches were only looking for the "ping-pong balls" (WIMPs). By looking for the "wrecking balls" (UHDM) using liquid argon, the DarkSide-50 team has opened a new door. They have provided a new way to hunt for the invisible architecture of our universe, helping us narrow down exactly what the "dark" part of our cosmos is actually made of.

Technical Summary

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

1. Problem Statement

Traditional dark matter searches primarily focus on Weakly Interacting Massive Particles (WIMPs), which are expected to undergo single elastic scattering events. However, an alternative candidate exists in the form of Nuclear Ultra-Heavy Dark Matter (UHDM). These are composite dark nuclei composed of many "dark nucleons."

Unlike WIMPs, UHDM candidates possess a large mass and a composite structure, which leads to two distinct physical phenomena:

• Multi-scatter signatures: Because of their high mass and large cross-sections, they are likely to scatter multiple times as they traverse a detector.
• Form Factor Suppression: Their finite physical size (radius $R_\chi$) affects the momentum transfer, potentially suppressing high-energy recoils and favoring low-energy interactions.

The challenge lies in designing a search strategy that can distinguish these multi-scatter, composite signals from standard single-scatter backgrounds and account for energy loss caused by the Earth's overburden.

2. Methodology

The study utilizes data from the DarkSide-50 experiment, a dual-phase liquid argon (LAr) time projection chamber (TPC) located at the Laboratori Nazionali del Gran Sasso (LNGS), Italy.

• Data Set: The analysis uses a 532-day low-radiation campaign using purified underground argon (UAr).
• Modeling UHDM: The researchers modeled the UHDM as a composite particle with a dark nucleon mass ($m_\chi$) of 10, 50, 100, and 500 GeV/$c^2$. They calculated the expected energy deposition by treating the UHDM as a collinear track of multiple energy deposits passing through the TPC.
• Overburden Accounting: Using the Verne toolkit, the team modeled how the Earth's material attenuates the UHDM velocity distribution before the particles reach the detector.
• Signal Selection:
  • The primary observable is the S1 signal (prompt scintillation).
  • The S2 signal (ionization) was not used for energy estimation because the electron drift time is comparable to the UHDM transit time, which would require complex, model-dependent recombination corrections.
  • Selection criteria included a drift time cut ($t_{drift} > 10\ \mu\text{s}$) to prevent merged S1/S2 signals and an energy ROI (Region of Interest) of 100–8000 photoelectrons (PE).
• Statistical Analysis: A $\mu$ (signal strength) fitter was employed using a likelihood model to compare the observed energy spectrum against a background model (primarily radiogenic sources like $^{39}\text{Ar}$, $^{85}\text{Kr}$, and PMTs).

3. Key Contributions

• First LAr TPC Search for UHDM: This represents the first time a dual-phase liquid argon TPC has been used to search for nuclear UHDM.
• Composite Modeling: The paper introduces a systematic approach to varying the dark nucleon mass ($m_\chi$), allowing the search to probe the internal structure of dark matter rather than just its total mass.
• Multi-scatter Optimization: The methodology specifically optimizes for the unique topology of multiple energy deposits within a single particle transit.

4. Results

The experiment found no significant excess of events over the predicted radiogenic background. Consequently, the team established 90% Confidence Level (CL) exclusion limits on the UHDM-nucleon scattering cross-section ($\sigma_{\chi,n}$).

• Mass Range: The limits cover a wide range of UHDM masses ($M_\chi$), from approximately $10^{17}$ to $10^{13}\ \text{GeV}/c^2$.
• Sensitivity Boundaries:
  • Lower Bound: Limited by the Earth's overburden (energy loss) and the detector's energy threshold.
  • Upper Bound: Limited by the "geometrical cross-section" (the physical size of the dark nucleus) and the detector's ability to resolve extremely high energy depositions (above 8000 PE).
  • Right-hand Bound: Limited by the total exposure and the decreasing flux of ultra-heavy particles.
• Dark Nucleon Mass Impact: The results demonstrate that as the dark nucleon mass increases, the visible parameter space shifts due to changes in the scattering dynamics and form factor suppression.

5. Significance

This work is significant because it expands the frontier of dark matter detection beyond the WIMP paradigm. By demonstrating that liquid argon TPCs are capable of searching for composite, multi-scattering dark matter, the DarkSide collaboration has provided a new tool for particle physics. This approach allows future experiments to not only search for the existence of dark matter but also to potentially constrain its internal composition (the mass of its constituent dark nucleons), providing a deeper understanding of the dark sector's fundamental properties.