Determination of the sensitivity of the DEAP-3600 experiment to supermassive charged gravitinos

This paper presents a feasibility study on the ability of the DEAP-3600 experiment to detect and characterize the signal topology of Planck-mass charged gravitino dark matter.

The Gist

The Cosmic Ghost Hunt: Searching for "Super-Heavy" Particles

Imagine you are standing in a dark, crowded room. You know there are people moving around, but you can’t see them. You can only tell they are there because, every once in a while, you feel a tiny breeze or hear a faint rustle.

In the world of physics, scientists are currently in that same "dark room." They know the universe is filled with something called Dark Matter—a mysterious substance that provides the "gravitational glue" holding galaxies together—but they haven't been able to actually see it yet.

This paper, written by Michał Olszewski, explores a very specific, wild idea: What if Dark Matter isn't a swarm of tiny, light particles, but a few incredibly massive, "super-heavy" ghosts?

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1. The Concept: The Cosmic Bowling Ball

Most scientists are looking for "WIMPs" (Weakly Interacting Massive Particles). Think of WIMPs like ping-pong balls—they are light, numerous, and move around constantly.

However, this paper looks at Gravitinos. If WIMPs are ping-pong balls, Gravitinos are heavy bowling balls. They are so massive (approaching the "Planck mass," a scale of weight so huge it's hard to imagine) that there aren't many of them. In fact, there might only be a handful in a huge area.

Because they have a tiny bit of electric charge, they don't just float through things; they leave a "trail" behind them, like a heavy sled being dragged through fresh snow.

2. The Detector: The Ultra-Pure "Snow Globe"

To catch these "bowling balls," you can't use a standard camera. You need something much more sensitive. The researcher is looking at an experiment called DEAP-3600.

Imagine a giant, ultra-pure glass sphere filled with Liquid Argon. This liquid is so clean that it’s like a "snow globe" where not a single speck of dust is allowed. Deep underground in a Canadian mine (to hide from the "noise" of cosmic rays from space), this detector waits for something to bump into the argon.

When a particle hits the liquid argon, it creates a tiny flash of light. The DEAP-3600 is designed to listen to the "rhythm" of these flashes.

3. The Challenge: The Speed Problem

Here is where the "detective work" gets tricky. The paper explains that how we catch these particles depends on how fast they are moving:

• The Slow Crawl ($v_E$): If the gravitino is moving slowly, its "trail" of light is so faint and spread out that the detector’s computer might mistake it for random background noise or simply ignore it. It’s like trying to hear a single person whispering in a noisy stadium.
• The Fast Streak ($v_S$): If the gravitino is moving faster, it creates a much clearer, more distinct "signature" of light. The researcher used computer simulations to show that if these fast-moving particles pass through the detector, they leave a very specific pattern of light that looks different from anything else.

4. The Verdict: Is it working?

The researcher concludes that while the current DEAP-3600 setup might struggle to catch the "slow" versions of these particles, it is perfectly capable of spotting the "fast" ones.

Even more excitingly, the paper suggests that because liquid argon detectors are so good at "hearing" these specific light patterns, the next generation of even larger detectors (like DarkSide-20k) will be like upgrading from a handheld radio to a massive satellite dish. They will have a much better chance of finally "seeing" these super-heavy cosmic ghosts.

Summary in a Nutshell

Scientists are checking if a massive, heavy type of Dark Matter exists. They are using a giant, ultra-pure tank of liquid deep underground to look for the specific "light trails" these heavy particles would leave behind. While it's a difficult hunt, we are building better "nets" every day to catch them.

Technical Summary

Technical Summary: Determination of the Sensitivity of the DEAP-3600 Experiment to Supermassive Charged Gravitinos

1. Problem Statement

The search for Dark Matter (DM) has primarily focused on Weakly Interacting Massive Particles (WIMPs) within specific mass windows. However, the lack of experimental discovery has motivated the study of alternative models, specifically Planck-mass dark matter.

This paper investigates a specific theoretical candidate: supermassive charged gravitinos predicted by extended supergravity theories. These particles are characterized by:

• Extreme Mass: $m \approx M_{Pl}$ (Planck mass).
• Fractional Electric Charge: Derived from $SU(3) \times U(1)_{em}$ symmetry.
• Low Abundance: Approximately $3 \times 10^{-14}$ particles per cubic meter.
• Unique Interaction: Due to their mass and charge, they do not undergo standard thermal equilibrium or frequent annihilations; instead, they interact with Standard Model (SM) matter by leaving a straight track of uniform ionization/excitation.

The core challenge is determining whether current and future liquid argon (LAr) detectors, specifically DEAP-3600, have the sensitivity to distinguish these extremely rare, slow-moving, and highly ionizing tracks from background noise.

2. Methodology

The author employs a computational and simulation-based approach to evaluate detector response:

• Simulation Framework: A custom superheavy charged gravitino generator was developed using the Geant4-based Monte Carlo and analysis tools specific to DEAP-3600.
• Modeling Physics: The simulation accounts for LAr scintillation (128 nm photons), isotropic emission from interaction points along a track, and the detector's temporal response based on DM velocity and LAr scintillation time constants.
• Velocity Scenarios: The study compares two velocity regimes:
  • $v_E \sim 30$ km/s (virial velocity relative to Earth).
  • $v_S \sim 230$ km/s (virial velocity relative to the Galaxy).
• Signal Discrimination: The study utilizes Pulse-Shape Discrimination (PSD), specifically the $F_{prompt}$ parameter (the ratio of prompt light to total light), to differentiate potential signals from electronic recoils (ER) and nuclear recoils (NR).

3. Key Contributions

• Feasibility Assessment: The paper provides a qualitative and quantitative assessment of the expected signal rates for DEAP-3600, LUX, XENONnT, DarkSide-20k, and ARGO.
• Signal Topology Characterization: It identifies that gravitino signals in LAr will appear as straight tracks of electronic recoil (ER) events rather than nuclear recoils.
• Detection Threshold Analysis: The work establishes the relationship between gravitino velocity and the detectability of the resulting scintillation light (measured in photoelectrons, PE).

4. Results

• Velocity Limitations: A critical finding is that gravitinos moving at $v_E$ (30 km/s) are undetectable by DEAP-3600. At this velocity, the number of produced scintillation electrons is too low ($\sim 250$), causing the signal to be lost in the background or excluded by the detector's trigger system.
• High-Velocity Detection ($v_S$): For faster gravitinos ($v_S \sim 230$ km/s), the signal is much stronger ($62,000$ electrons), producing $10^2$ to $10^4$ photoelectrons.
• Waveform and PSD Signature: Simulated waveforms for $v_S$ gravitinos show a signal distributed over a $\sim 6\ \mu\text{s}$ window. The $F_{prompt}$ values for these events fall between 0 and 0.1, placing them in the ER band. While this distinguishes them from some backgrounds, they reside in a low-event-rate region of the signal plot.
• Expected Event Rates: For DEAP-3600, the expected signal rate is approximately 0.15 events per 820 days of Run II data.

5. Significance

The paper concludes that while the current DEAP-3600 dataset is primarily useful for setting exclusion limits on high-velocity gravitino cross-sections, it serves as a vital proof-of-concept.

The study highlights that large-scale liquid argon (LAr) detectors (like the upcoming DarkSide-20k and ARGO) possess a significant sensitivity advantage over liquid xenon (LXe) detectors due to their larger dimensions and superior PSD capabilities. This work paves the way for a dedicated gravitino search analysis using the simulated signal models developed herein.