SENSEI at SNOLAB: Single-Electron Event Rate and Implications for Dark Matter

Following a major hardware upgrade at SNOLAB, the SENSEI experiment achieved an order-of-magnitude reduction in single-electron background rates to $(1.39 \pm 0.11) \times 10^{-5}$ e$^-$/pix/day, enabling tighter constraints on sub-GeV dark matter and validating that previous higher rates were likely caused by light leaks in the older sensor tray design.

The Gist

The Big Picture: Hunting Ghosts with Super-Sensitive Cameras

Imagine you are trying to catch a ghost. But this isn't a spooky sheet; it's Dark Matter, a mysterious substance that makes up most of the universe but refuses to interact with normal stuff. Scientists believe that sometimes, a tiny speck of dark matter might bump into an electron inside a silicon chip, giving it a tiny "kick" that creates just one single electron.

The SENSEI experiment is like a super-powered camera designed to catch that single electron. The problem? The camera is so sensitive that it often gets "false alarms." These alarms come from heat, stray light, or electronic noise, which also look like a single electron. To find the ghost (dark matter), the scientists had to make their camera so quiet that it could hear a pin drop in a library.

The Upgrade: From a Leaky Bucket to a Sealed Vault

The paper describes a major upgrade the team made in 2023. Think of their old setup like a bucket with holes in it. Even though they were trying to keep the water (the signal) inside, light and heat were leaking in through the cracks, creating a lot of "noise" (fake electron events).

What did they fix?

1. New Trays: They replaced the copper trays holding the cameras with a brand-new design. The old trays had gaps and open corners (like a sieve), letting in invisible infrared light from the warm walls of the lab. The new trays are like a sealed, light-tight vault.
2. More Sensors: They added 16 new sensors to the mix, making the "net" bigger.
3. Better Cooling: They kept the cameras extremely cold (about -128°C) to stop the silicon itself from getting jittery and creating fake signals.

The Result: A Whisper in a Hurricane

Before this upgrade, the camera was hearing a constant hum of noise. After the upgrade, the noise dropped dramatically.

• The Old Noise: Imagine a crowded room where everyone is shouting. You can't hear the person you are looking for.
• The New Noise: The room is now a silent library. The team measured the background noise (the "single-electron rate") and found it was 10 times lower than ever before.

They measured a rate of about 1.39 single electrons per pixel per day. To put that in perspective, if a pixel were a grain of sand, they are now counting the number of times a single grain of sand spontaneously jumps up on a beach, and they found it happens incredibly rarely. This is the quietest silicon detector in the world.

The "Light Leak" Mystery Solved

The scientists had a hunch that their previous experiments (done in a different location called MINOS and an earlier run at SNOLAB) had too much noise because of light leaks.

To prove this, they went back to the MINOS lab and did a fun experiment:

• They took a camera and shined an LED light at it through the gaps in the old tray design.
• Result: The camera went crazy with fake signals. It was like shining a flashlight into a dark room and seeing the dust motes dance everywhere.
• Then, they swapped in the new, sealed trays and taped up the gaps.
• Result: The fake signals dropped by more than half.

This confirmed that their previous "noise" was actually just infrared light (heat radiation) sneaking in through the cracks of the old trays, tricking the camera into thinking it saw dark matter.

Why Does This Matter? (The Dark Matter Connection)

Now that the camera is so quiet, the scientists can set much stricter rules on where dark matter could be hiding.

• The Analogy: Imagine you are looking for a specific type of bird in a forest. If the forest is full of noisy squirrels (background noise), you might think you saw the bird when it was just a squirrel. But if you silence the squirrels, and you still don't see the bird, you can say with much more confidence: "Okay, this bird definitely doesn't exist in this part of the forest."

By lowering the noise, the SENSEI team has ruled out many theories about what dark matter might be. They have pushed the "search boundaries" further than anyone else has before. If dark matter exists and interacts with electrons in the way they are looking for, they are now much closer to finding it. If they don't find it, they have proven that those specific theories are wrong.

The Bottom Line

The SENSEI team at SNOLAB fixed the "leaks" in their experiment, turning a noisy, leaky bucket into a silent, sealed vault. This allowed them to hear the faintest whisper of a single electron ever recorded. While they haven't caught the dark matter ghost yet, they have cleared the room of all the distractions, making the hunt for the real thing more serious and promising than ever before.

Technical Summary

1. Problem Statement

The search for sub-GeV dark matter (DM) using silicon-based detectors relies on detecting the tiny energy deposits (1–few electrons) produced when dark matter scatters off electrons. The primary limitation in these searches is the single-electron ($1e^-$) background rate.

• The Challenge: Unlike higher-energy events, $1e^-$ backgrounds cannot be modeled independently or subtracted because their rate is currently unknown and dominates the signal region. Coincident $1e^-$ events in neighboring pixels can mimic higher-energy signals, but the fundamental limit is the intrinsic $1e^-$ rate.
• Previous Limitations: Previous SENSEI runs achieved rates around $1.59 \times 10^{-4} \, e^-/\text{pix/day}$. A significant portion of this background was suspected to be caused by light leaks (blackbody radiation) entering the detector through gaps in the copper trays housing the Skipper-CCDs, rather than intrinsic detector noise.

2. Methodology

The authors conducted a new science run at SNOLAB (a deep underground laboratory) using an upgraded SENSEI apparatus.

• Hardware Upgrades:
  • New Trays: Replaced the copper trays housing the CCDs with a new, light-tight design featuring closed corners and improved shielding to prevent blackbody radiation leaks.
  • Sensor Deployment: Deployed 16 new sensors (totaling 22 CCDs) with a total active mass of $\sim 48$ grams.
  • Readout Strategy: Implemented "hardware binning" during readout, summing 32 rows into a single "superpixel" to reduce the impact of serial register spurious charge.
• Data Acquisition:
  • Collected data between November 2023 and February 2024.
  • Utilized a "blind" analysis strategy: Data was split into "commissioning" (used to optimize cuts) and "hidden" (used for final results) datasets.
  • Exposure times varied (0, 2, 6, and 20 hours) to distinguish between exposure-dependent rates (signal + thermal/intrinsic backgrounds) and exposure-independent densities (spurious charge/amplifier light).
• Analysis Techniques:
  • Applied fiducial cuts (masks) to remove hot pixels, columns, and bleeding zones.
  • Used a double-Gaussian fit on the charge histogram to extract the number of $1e^-$ events and noise.
  • Identified "Golden" and "Witness" quadrants to monitor cryocooler performance and data quality, masking images with elevated noise.
• Hypothesis Testing (MINOS Comparison):
  • To verify the light-leak hypothesis, the team performed a controlled test at the MINOS cavern (Fermilab).
  • They installed identical Skipper-CCDs in the old tray design (with known light leaks) and the new design.
  • They illuminated the trays with an LED to simulate light leaks and measured the resulting $1e^-$ rates before and after replacing the trays.

3. Key Contributions

• Record-Breaking Sensitivity: Achieved the lowest single-electron event rate ever recorded in a silicon detector and the lowest for any photon detector in the near-infrared-ultraviolet range.
• Decomposition of Backgrounds: Successfully separated the $1e^-$ rate into exposure-dependent and exposure-independent components, allowing for a more precise determination of the intrinsic detector noise.
• Light-Leak Verification: Provided compelling experimental evidence that light leaks from blackbody radiation were the dominant source of the elevated $1e^-$ rates in previous runs, validating the new tray design.
• Dark Matter Constraints: Set the most stringent limits to date on sub-GeV dark matter scattering off electrons via light mediators and dark photon absorption.

4. Results

• Single-Electron Rate:
  • Measured an exposure-dependent rate of $(1.39 \pm 0.11) \times 10^{-5} \, e^-/\text{pix/day}$ in the "Golden Quadrant."
  • This corresponds to $(39.8 \pm 3.1) \, e^-/\text{gram/day}$.
  • This represents an order-of-magnitude improvement over the previous best result ($1.59 \times 10^{-4} \, e^-/\text{pix/day}$).
  • The 90% confidence level upper bound is set at $1.53 \times 10^{-5} \, e^-/\text{pix/day}$.
• Exposure-Independent Density:
  • The exposure-independent density was measured at $6.94 \pm 0.85 \times 10^{-5} \, e^-/\text{superpix/image}$.
• Light-Leak Hypothesis Confirmation:
  • In the MINOS test, the old tray design yielded a rate of $\sim 8 \times 10^{-5} \, e^-/\text{pix/day}$.
  • After switching to the new closed-corner trays and sealing edges with copper tape, the rate dropped to $3.43 \times 10^{-5} \, e^-/\text{pix/day}$ (a factor of $>2$ improvement).
  • Visual evidence (Fig. 3) showed a distinct pattern of light leakage through tray edges in the old design, which was significantly suppressed in the new design.
• High-Energy Background:
  • The background rate in the 500 eV–10 keV range improved to $\sim 50$ events/kg/day/keV, a factor of 3 lower than the previous SNOLAB run, without dedicated high-energy background reduction efforts.

5. Significance

• Dark Matter Sensitivity: The reduction in the $1e^-$ background rate directly translates to a significantly improved sensitivity for sub-GeV dark matter. The new constraints (Fig. 4) exclude a larger parameter space for dark matter-electron scattering cross-sections ($\sigma_e$) and dark photon kinetic mixing parameters ($\epsilon$), particularly for dark matter masses below 10 MeV.
• Detector Physics: The study conclusively demonstrates that environmental factors (specifically blackbody radiation leaking through mechanical gaps) can dominate the noise floor in ultra-low-background experiments. This provides a critical roadmap for future detector designs, emphasizing the need for rigorous light-tightness in cryogenic environments.
• Future Outlook: The results validate the current design for the next generation of SENSEI detectors. The collaboration plans to further push these boundaries with cleaner shields, improved packaging, and advanced analysis techniques to continue the search for light dark matter.