First Limits on Light Dark Matter Interactions in a Low Threshold Two Channel Athermal Phonon Detector from the TESSERACT Collaboration

The TESSERACT Collaboration reports the first limits on light dark matter interactions using a low-threshold, two-channel athermal phonon detector operated above ground, achieving the best energy resolution for such devices and setting the most stringent constraints to date for dark matter masses between 44 and 87 MeV/$c^2$.

The Gist

The Big Picture: Hunting for "Ghost" Particles

Imagine the universe is filled with invisible "ghosts" called Dark Matter. We know they are there because they have gravity (they hold galaxies together), but they don't shine, reflect light, or interact with normal matter in any way we can easily see.

For decades, scientists have been trying to catch these ghosts. Most experiments have been looking for "heavy" ghosts (like bowling balls). But recently, physicists started wondering: What if the ghosts are tiny, like dust motes?

This paper is about a new, ultra-sensitive experiment that caught a glimpse of these "tiny dust mote" ghosts, or at least set the strictest rules yet on where they could be hiding.

The Detective: A Super-Sensitive Silicon Ear

The team built a detector that is essentially a silicon ear listening for the faintest whisper in the universe.

• The Material: They used a tiny piece of silicon (about the size of a postage stamp and as thick as a credit card).
• The Mechanism: Inside this silicon, they placed 50 tiny sensors called Transition Edge Sensors (TES). Think of these sensors as incredibly sensitive thermometers.
• The Goal: If a Dark Matter particle hits the silicon, it creates a tiny "ripple" of heat (called an athermal phonon). It's so small that it would barely warm up a cup of coffee, but these sensors can detect it.

The Problem: The "Low Energy Excess" (The Static Noise)

The biggest problem with hunting for these tiny ghosts is noise.
Imagine you are trying to hear a pin drop in a room where a construction crew is drilling. You can't hear the pin.

In previous experiments, scientists saw a lot of "noise" at very low energy levels. They called this the "Low Energy Excess." It was like static on a radio. They didn't know if this static was:

1. Real Dark Matter hits.
2. The detector itself getting stressed and creaking (like a floorboard settling).
3. Tiny defects in the metal sensors vibrating.

Because they couldn't tell the difference, they couldn't claim they found Dark Matter.

The Solution: The "Two-Channel" Trick

This is where the TESSERACT team got clever. They didn't just use one ear; they used two ears (two channels) listening to the same silicon chip.

Here is the analogy:

• Scenario A (The Ghost): If a Dark Matter particle hits the middle of the silicon chip, it sends ripples out in all directions. Both "ears" (channels) hear the sound at the exact same time, with roughly the same volume.
• Scenario B (The Noise): If the noise comes from a specific sensor film (like a loose wire vibrating), it only hits one ear. The other ear hears nothing.

By comparing the two ears, the team could instantly tell the difference. If both ears heard a sound together, it was a candidate for Dark Matter. If only one ear heard it, they could ignore it as noise.

The Results: The Quietest Room in the World

Because they could filter out the noise so effectively, they achieved something incredible: The best energy resolution ever recorded for this type of detector.

• The Analogy: Imagine trying to measure the weight of a single grain of sand on a scale that usually weighs elephants. Most scales would just say "0." This detector was so precise it could tell you the grain of sand weighed exactly 0.0003 grams.
• The Number: They measured energy with a precision of 361.5 milli-electron-volts. That is incredibly tiny.

What Did They Find?

They ran the detector for 12 hours (a short time for these experiments) and looked for the "tiny dust mote" ghosts.

1. No Ghosts Found (Yet): They didn't find a confirmed Dark Matter particle.
2. But they set a "No Trespassing" sign: Because their detector is so quiet and sensitive, they can now say with high confidence: "If Dark Matter exists with a mass between 44 and 87 MeV/c², it cannot be interacting with us as strongly as we thought."

They have ruled out a huge chunk of the "search space" for these light particles. It's like saying, "We checked the entire basement, and if the ghost is there, it's much quieter than we thought."

Why This Matters

This is a breakthrough for two reasons:

1. Sensitivity: They probed the lowest mass of Dark Matter ever tested in a direct detection experiment.
2. Methodology: They proved that using two channels to cancel out noise is a winning strategy. This opens the door for future experiments to use even bigger detectors to hunt for these elusive particles.

In summary: The TESSERACT team built a super-quiet, two-eared listening device made of silicon. They used it to listen for the faintest whispers of the universe. While they didn't catch the "ghost" this time, they proved the room is quiet enough that if the ghost is there, it's hiding in a very specific, tiny corner that they have now mapped out.

Technical Summary

1. Problem Statement

Direct detection of sub-GeV (sub-GeV/c²) Dark Matter (DM) is challenging because the expected nuclear recoil energies are extremely low (in the eV range). Previous experiments have been hindered by the "Low Energy Excess" (LEE), a background of unknown origin that mimics low-energy DM signals.

• The Specific Challenge: A subclass of LEE backgrounds has been identified as strongly coupling to the sensor films (aluminum fins) of the detector rather than the bulk substrate. These backgrounds produce signals primarily in a single readout channel, whereas genuine DM interactions in the silicon substrate are expected to produce "shared" signals across multiple channels.
• Goal: To establish the first stringent constraints on spin-independent DM-nucleon interactions for masses between 44 and 87 MeV/c² by discriminating against these sensor-coupled backgrounds using a multi-channel approach.

2. Methodology

Detector Architecture

• Target: A high-resolution silicon crystal (1 cm² area, 1 mm thick, 0.233 g mass).
• Sensors: 50 Tungsten Transition Edge Sensors (TESs) coupled to aluminum athermal phonon collection fins. This forms the QET (Quasiparticle-trap-assisted Electrothermal-feedback Transition edge sensor) architecture.
• Readout: The 50 QETs are wired into two independent channels (25 QETs each), read out separately by DC SQUID array amplifiers.
• Operation: The detector operates at ~48 mK in a dilution refrigerator. It was operated above ground (two floors below ground at UC Berkeley) without special shielding against high-energy backgrounds, as the focus is on low-energy discrimination.

Signal Discrimination Strategy (Two-Channel Technique)

The core innovation is the use of two-channel coincidence to distinguish DM from background:

1. DM Events (Shared): A DM particle scattering in the silicon substrate generates athermal phonons that propagate to both channels. These events produce coincident pulses with roughly equal amplitude in both channels (a "shared" event).
2. Background Events (Singles): Backgrounds coupling to the sensor films (e.g., stress relaxation, sensor defects) deposit energy locally. These produce sharp rise-time pulses in only one channel ("singles" events).

Data Analysis Pipeline

• Optimal Filtering (OF): The team developed an $N \times M$ Optimal Filter (where $N=2$ channels and $M$ signal shapes) to extract signal amplitudes and start times from noisy data streams.
• Event Classification:
  • A 2 $\times$ 2 OF is used to reconstruct energy in both channels independently.
  • $\delta\chi^2$ Statistics: Three 2 $\times$ 1 OFs are fitted to each event (assuming: shared topology, left-channel singles, right-channel singles). The differences in $\chi^2$ values ($\delta\chi^2_{SL}$, $\delta\chi^2_{SR}$) determine if an event is "shared-like" (DM candidate) or "singles-like" (background).
• Salting Technique: To measure efficiency and energy resolution without bias, the team injected "salt" pulses (simulated ideal signals) into the raw data stream. This allowed for the precise modeling of the detector's response to sub-threshold events and the calculation of the net differential sensitivity.

3. Key Contributions

1. Record-Breaking Energy Resolution: The detector achieved a root-mean-square (r.m.s.) baseline energy resolution of 361.5 ± 0.4 meV. This is the best resolution reported to date for any athermal phonon detector, enabling the detection of extremely low-energy recoils.
2. Novel Background Rejection: The successful implementation of a two-channel coincidence requirement effectively rejected sensor-coupled backgrounds (singles), which had previously plagued low-threshold searches.
3. Lowest Mass Probed: By utilizing the "salting" technique and excellent resolution, the experiment probed DM masses as low as 44 MeV/c², the lowest mass ever probed by a particle-like DM direct detection experiment.
4. Above-Ground Operation: Demonstrated that high-sensitivity DM searches can be conducted above ground if the detector threshold is low enough and background discrimination is robust, bypassing the need for deep underground facilities for this specific mass range.

4. Results

• Exposure: 0.233 g $\times$ 12 hours.
• Exclusion Limits:
  • Established the most stringent constraints on spin-independent DM-nucleon cross-sections for masses between 44 MeV/c² and 87 MeV/c².
  • Lowest Cross-Section: Reached a cross-section limit of $4 \times 10^{-32}$ cm² at a DM mass of 87 MeV/c².
  • Lowest Mass Limit: Set a constraint of $4.67 \times 10^{-30}$ cm² at 44 MeV/c².
  • Best Sensitivity: The most stringent bound was found at 500 MeV/c² with a cross-section of $6.56 \times 10^{-35}$ cm².
• Pileup Constraint: The analysis excluded regions where DM interaction rates would cause significant signal pileup (above ~5.5 kHz), ensuring the validity of the linear differential rate modeling used.

5. Significance

This work represents a major milestone in the search for Light Dark Matter (LDM):

• Parameter Space Exploration: It opens a new window into the sub-GeV mass regime, testing models such as ELDERs, SIMPs, and freeze-in DM that were previously inaccessible to traditional WIMP detectors.
• Technological Validation: It validates the TESSERACT initiative's approach of using low-threshold, multi-channel superconducting sensors to overcome the "Low Energy Excess" background problem.
• Future Outlook: The results demonstrate that with further improvements (e.g., using Gallium Arsenide or superfluid Helium targets as mentioned in the conclusion), the sensitivity to even lower masses and higher cross-sections can be achieved, potentially revolutionizing the landscape of direct DM detection.

In summary, the TESSERACT collaboration has successfully demonstrated that a low-threshold, two-channel athermal phonon detector can achieve world-leading energy resolution and background rejection, setting the first meaningful limits on dark matter interactions in the 44–87 MeV/c² mass range.