How Silicon Crystals Interfere with the Quest for Dark Matter

The detectors are meant to remain quiet. Cooled to just above absolute zero, buried deep underground, and shielded from cosmic rays, these highly sensitive devices are intended to capture the universe’s most fleeting particles. However, researchers at Texas A&M University have found that the crystals themselves are quite vocal. Uncontrolled energy surges keep occurring in bulk silicon substrates, generating phantom signals that closely resemble what scientists seek: dark matter.

The phenomenon, discussed in Applied Physics Letters, revolves around athermal phonons, minute packets of vibrational energy released by silicon without any external trigger. These energy bursts disrupt the superconducting sensors, creating low-energy background noise that has hindered experiments like TESSERACT and SuperCDMS over the years. This is particularly exasperating because the interference looks strikingly similar to the signal from a WIMP, or Weakly Interacting Massive Particle, a prime candidate for dark matter.

The noise that refuses to dissipate

Experimental particle physicist Rupak Mahapatra, who heads the research, has dedicated decades to perfecting cryogenic detectors capable of detecting interactions that may occur once in a decade. His team tracked phonon collection efficiency over several weeks as temperatures fell from 20 millikelvin to 6 millikelvin, well below the 2.7 Kelvin background temperature of space itself. The bursts continued unabated. Even mechanical disruptions, such as the simple act of banging the refrigerator containing the detector, excited narrow resonance peaks in the data.

The silicon seems to be either relaxing or internally shifting, potentially within the aluminum films applied to its surface or within the crystal lattice itself. These localized energy emissions induce quasiparticle poisoning in the sensors, overpowering the faint signals that researchers are pursuing. It’s akin to trying to catch a whisper in a room where the walls unpredictably creak and moan.

“The search requires extraordinarily sensitive sensing technologies and could lead to advancements we can hardly conceive of today,” Mahapatra notes.

Dark matter and dark energy collectively account for about 95% of the universe, yet neither emits, absorbs, nor reflects light. Scientists infer their presence through gravitational effects: dark matter serves as unseen scaffolding that binds galaxies, while dark energy propels the universe’s accelerating expansion. Mahapatra compares the challenge to describing an elephant by only feeling its tail. Gravity indicates that a substantial mass exists, but direct observation is still unattainable.

When the instrument becomes part of the experiment

Recognizing the internal noise of silicon is a step forward, even if it complicates the quest. By grasping how substrates misbehave at extreme cold, researchers can create purer materials or improve shielding techniques. Mahapatra highlighted in prior work that no individual experiment will crack the dark matter enigma. Direct detection must be paired with indirect astrophysical searches and collider-based methods, with each technique balancing the limitations of the others.

The discovery of phonon bursts may have wider implications extending beyond cosmology. Innovations aimed at mitigating this noise could have applications in quantum computing, medical imaging, or any field that demands measurement on the frontier of physical sensitivity. For now, though, the pressing challenge is to differentiate the universe’s whispers from the murmurs produced by the detectors themselves. When a genuine dark matter interaction is finally detected, scientists must be absolutely certain that it isn’t mere silicon conversing with itself.

Applied Physics Letters: 10.1063/5.0281876