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Radioactive radon settles on detector components and creates interference that mimics signals from dark matter particles.
Abstract
The dual-phase xenon time projection chamber (TPC) is a powerful instrument for spotting rare interactions, like dark matter particles scattering off nuclei. Thanks to built-in amplification of ionization signals, a dual-phase TPC can be sensitive to events with just a few electrons—a must-have for searching for light dark matter. However, detection based only on ionization at low energies is limited by a surplus electron background seen in all experiments. The main culprit: the settling of radon-222 decay chain isotopes onto the surfaces of high-voltage grids that span the entire TPC cross-section. A first-principles model of this background is presented, with predictions that match data from LZ and LUX. Strategies for suppressing the background in future dual-phase TPCs are discussed, along with the model's potential for analyzing dark matter searches using the ionization channel.
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A dark matter detector is like a room where you're trying to hear a whisper while static electricity crackles nearby. This crackle is produced by radon atoms—a radioactive gas that seeps out from materials and air. Settling on metal grids inside the liquid xenon chamber, they knock out electrons when decaying, mimicking the signal from a dark matter particle.
Physicists built a model of this noise and compared it with data from the LUX and LZ experiments. The calculations matched, confirming the culprit. Now it's clear: to hear the whisper, you need to get rid of the crackle—either by cleaning the components more thoroughly or by changing the design, since the sought-after events carry energy of just a few electrons.
A curious twist: radon is born from uranium, which is present in trace amounts in the materials of the detector itself—the setup creates its own interference.
The interference distorts the energy spectrum—the distribution of electrons where physicists look for anomalies pointing to new physics beyond the standard model. Eliminating this background will bring us closer to solving the mystery of the Universe's hidden mass.
🎯 Radon, which hinders the search for dark matter, is the same gas that accumulates in basements and is the second leading cause of lung cancer.
