Study of Argon from Underground Sources for Dark Matter Detection

Abstract

There has been accumulating evidence for the existence of dark matter in the universe, but its nature remains a deep mystery. Weakly Interacting Massive Particles (WIMPs) are likely dark matter candidates, which may scatter with nuclei and get detected. Several experiments have been launched in attempts to detect WIMP-induced nuclear recoils, but no conclusive detection has been confirmed. Now it is generally believed that ton-scale detectors are required for sensitive WIMP detection, and this may go beyond the reach of traditional low background technologies. Detectors using noble liquids such as argon offer this desired scalability, and argon has also been shown to have excellent background rejection power from scintillation pulse shape analysis. The problem with argon, however, lies in the cosmogenic <super>39</super>Ar isotope in atmospheric argon; it restricts the ultimate size of argon Time Projection Chambers (TPCs) and also reduces the detectors' WIMP sensitivity.

The DarkSide collaboration has successfully located underground sources of argon with extremely low <super>39</super>Ar levels that are below the sensitivity of all known measurement techniques. Therefore, we developed a low background liquid argon detector to measure the <super>39</super>Ar concentration in this underground argon. The detector was constructed with radio-clean materials and was carefully shielded from backgrounds. Using the data acquired in an underground laboratory (1450 m.w.e.) combined with Monte Carlo techniques, we obtained an <super>39</super>Ar limit in samples of underground argon at 0.65% of the concentration in atmospheric argon, 10 times better than the previously best result. This measurement demonstrates the possibility of achieving ultra-low background levels in argon-based detectors, and may extend the mass limit of argon TPCs by 45 times. Multi-ton detectors using underground argon are expected to cover a large fraction of the WIMP parameter space predicted by popular WIMP theories. I also discuss the possible application of the underground argon in the detection of low mass WIMP interactions and coherent neutrino scattering.