The collaboration that built and runs the Large Underground Xenon (LUX) experiment, operating in the Sanford Underground Research Laboratory, has released its first results in the search for weakly interacting massive particles (WIMPs) – a favoured candidate for dark matter.
The LUX detector holds 370 kg of liquid xenon, with 250 kg actively monitored in a dual-phase (liquid–gas) time-projection chamber measuring 47 cm in diameter and 48 cm in height (cathode-to-gate). If a WIMP strikes a xenon atom it recoils from other xenon atoms and emits photons and electrons. The electrons are drawn upwards by an electrical field and interact with a thin layer of xenon gas at the top of the tank, releasing more photons. Light detectors in the top and bottom of the tank can detect single photons and so the two photon signals – one at the interaction point, the other at the top of the tank – can be pinpointed to within a few millimetres. The energy of the interaction can be measured precisely from the brightness of the signals.
The detector was filled with liquid xenon in February and the first results, for data taken during April to August, represent the analysis of 85.3 live days of data with a fiducial volume of 118 kg. The data are consistent with a background-only hypothesis, allowing 90% confidence limits to be set on spin-independent WIMP–nucleon elastic scattering with a minimum upper limit on the cross-section of 7.6 ×10–46 cm2 at a WIMP mass of 33 GeV/c2. The data are in strong disagreement with low-mass WIMP signal interpretations of the results from several recent direct-detection experiments.