Nearly 400 days of data taken by the XENON collaboration were used to look for the telltale signature of dark matter, an event rate that varies periodically over the course of a year.
The null result of this search – the first of its kind using a liquid-xenon detector – strongly challenges dark-matter interpretations of the annual modulation observed by the DAMA/LIBRA experiments. Both subterranean experiments are operated at the Laboratori Nazionali del Gran Sasso (LNGS).
An annually varying flux of dark matter through the Earth is expected due to the Earth’s orbital motion around the Sun, which results in a change of relative velocity between the Earth and the dark-matter halo thought to encompass the Milky Way. The observation of such an annual modulation is considered to be a crucial aspect of the direct detection of dark matter.
The DAMA/LIBRA experiments have observed an annual modulation of the residual rate in their sodium-iodide detectors since 1998. However, previous null results from several experiments searching for dark-matter-induced nuclear recoils, including XENON100, have challenged such an interpretation of the DAMA/LIBRA signal.
An alternative explanation, that the DAMA/LIBRA signal is instead due to dark-matter interactions with electrons, is challenged strongly by the new results from XENON100. In studies recently published in Science and Physical Review Letters, three models that predict dark-matter interactions with electrons were considered. The very low rate of electronic recoils in XENON100 allowed these models to be ruled out with high probability.
The studies highlight the overall stability and low background of XENON100, a landmark performance achieved with this type of technology so far. Liquid-xenon detectors continue to lead the field of direct dark-matter detection in terms of their sensitivity to these rare processes. The commissioning of the next generation of XENON experiments at the underground site in LNGS is nearing completion. The detector, XENON1T, is expected to be 100 times more sensitive than its predecessor, and will hopefully shed more light on the elusive nature of dark matter.