Have We Really Found Dark Matter?
How SABRE aims to reveal the source of the DAMA collaboration’s mysterious signal
Dark matter makes up roughly 85% of the matter in the Universe. Although we are unable to see it, as it does not interact with light, observing its gravitational effects on objects such as galaxies and galaxy clusters provides us with evidence for its existence. Without the extra mass due to dark matter, entire galaxies would fly apart. Currently, the most popular candidate for dark matter is the Weakly Interacting Massive Particle (WIMP).
Since the 1990s, more and more experiments have joined the search for the WIMP, but the particles still evade detection. That is with the exception of the Italian DAMA collaboration, which, for the last decade, has been claiming to have found the elusive particle. However, their results do not match up with other experiments, which throws doubt upon their validity.
The Milky Way, like other spiral galaxies, comprises a thin “Galactic disk”, which consists of stars and gas, and a spherical “halo”, which is primarily made up of dark matter. The rotation of the Galactic Disk through the dark matter halo should result in a “WIMP wind” on Earth. Although WIMPs interact very weakly, this flow of particles is so large that we should be able to observe some of them scattering off nuclei. In fact, it is thought that trillions of these particles pass through our bodies every second. The method of looking for events where WIMPs collide with nuclei inside a detector is known as “direct detection”.
DAMA/LIBRA is a direct detection experiment, which releases energy that is converted into light when a WIMP collides with the sodium iodide target. However, unlike many detectors such as SuperCDMS, it searches for an “annual modulation” of the number of events. The WIMP wind would actually vary throughout the year, due to the orbital motion of the Earth around the Sun. The Earth is moving fastest in the direction of rotation of the Galactic Disk in early June, so the speed of the WIMP wind would be maximal at this time of year. Conversely, the Earth moves fastest in the direction opposite to the disk rotation in early December, so the wind would be at a minimum speed.
Ordinary direct detection experiments like SuperCDMS aim to observe a number of events large enough that we can be confident that they are not the result of background noise from other types of particle. This background radiation can cause collisions with electrons in the target material, which can be difficult to distinguish from the nucleus recoils due to WIMPs. DAMA/LIBRA, on the other hand, observes how the number of events varies throughout the year over several annual cycles. Levels of background radiation are thought to stay the same throughout the year, so do not contribute to the modulating signal.
Even if the background signals do not experience an annual modulation, we still need to minimise noise that would swamp the tiny dark matter signal by siting the experiment deep underground at the Laboratori Nazionali del Gran Sasso in Italy. This shields the detectors from highly penetrating charged particles known as muons, which can be generated by cosmic rays. The experiment consists of 25 detectors, but only “single-hit” events, which are seen in just one detector, are attributed to WIMP interactions, as events due to background radiation are more likely to appear in multiple detectors. Since 1997, DAMA has produced results which show an annual modulation consistent with WIMPs of a certain mass.
Because these results have not been found by other direct detection experiments, many physicists doubt their validity and have been looking for other explanations for the signal. Some speculated that the annual modulation was due to problems with the apparatus, and might disappear when the equipment was upgraded, but this was not the case. Another possible explanation would be seasonal variations in the conditions at Gran Sasso that are unrelated to WIMPS. Although the DAMA/LIBRA detector is buried deep beneath the Earth’s surface in order to minimise temperature fluctuations and effects of sunshine, there may be other seasonal variations that have not been accounted for.
To test this, two detectors will be built in the Northern and Southern hemispheres. If the annual modulation is opposite in the two detectors, so that the peaks in one set of results matches up with the troughs in the other, the seasonal variation explanation is supported, but if the annual modulation of the signals matches up, this theory can be discounted, leaving the WIMP wind as a viable explanation of DAMA’s results.
The two detectors are known collectively as the Sodium Iodide with Active Background Rejection Experiment (SABRE). The detector in the Northern hemisphere is located at the home of DAMA in Italy, whereas the detector in the Southern hemisphere will be sited at the Stawell Underground Physics Laboratory (SUPL) in Australia. Construction at SUPL has been delayed for several years due to the closing of Stawell gold mine, but is now set to go ahead, with $5 million of the Australian federal budget allocated to SUPL.
With other experiments such as ANAIS and COSINE-100 already beginning to produce data to be compared with DAMA/LIBRA, and SABRE soon starting to explore seasonal variations, we are getting ever closer to uncovering the truth behind the mysterious modulating signal and the nature of dark matter.
