Uncovering dark matter is possible, but not simple

The latest results from an underground detector in Japan looking for dark matter have reignited the debate about its identity.

“Using different technology than other xenon detectors, XMASS may see signals that other experiments cannot observe,” the researcher says.

No one has ever seen dark matter. But scientists know it exists because, by doing the math, they have found there is a larger than expected amount of gravity keeping the universe and everything inside it together. The most likely explanation is the existence of invisible matter that has not yet been detected.

Yoichiro Suzuki, a principal investigator at the Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU), has led the development and use of XMASS (xenon detector for weakly interacting massive particles). This is one of a handful of detectors in the world built to detect weakly interacting massive particle (WIMP) signals, a popular theoretical notion that predicts the nature of dark matter.

XMASS is able to collect highly accurate statistical data. It is unique because it is the first dark matter detector to be placed inside a water tank, where water acts as a shield against subatomic particles such as high-energy muons and other interferences from the atmosphere. If a WIMP collides with atomic nuclei inside the 800-kilogram liquid xenon detector, its interaction is picked up by XMASS as a signal.

“Using different technology than other xenon detectors, XMASS may see signals that other experiments cannot observe,” says Dr Suzuki.

In September 2015, XMASS researchers announced their latest results after 16 months of recording data. Their findings did not produce convincing evidence in line with another underground project, the DAMA-LIBRA experiment in Italy, which claimed it had discovered dark matter. Previously, DAMA-LIBRA researchers announced they had found evidence of annual modulation: seasonal fluctuations in dark matter signals as the earth moves through a band of dark matter particles while circling the sun.

While XMASS found a weak modulation effect, the team says the result could be explained by background fluctuation. But no one will know for certain what these fluctuations actually represent until more data is taken.

Did you know?
In November 2015, the 2016 Breakthrough Prize in Fundamental Physics was awarded to seven scientists representing five neutrino research projects around the world, including Kavli IPMU Project Professor Yoichiro Suzuki and the 2015 Nobel Laureate and Institute for Cosmic Ray Research Director Takaaki Kajita. These two have worked together as members of the Super-Kamiokande research team.

Did you know?
XMASS is an 800kg dark matter detector located one kilometre inside an old mine at the University of Tokyo’s Kamioka Observatory in central Japan. Its isolated location keeps out cosmic rays, which create too much noise for researchers trying to see the liquid xenon inside the detector interact with WIMPs. The XMASS project is a collaboration between 12 institutions in Japan and Korea. It is currently operated by the University of Tokyo Institute for Cosmic Ray Research.

For further information contact:
Motoko Kakubayashi
Kavli Institute for the Physics and Mathematics of the Universe
E-mail: [email protected]