An experiment to test whether dark matter signals change throughout the year has found contradicting results to a previous experiment, announced an international team of researchers at the TAUP2015 conference in Italy this week.

There is strong evidence that dark matter exists in our universe, but no one has ever detected it. Among many candidates, it is thought to be some sort of particle, with the best candidate to date being a WIMP (weakly interacting massive particle). Positive results have already been reported in the past by the DAMA/LIBRA experiment in Italy, which had found evidence a dark matter signal modulated with a period of one year.

The XMASS (Xenon detector for weakly interacting MASSive particles) experiment in Kamioka, Japan, involving Kavli IPMU (Kavli Institute for the Physics and Mathematics of the Universe) Project Professor Yoichiro Suzuki, spent 16 months taking data using the XMASS detector. A dark matter model independent analysis found a weak modulation effect, but researchers say this could be explained by background fluctuation. In the model dependent analysis assuming standard WIMPs, researchers constrained the deposited energy spectrum to the expected WIMP recoil signal, but found their results differed significantly from those found in the DAMA/LIBRA experiment.

This is the first extensive search exploiting the annual modulation caused by the Earth’s rotation around the Sun.

In order to reduce systematic uncertainty and improve the XMASS result, the researchers will continue to take more data.

PIO Contact:
Kavli IPMU Press Office
+81 (0)4-7136-5977
press@ipmu.jp

Science Contact:
Yoichiro Suzuki
XMASS Collaboration Spokesperson
Deputy Director, Principal Investigator, Project Professor
Kavli Institute for the Physics and Mathematics of the Universe
The University of Tokyo Institutes for Advanced Study, The University of Tokyo
yoichiro.suzuki@ipmu.jp

XMASS website:
http://www-sk.icrr.u-tokyo.ac.jp/xmass/index-e.html

TAUP (Topics in Astroparticle and Underground Physics) 2015 website:
http://taup2015.to.infn.it/

The XMASS program is an inter-university cooperative research program at Kamioka underground Observatory. The XMASS experiment at Kamioka in Japan holds 832 kg of liquid xenon as a target material for the interaction of the dark matter particles. The detector is sensitive not only the nuclear recoil signal from WIMPs, but also the electron and gamma signals emitted from, for example, interactions with electrons in the Xe atoms of other candidates of dark matter.

XMASS Collaboration Members:
* Kamioka Observatory, Institute for Cosmic Ray Research, the University of Tokyo
* Kavli Institute for the Physics and Mathematics of the Universe, the University of Tokyo
* Information and Multimedia Center, Gifu University
* Department of Physics, Kobe University
* Miyagi University of Education
* Solar Terrestrial Environment Laboratory, Nagoya University
* Kobayashi-Masukawa Institute for the Origin of Particles and the universe, Nagoya University
* Department of Physics, Tokai University
* Department of Physics, Tokushima University
* Department of Physics, Faculty of Engineering, Yokohama National University
* Center for Underground Physics, Institute for Basic science, Korea
* Korea Research Institute of Standards and Science, Department of Physics, Korea