A larger detector with improved photo-sensors

The ~1 ton detector has not yet found evidence for dark matter particle interactions. This could simply be due to the detector being too small for the unknown true event rate. To explore lower event rates, a larger target mass is necessary. With a 5 ton liquid xenon target and a reduced background rate XMASS-1.5 will significantly improve the sensitivity of our dark matter searches.

It can be due to the smallness of the detector and thus the event rate caused by dark matter particles is too small. It is necessary to increase an expected rate of dark matter signal by increasing target mass of a detector and to reduce noise events (background events) for dark matter signal. XMASS-1.5 is the project to realize significant improvement on its sensitivity using 5ton of liquid xenon target.

Changing the shape of the photo-sensors (photomultiplier tubes or PMTs for short) is the most important measure with regard to reducing the background rate in this next generation detector. As shown in Figure 1 their photo-sensitive surface will be dome-shaped. The important difference to the current flat surface PMT design is that the new PMTs will be able to detect light arriving from the side. The development of these new PMTs is almost complete; final details of the new design are currently being evaluated on samples already delivered.

A final, much larger detector: A multi-purpose discover machine

The XMASS project in its final stage not only aims to detect dark matter particles, but also to significantly improve the measurement of solar pp-neutrinos and discover neutrinoless double beta decay in xenon.

XMASS-II will contain more than 20 tons of liquid xenon, allowing it to measure 10^-20 pp-neutrino interactions per day and down to much lower energies than other pp-neutrino detectors. Discovering neutrinoless double beta decay would clarify the origin of neutrino mass and simultaneously provide an absolute measurement of this mass.

This future XMASS-II detector will be a true multi-purpose detector and discovery tool from which we can expect far reaching physics results.