There is so much evidence of the existence of dark matter in the Universe. However, all the evidence is acquired through gravitational effect so far and therefore the nature of dark matter is not understood very well. Various models have been proposed to explain the dark matter. In order to make a direct detection of such dark matter in terrestrial experiments, it needs at least one interaction other than the gravitational one. Well-motivated models incorporating the dark matter particle as a weakly interacting thermal relic are provided by various extensions of the standard model of particle physics. Such a WIMP fits the cold dark matter (CDM) paradigm.
On the other hand, simulations based on this CDM scenario expect a richer structure on galactic scales than those observed. Furthermore, there is so far no evidence of super-symmetric particles by the high-energy collider experiments therefore, it is important to investigate various types of dark matter candidates. These facts strengthen an interest to consider lighter and much more weakly interacting particles such as bosonic super-WIMPs, a candidate for warm dark matter. If the mass of the super-WIMPs is above 3 keV, there is no conflict with structure formation in the Universe. Bosonic super-WIMPs are experimentally attractive since their absorption in a target material would deposit energy essentially equivalent to the super-WIMP’s rest mass.
The XMASS experiment has conducted the direct search for such bosonic super-WIMPS, especially the vector and the pseudoscalar cases, in the mass range between 40 and 120 keV. XMASS is a liquid xenon detector using about 1 tons of liquid xenon as target material. With the use of 165.9 day of data, no significant excess above background was observed in the fiducial mass of 41 kg. The present limit for the vector super-WIMPs, the first such measurement, has excluded the possibility that such particles constitute all of dark matter (Fig. 1). The absence of a signal also provides the most stringent direct constraint on the coupling constant of pseudo-scalar super-WIMPs to electrons. This unprecedented sensitivity was achieved exploiting the low background nature of the detector at a level 10−4day−1kg−1keVee−1.
This result was published in September 19th issue of Physical Review Letters as an Editors’ Suggestion.
Figure 1: Limits on coupling constants for (a) electrons and pseudoscalar bosons and (b) electrons and vector bosons at 90% C.L. (thick red line). The black line corresponds to the coupling constant required to reproduce the observed dark matter abundance. The dotted lines and dashed lines are constraints from other experiments or astrophysical arguments.