The DARWIN Project

One of the most exciting topics in physics today is the nature of Dark Matter in the Universe. Although indirect evidence for cold dark matter is well established, its true nature is not yet known. The most promising explanation is Weakly Interacting Massive Particles (WIMPs), since they would naturally lead to the right abundance and since they are part of many of the required extensions of the Standard Model of particle physics. WIMPs, could be detected directly by their collisions with nuclei in underground experiments and such a discovery would be a milestone in physics. However, since the predicted signal rates are much lower than 1 interaction per kg of target material and day, large detector masses and ultra-low backgrounds are necessary ingredients of any experiment aiming to discover WIMPs.

Noble Liquid Detectors

Results from noble liquid detectors have recently shown that they are among the most promising technology to push the sensitivity of direct WIMP searches far beyond existing limits into the physically regime of theoretical predictions. Liquid argon (LAr) and xenon (LXe), having high charge and light yields for nuclear recoils expected from WIMP-nucleon scattering, are excellent WIMP targets. A noble liquid Time Projection Chamber (TPC) can offer a large, self-shielding, homogeneous and position sensitive WIMP detector. The relative size of the charge and light signals, as well as their timing allows efficient discrimination against electron recoil events, and good spatial resolution allows the identification of the neutron background.

The XENON10 and WARP experiments at the Gran Sasso Underground Laboratory (LNGS) were successful demonstrators, reaching competitive limits on the spin-independent WIMP-nucleon cross section of ~10^-43cm² and ~10^-42cm², respectively. The XENON100 and WARP-140kg experiments, 100kg-scale LXe and LAr TPCs, are under final commissioning at LNGS; the ArDM detector, with a LAr WIMP target of 850 kg, is under commissioning at CERN, with the goal of underground operation by 2010. While the aimed sensitivities are around 10^-45cm², a few events per year would be detected for a spin-independent cross section at the level of 10^-44cm².

The Way to a ton-scale Detector...

This project coordinates the European groups active in this field, bringing in new and valuable expertise from closely related fields, towards the R&D and design study for a ton to multi-ton scale LAr and LXe dark matter search facility. For a ton-scale liquid xenon TPC, some of the European groups are already active members of the XENON collaboration, which is moving towards XENON1T. To achieve similar sensitivity with a liquid argon TPC, the scale has to be multi-ton. Simultaneous charge and light readout provide the baseline design, with the added synergy offered by novel sensor technologies.

To convincingly demonstrate the dark matter nature of a signal, a measurement of the interaction rate with multiple targets is mandatory. Operating a LAr and a LXe target under similar experimental conditions would allow to measure the dependence of the rate with the target mass and to distinguish between spin-independent and spin-dependent couplings (⁴⁰Ar has no spin, while natural xenon contains ¹²⁹Xe and ¹³¹Xe). There are many common aspects to a LAr and a LXe dark matter TPC, starting from the cryostat design, to charge and light readout, to the purification of the noble liquids, to the HV systems required for charge drift and uniform field, use of ultra-low radioactivity materials and shields and the underground infrastructure and safety aspects, to mention just a few. This calls for a coordinated R&D and detector design effort among the groups spread out in different European countries, with a clear added value from close collaboration and interchange of ideas, information and results among these groups. Such an effort is in the spirit of the ASPERA roadmap, where it was recognized that strong coordination among the groups in Europe is necessary in order to build large infrastructure for particle astrophysics, in this case specifically for the direct detection of dark matter particles.

DARWIN and the ASPERA Roadmap