The WARP programme (pdf) is a graded programme intended to search for Cold Dark Matter in the form of weakly interacting, massive sub-atomic sized particles known as WIMP's (Weakly Interacting Massive Particles). These particles may produce via weak interactions nuclear recoils in the energy range 10–100 keV. A cryogenic noble liquid detector such as Argon, unlike an ordinary scintillator, permits the simultaneous detection of both ionisation and scintillation.
Already in 1993 the ICARUS collaboration, in the framework of a systematic study of ultra-pure noble liquids, pointed out for the first time that this property could lead to a unique signature for the energy deposited in the medium by the recoiling nucleus. It was then shown experimentally that, due to the high ionisation density of the recoiling nucleus, the final ionisation signal is strongly suppressed because of electron recombination, while scintillation light will still be produced from the excited states. Therefore electron mediated background events due to cosmic rays and the residual radioactivity in the detector elements and in the regions surrounding the detector itself may be easily rejected, since they generally deposit energy in the liquid close to minimum ionisation.
Initial tests were performed with Xenon. We have switched to Argon, which has even more favourable features and which can be produced in large volumes at a low cost, as already proven by the ICARUS experiment.
The main concept of the inner double phase argon detector is sketched on the right. When a particle interacts in the liquid region excitation and ionization occur
The aim of experiment is to detect nuclear recoils and their spectrum, possibly induced by WIMPs interactions
in the detector, in the energy range 10-100 keV: in this range the substantial background induced by gammas and electrons is strongly
suppressed by the proposed techniques that are used as discrimination methode.