The missing mass problem
 
The gravitational behaviour of the Universe suggests that there is more matter present than is immediately detectable by the emanating radiation. It is estimated that ‘Dark Matter’ comprises about 90% of matter in the Universe in the form of weakly interacting massive particles (WIMPs). Such exotic particles have not yet been discovered but are predicted to exist by extensions of the Standard Model of Particle Physics used to describe matter and the forces in nature. It is also predicted that they might be detected directly by the low-energy recoils of atomic nuclei suffered on a WIMP collision.
A new kind of detector and observatory has recently been established with the aim of detecting WIMPs. The predicted rate of a nuclear recoil is less than 1 per 10 days kg⁻¹ of detector material. The equipment used as the detector is known as the DRIFT (Directional Recoil Identification From Tracks). It comprises 1 m³ of gas at low pressure with two multiwire proportional chambers (see earlier) to detect the tracks of ionization left by any nuclear recoil. At present, the readout from the system allows a two-dimensional track projection but future developments should provide a fully three-dimensional capability.
The principle behind the observations is that the directions of the recoils should change on a regular cycle following the Earth’s rotation. The motion of the Solar System through the Galaxy effectively induces a WIMP wind from the direction of travel, this being at an angle ∼42^◦ to Earth’s rotational pole. The wind vector, therefore, changes direction according to the rotational position of the sited detectors. For DRIFT at Boulby, UK (N hemisphere), the average recoil direction should vary from ‘downwards’ through the detector at midnight to ‘southwards’ at midday. Such a recorded signature will provide unique evidence for the existence of WIMPs.
The DRIFT system at Boulby is deep underground in an old coal mine to reduce interference from cosmic rays and other sources of noise. Although the observing location seems to be far away from ‘Astronomy’ this experimental research is most fundamental to our understanding of the Universe.