Dark matter detected?

The Cryogenic Dark Matter Search (CDMS) project is announcing on Friday 18th December, the possible detection of two dark matter particles. However, the chances of a false positive here are estimated to be 23%, which is rather high compared to the threshold of 5% typically employed in measurement science, and extremely high compared to the threshold of 0.1% stipulated by the CDMS team.

The CDMS detector resides in an abandoned iron mine in Minnesota, (to minimise the background neutron radiation from cosmic rays), and consists of germanium discs cryogenically cooled to an extremely low temperature. The discs are coated with phonon sensors, designed to detect the tiny vibrations in such a crystalline solid. When the crystals are cooled to a very low temperature, the vibrational background of the solid is virtually eliminated, and it is possible to detect the phonons created by incoming particles colliding with the atoms in the solid.

These phonons, however, can be created by background gamma radiation colliding with the electrons in the solid, or by hypothetical dark matter particles colliding with the nuclei of the atoms. The two different processes create different patterns of ionisation, hence the detection of dark matter particles requires the simultaneous detection of the phonons and the ionisation in the crystals. (See Dark Side of the Universe, Iain Nicolson, p84-85).