Terrestrial searches for new stable particles

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Abstract
If new types of stable charged particle exist they could be present at a low concentration in terrestrial materials, either surviving from the early universe, or accumulated from cosmic-ray interactions during the Earth's lifetime. In the case of electrically neutral particles, these would be most likely to exist freely in space, but could have clustered gravitationally with stellar matter during the evolution of the universe and could account for the unidentified dark matter already known to comprise 90% of the mass of our galaxy. Some of the main experiments which have been carried out to search for charged particles in terrestrial materials, and planned experiments to search for galactic dark matter particles are described. A brief summary of the present status of particle physics is given, to illustrate the possibilities for the existence of new stable particles. A sensitive search for heavy charged particles, in the form of anomalous hydrogen-like atoms in natural water, has yielded a null result of importance for the theory of ‘supersymmetry’. Stimulated by the quark model, a variety of searches have been made for free fractional charge, also with null results, although these fractional-charge searches have still not reached the very low concentration levels which would result from production by cosmic rays. Plans for dark-matter experiments include the development of new microwave detectors for very light particles (axions) and sensitive low-temperature detectors for heavy particles (photinos). This account is presented mainly from the viewpoint of the author's own research programme, and is intended to give some insight into the motivation for the experiments, how the experimental ideas evolved, and some of the unusual techniques which had to be developed, or are still being explored, to enable these sensitive searches to be carried out.