A brief review of the search for isolatable fractional charge elementary particles

Since the initial measurements of the electron charge a century ago, experimenters have faced the persistent question as to whether elementary particles exist that have charges that are fractional multiples of the electron charge. In the standard model of particle physics the quarks are such particles, but it is assumed that quarks cannot be individually isolated, the quarks always being confined inside hadrons. This paper is a brief review of the present status of searches for isolatable fractional charge particles such as a lepton-like particle with fractional charge or an unconfined quark. There have been a very large number of searches but there is no confirmed evidence for existence of isolatable fractional charge particles. It may be that they do not exist, but it is also possible that they are very massive or that their production mechanisms are very small so that they have been missed by existing searches. Therefore the aim of this review is to urge (a) the invention of ways to substantially increase the range of known search methods and (b) to urge the invention of new search methods for isolatable fractional charge particles.