High-energy interactions in the region of extreme multiplicities

Hadron and nuclear interactions at high energies in the region of extreme multiplicities (much higher than mean multiplicities) are reviewed. Collective phenomena such as Bose-Einstein condensation, Cherenkov gluon radiation, clustering, and an excess yield of soft photons may manifest themselves in this region. The phenomenological gluon-dominance model developed for describing extreme multiplicities predicts the restrictions on their values and the type of hadronization mechanism and makes it possible to estimate thee size of the hadronization region. The status of the research within the Thermalization project at the U-70 accelerator of the Institute for High Energy Physics (IHEP, Protvino) is reported, and information about searches for new collective phenomena and about studies of known ones in the region of extreme multiplicities is given.