Recent Theoretical Considerations on Ultra High Energy Nuclear Collisions

Recent theoretical works on ultra high energy nuclear collisions (cosmic ray jets) are surveyed. In view of the recent achievements of the relativistic Regge pole theory in the accelerator region and its further potentiality, the present review is mainly concerned with applications of this theory to ultra high energy nuclear interactions, including, however, a closely related phenomenological approach, too. The topics covered are: i) behaviour of cross sections at ultra high energy, ii) a necessary condition – tentatively called “shadow criterion” – imposed on any model of the multiple particle production, and two models which satisfy this requirement, iii) extension of the Regge pole theory to the production processes, and iv) general phenomenological considerations on the chain model.