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I present the measurements, analysis and results obtained with the NA34 experiment at the CERN European laboratory, on the global characteristics of particle production in ultra-relativistic oxygen-nucleus collisions, at incident energies of 60 and 200 GeV/nucleon. The observed properties of the particle flow are analysed in the framework of phenomenological models inspired by quantum chromodynamics. From this analysis, novel information emerges concerning the mechanisms and space-time evolution of the soft-hadronic processes in short-lived extended volumes of matter at extreme densities and temperature. We consequently study the parameters and critical conditions under which nuclear collisions could allow the observation of a phase transition from hadronic matter towards a new state of deconfined quark-gluon plasma matter.

The development of a neural network based electron finder designed for the ZEUS calorimeter is described. A study of elastic and inelastic J/ψ meson production in electron-proton collisions at HERA using the ZEUS detector is presented. During the 1993 and 1994 running periods, the ZEUS detector collected ~ 500 nb^-1 and ~ 3 pb^-1 of data respectively. The direct inelastic J/ψ photoproduction cross-section is measured for both running periods and an attempt is made to extract the gluon density inside the proton using the 1994 inelastic J/ψ sample.

This thesis is concerned with non-adiabatic effects in alkali/halogen collision systems. After a general survey of non-adiabatic molecular collision theory, and its application to such systems, calculations have been performed relating to three topics : (i) Vibrational energy distributions resulting from electron transfer collisions between alkali metal atoms and halogen molecules are often treated by the multi-curve crossing approach to the classical path approximation, in the form of two approximations, one valid at high collision energy, the other at low energy; the performance of these is evaluated over a wide energy range by comparison with the results from an "exact" multi-curve crossing approach. (ii) Classical trajectory calculations for reactive alkali / halogen collisions are performed using a simple ionic potential energy surface, developed earlier to model the weakening of the halogen bond by the metal ion immediately after the electron transfer. Detailed comparison is made between the results and those from experiment; good agreement is obtained for K/I₂ and comparison of the results for K/I₂ and Cs/I₂ indicates that this effect is not merely dependent on the charge of the ion. (iii) The population of ground and excited sodium atoms resulting from Na/I charge neutralisation collisions is investigated using the multi-curve crossing approach. Although it has been suggested that population inversions may be produced by such a process, it is shown that this is only possible at extremely high collision energies.