Correlations in multi-fragment events

The properties of excited nuclear matter and the quest for a phase transition which is expected to exist in this system are the subject of intensive investigations. High-energy nuclear collisions between finite nuclei which lead to matter fragmentation are used to investigate these properties. The present report covers effective work done on the subject over the two last decades. The analysis of experimental data is confronted with two major problems, the setting up of thermodynamic equilibrium in a time-dependent fragmentation process and the finite size of nuclei. The present status concerning the first point is presented. Simple classical models of disordered systems are derived starting with the generic bond percolation approach. These lattice and cellular equilibrium models, like percolation approaches, describe successfully experimental fragment multiplicity distributions. They also show the properties of systems which undergo a thermodynamic phase transition. Physical observables which are devised to show the existence and to fix the order of critical behaviour are presented. Applications to the models are shown. Thermodynamic properties of finite systems undergoing critical behaviour are advantageously described in the framework of the microcanonical ensemble. Applications to the designed models and to experimental data are presented and analysed. Perspectives of further developments of the field are suggested.

We investigate the thermodynamic content of a cellular model which describes nuclear fragmentation as a process taking place in an excited disordered system. The model which reproduces very well the size distribution of fragments does not show the existence of a first order phase transition.

We investigate an approach to the description of nuclear fragmentation relying on the concept of disorder which may characterise a fragmenting system of particles interacting by means of two-body short and long range potentials. We introduce different criteria in order to define clusters of bound nucleons. We work out the fragment content of events generated by means of numerical simulations. We analyse the moments of mass and charge distributions and related observables, as well as the energy content of the system. We discuss the outcome of the calculations in terms of the concept of universality which has been introduced in the field of nuclear fragmentation in the framework of percolation models.

The outcome of the fragmentation of finite nuclei is described in terms of a disordered system of interacting nucleons. Clusters are identified by means of different bond criteria for neighbouring particles. We discuss the possible universality properties of the system which are reflected in the moments of mass distributions and related physical observables. Results are also analyzed with reference to bond percolation approaches which predict these universality properties.

A review is made of the present status concerning the production of nuclei above 5 MeV temperature. Considerable progress has been made recently on the understanding of the formation and the fate of such hot nuclei. It appears that the nucleus seems more stable against temperature than predicted by static calculations. However, the occurence of multifragment production at high excitation energies is now well established. The various experimental features of the fragmentation process are discussed.

We discuss conditions and mechanisms through which intermittent fluctuations could be generated in final charge or mass distributions obtained in the fragmentation of complex systems. We analyse the specific case of power-law distributions and the results obtained by means of a class of break-up processes which are governed by non-linear rate equations.