Revealing intermittency in nuclear multifragmentation with 4 pi detectors.

The distortion on the intermittency signal, due to detection efficiency and to the presence of pre–equilibrium emitted particles, is studied in a schematic model of nuclear multi-fragmentation. The source of the intermittency signal is modeled with a percolating system. The efficiency is schematized by a simple function of the fragment size, and the presence of pre–equilibrium particles is simulated by an additional non–critical fragment source. No selection on the events is considered, and therefore all events are used to calculate the moments. It is found that, despite the absence of event selection, the intermittency signal is quite resistant to the distortion due to the apparatus efficiency, while the inclusion of pre–equilibrium particles in the moment calculation can substantially reduce the strength of the signal. Pre–equilibrium particles should be therefore carefully separated from the rest of the detected fragments, before the intermittency analysis on experimental charge or mass distributions is carried out. 1 In high energy proton–nucleus and nucleus–nucleus collisions, as well as in heavy– ion reactions at intermediate energies, events with very large charge particle multiplicities are observed. The theoretical analysis of these events is hindered by the complexity of the reaction mechanisms, and often biased by model dependent assumptions. In the experimental data large fluctuations in various physical quantities are apparent, either in each event, either from one event to another. In recent years it was realized [1-3] that these fluctuations are quite useful for the study of the nuclear dynamics and the reaction mechanisms. They can be analyzed in terms of global variables, whose trends can be used to characterize high multiplicity events in an essentially model independent way. In particular, the intermittency analysis [2,3] has proven to be one of the most powerful and promising method to this respect. Intermittency patterns have been experimentally observed, both for rapidity charge particle distributions in high energy proton–nucleus collisions [2], and for fragment charge distributions in nuclear emulsion data on projectile multifragmentation in peripheral heavy–ion collisions at energies around 1 GeV/A [4]. The intermittency analysis of the fluctuations is essentially a multifractal analysis of a given distribution. This method has been developed in many fields of physics, ranging from hydrodynamics to astrophysics. For the problem in exam, the intermittency analysis is performed by studying the behaviour of the moments of the distribution, as one varies the resolution δs with which the distribution itself is considered. Only event to event fluctuations …