"EMU-15" - an investigation of collective effects and peculiarities of the secondary-particle distributions in collisions of 32 TeV/nucleus incident lead nuclei with the target ones.

The main goal of the experiment is the study of possible signals from sub-hadronic states of high-temperature and high-density nuclear matter. The experiment was made in 1996 at CERN accelerator SPS by EMU-15 collaboration which incorporated the Russian scientists only and, in this respect, it was unique in CERN.

The experimental device of EMU-15 is a cylinder emulsion chamber with thin (400 microns) lead target and 38 emulsion layers of 50 microns each with base width 25 microns. One nuclear emulsion layer is placed just in front of the lead target, and the others are placed behind it. The chamber length is 260 mm and diameter is 95mm. In course of the experiment, the chambers were placed in the 2-Tesla transverse magnetic field, so that the lead target and the nuclear emulsions were oriented normally to the beam. The total lead nucleus irradiation of each chamber was 10000 particles.

The main methodic problem inherent in these investigations is associated with an enormous number of measurements, which are to be perfofmed in nuclear emulsion. For example, the full treatment of 100 high multiplicity events in 38 emulsion layers asks for about ten million measurements of the secondary particle track coordinates. Evidently, such work can be done in a reasonable time interval only making use of some completely automated instrument of the PAVICOM type.

The scheme of processing of each EMU-15 event is as follows:

  1. Image matching.
  2. Clustering (separation of individual blobs or groups of blobs).
  3. The interaction vertex localization.
  4. Tracking.

The first step of the data processing consists of the interaction image reconstruction in a single layer, i. e. at a fixed depth of emulsion. The Figure presents the event reconstructed in an emulsion layer obtained by one-by-one matching of 25 fields of view with total scanned square 1456 pixels x 1456 pixels.

After the binarization process and cutting off background, the emulsion blob clasterization is to be made. At this step, all the verging dark pixels are combined into clusters and centre-of-gravity coordinates for each cluster are found. The data file with information about centre-of-gravity coordinates is the result of processing clustering code.

The goal of the tracking code is to reconstruct the secondary particle tracks and their escape angles according to the cluster centre-of-gravity data. These calculations result in determination the interaction vertex coordinates.

The total duration of a single event processing at PAVICOM facility is about 8 hours only.

The processed experimental data make it possible studying peculiarities of particle angle distributions, search for unique low-probability events and multiparticle correlations which can be expected to occur under hadronization of the excited sub-hadronic matter. The distributions are examined in terms of the elaborated mathematical approach to find out the signals specific for quark-gluon plasma or some other states with unconfined coloured particles (for example, with massive constituent quarks, if they are quite stable quasiparticles).