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Conclusions

This chapter summarizes the main results and conclusions of the thesis.

The single particle measurements of charged kaons and pions (Chapters 4 and 5), and the multiparticle texture analysis (Chapters 6 and 7), both represent searches for hadronic signatures of a phase transition. However they are quite different in terms of the method and, what is more relevant to this chapter, in terms of the physics of the signature.

The strangeness enhancement or, in a more elaborate language, chemical equilibration as a phase transition signature represents an attempt to use hadronic data (highly compressed down to the number of particles of certain identity with certain $p_T$ and $y$, produced per collision) to identify the nature of the degrees of freedom responsible for the hadron production.

The texture signatures of a phase transition exploit analogies between the multiparticle physics of the heavy ion collision and that in the macroscopic condensed matter systems. This is inspired by the notion of universality in the phase transitions, explored with a great success in the condensed matter physics over the past decades. Technically, the texture analysis carried out here relies on the recent advances in the presently booming area of industrially-applicable mathematics of efficient image processing. Here, the ``smart'' multi-step (multi-scale !) data compressionitself becomes, in a sense, the process of measurement, and the results of the compression steps throughout the process turn out to be related with the multiparticle degrees of freedom.

The results of our research can be summarized as follows:

• a considerable enhancement of $K^+$ production over $\pi^+$ is observed as compared to $p+p$ collisions at the same energy. This is not necessarily a sign of a QGP phase transition: alternative explanations exist, as has been discussed in detail in Chapter 5. In the modern phenomenology, the issue of chemical equilibration, rather than strangeness enhancement by itself, became the subject of investigation. This has also been discussed in Subsection 5.3, and some caveats have been made. Letessier and Rafelski [73] claim on the basis of a statistical model fit to the hadron abundance data that the QGP is formed in the $Pb+Pb$ collisions. The value of our measurement [26] lies in the fact that it provides the quantitative information required for such studies. I believe that the enhanced $K^+$ production in $Pb+Pb$ should be ascribed to the hadron gas rescattering.
• no signal of non-trivial dynamic texture has been revealed by the Discrete Wavelet Transform analysis of the $Pb+Pb$ data, within the sensitivity limit dominated by systematic errors due to the imperfections of the experimental apparatus. I personally was impressed by the sensitivity of this novel method despite the experimental imperfections. Viewed by this method, the final states of the $Pb+Pb$ collisions appear to be fairly homogeneous, so that if the phase transition takes place, it is either smooth or its effects are moderated by the hadron gas rescattering which then needs to be taken seriously.

Clearly, each of the two results leaves a lot of freedom in the interpretation and they do not contradict each other. Does the ``negative'' dynamic texture result contradict other CERN data, such as $J/\Psi$ suppression[108], where the strongest claim for the QGP evidence was made ? No, because neither of the results is logically a sufficient condition of existence/non-existence of QGP in the $Pb+Pb$ collisions, and the strongest claim just mentioned in fact merely states that deconfinement is a ``natural explanation'' of the $J/\Psi$ data.

Finally, I wish to express my intuitive feeling that much of the ambiguities present in the interpretations of the heavy ion data may be a consequence of too high a degree of data compression, applied to made the data analyzable by human brain, following a cultural heritage of the previous epoch of scarce computing power. Consequently, the ``smart'' compression algorithms, and the analysis techniques based on them, however technical this may sound for the last sentence of a PhD thesis, may well be one of the most promising paths of further development in the heavy ion experiment.