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Final Activity Report Summary - EHP (Experimental hadronic physics from elementary to nuclear collisions)

The strong interaction, which binds the building blocks of atomic nuclei, is the most predominant interaction type in the Universe. It acts in the core of stars driving their energy production, and transforms nuclei to build up our present world. Few microseconds after the "Big Bang" the strong interaction dominated the evolution of the then very hot Universe.

For half a century up to today, understanding the nature of the strong interaction has been one of the key issues of fundamental physics research, identifying the substructure of all strongly interacting particles (hadrons) in form of confined quarks and gluons. The underlying theory, Quantum Chromodynamics (QCD), being part of the Standard Model of particle physics, explains why the true nature of these constituents can only be revealed when they receive a high momentum transfer in a minute fraction of all the interactions.

In the dominant "soft" processes hadrons seem to act with all their components coherently - with a dynamics uncalculable from QCD. It is therefore an experimental challenge to constrain the large number of proposed approximations to QCD.

The two-year project aimed at such an experimental study in the non-perturbative sector of QCD. Specifically, its objectives were to study all sectors of soft hadronic interactions simultaneously with the same detector in order to obtain consistent high quality data set, and to establish links especially between the most elementary hadron+hadron collisions and the more complex interactions involving nuclei.

The NA49 experiment collected a data set of 5 million p+p and 0.5 million p+C events, unprecedented in these reactions in the range of 17 GeV center-of-mass energy, and new methods of data analysis techniques had to be developed, in order to ensure that the systematic uncertainties are well below the statistical fluctuations, thus maximally exploit the large event sample. These results are foreseen to serve as the basis in all future considerations for these reactions, both on the phenomenological and on the theoretical level.

The results revealed distinct structures in the pion production distribution in p+p interactions, which was attributed to hadronic resonances. The NA49 results indicate in this context that the non-perturbative domain extends much further into the claimed perturbative realm than expected before. Similar conclusions were drawn from the most recent results obtained at the RHIC storage ring at a decisively higher range of interaction energy. Thus the approach pursued by this project, which will allow a detailed study of the junction between soft and hard processes, brings deeper understanding of hadronic production.

The comparative study of p+C and p+p interactions led to a two-component picture of particle production in hadron-nucleus interactions, which predicts an independent fragmentation of the target and the projectile. The projectile, which undergoes multiple collisions, suffers a violent interaction, and so provides basic information on soft QCD processes. In the course of the project, this superposition mechanism has been quantitatively proven, using all the available information, and only relying on isospin symmetry.

The present project brought the soft hadronic interactions on a new level of precision, and pointed out the importance of hadron-nucleus interactions, as the ideal laboratory to study hadronic systems that undergo multiple collisions. This fact has only recently been recognised at the RHIC collider, and in the p+A program of the CERN LHC. Using high precision and complete experimental results provided by the present project, it was demonstrated how model-independent information can be built up. This shows that inproved phenomenological understanding and important constraints on models describing the strong interaction can be obtained even in the absence of reliable theoretical predictivity.