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Our understanding of the structure of hadrons has developed during the seventies and early eighties from a few vague ideas to a precise theory, Quantum Chromodynamics, that describes hadrons as made of elementary partons (quarks and gluons). Deep inelastic scattering of electrons and neutrinos on nucleons and electron–positron collisions have played a major role in this development. Less well known is the role played by hadron collisions in revealing the parton structure, studying the dynamic of interactions between partons and offering an exclusive laboratory for the direct study of gluon interactions. The present article recalls the decisive contributions made by the CERN Intersecting Storage Rings and, later, the proton–antiproton SPS Collider to this chapter of physics. 1. Preamble In the mid-sixties, when the ISR were being born, the idea that hadrons could be composite particles was still far from being generally accepted. Summer school lectures were giving as much weight to bootstrap ideas 1 as to the newly born quark model. 2 We remember a seminar by C. N. Yang 3 at CERN, just before the ISR first collisions, introducing the concept of limiting fragmentation, which we were religiously listening to in the hope that it could give us an idea of what to expect from our imminent exploration of the high energy territory. In spite of the spectacular success of Gell-Mann’s eightfold way, the quark model had to face two very strong counter-arguments: the failure of many quark search experiments to find any hint for fractional charges and the apparent incompatibility of the quark model with Fermi–Dirac statistics. Indeed, we did not know about colour, nor about the peculiar behavior of the strong force to get weaker at short distances. The light would come from SLAC at the very end of the decade, with deep inelastic electron scattering soon followed by SPEAR and its harvest of revolutionary results. If hadrons are composite, it should be possible to understand hadron collisions in terms of interactions and rearrangements of the constituents, the so-called partons, and, in particular, to eject one of them, as in nuclear physics with (p,2p )o r (p,pn) reactions. It is indeed possible, but it took a decade to reach this goal. Hadrons are very different from nuclei, which can be qualitatively described classically in |
