An ISR Discovery: The Rise of the Proton–Proton Cross-Section

Abstract

The Intersecting Storage Rings (ISR) were the first hadron collider ever built, providing proton–proton collisions at centre-of-mass energies as high as 62 GeV, almost five times larger than any previous accelerator. When in 1971 the ISR began operation the Reggepole approach dominated and the proton–proton total cross-section was expected to have already reached a finite asymptotic value. However, ISR experiments found that the cross-section was rising by 10% between 22 and 62 GeV, while the interaction radius was increasing by 5%, a trend that continues up to the hundred times larger energies available at the Large Hadron Collider. In order to accurately measure the total and elastic cross-sections, new experimental methods — uniquely adapted to the environment of a hadron collider — had to be developed; they are described in the central part of this paper, which closes with a review of the data obtained at the LHC since they put in a wider perspective the forty years old ISR results.