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

The Intersecting Storage Rings (ISR) were the first hadron collider ever built, providing proton–proton collisions at centre-of-mass energies as high as 62GeV, 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.