HERA Legacy

HERA collided circulating beams of electrons and protons at energies of 27.6 GeV and 920 GeV, respectively, and quickly produced surprising results: the gluon turned out to be more abundant in the proton than previously thought. At low energies, the electromagnetic interaction of the point-like electron with the charged constituents inside the nucleon probes its structure, while at high momentum transfers the charged and neutral currents become equally strong, an observation often referred to as electroweak unification and observed for the first time at HERA.

The structure of the proton itself is governed by the strong interaction and, in the case of HERA, is parameterised by three structure functions that can be related to the abundance of quarks and gluons inside the proton. Their evolution is fully described by QCD, the theory of the strong interaction. The observed scaling violation is a direct consequence of the scale dependence of the coupling strength in the strong interaction. The distribution of the partons cannot (yet) be calculated from first principles. The H1 and ZEUS collaborations have meticulously mapped the structure of the proton over a wide kinematic range.

The discovery of a class of events with large rapidity gaps between the colliding proton and other particles was indicative of the diffractive process previously observed at low energies. The surprisingly large rate of these events was evidence that the high gluon content of the nucleon facilitated colourless exchange of gluon pairs at high energy, which could be understood in terms of a diffractive structure function.

Detailed knowledge of these structure functions has laid the foundation for precise interpretations of measurements at the LHC and at future hadron colliders at the electroweak scale and beyond.

The talk will recall some of HERA's experimental hurdles and highlight other key results in addition such as the search for leptoquarks and the hadronic nature of the photon.