HERA (German: Hadron-Elektron-Ringanlage, English: Hadron–Electron Ring Accelerator) was a particle accelerator at DESY in Hamburg. It was operated from 1992 to 30 June 2007. At HERA, electrons or positrons were brought to collision with protons at a center-of-mass energy of 320 GeV.
HERA was used mainly to study the structure of protons and the properties of quarks, laying the foundation for much of the science done at the Large Hadron Collider (LHC) at the CERN particle physics laboratory today. HERA is the only lepton–proton collider in the world to date and was on the energy frontier in certain regions of the kinematic range.
To collide protons with either electrons or positrons, HERA used mainly superconducting magnets, which was also a world first. At HERA, it was possible to study the structure of protons up to 30 times more accurately than before. The resolution covered structures 1/1000 of the proton in size, facilitating many discoveries concerning the composition of protons from quarks and gluons.
The HERA tunnel runs 10 to 25 m below ground level and has a circumference of 6.3 km and an inner diameter of 5.2 m. For the construction, the same technology was used as for the construction of subway tunnels. Two storage rings were located on top of each other inside the tube. One accelerated electrons to energies of 27.5 GeV, the other one protons to energies of 920 GeV in the opposite direction. Both beams completed their circle nearly at the speed of light, making approximately 47000 revolutions per second.
There are four interaction regions around the ring, which were used by the experiments H1, ZEUS, HERMES and HERA-B. All these experiments were particle detectors run by international groups of researchers. These groups developed, constructed and ran the multi-storey, complex measurement devices in many years of cooperative work and evaluated enormous amounts of data.
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