HERA accelerator ring. View of part of the HERA accelerator at DESY (Deutsche Elektronen Synchro- tron), Hamburg, Germany. HERA (Hadron Electron Ring Accelerator) consists of a pair of accelerator rings mounted in a 6.3km circumference tunnel. The upper (cream-coloured) ring carries protons, whilst the lower (pink) ring carries electrons in the other direction. At two points on the ring the beams cross providing proton/electron collisions at an energy of 850 GeV. Two large detectors (H1 and ZEUS) will then study these collisions. Data from these detectors will provide information about the quarks inside protons and the gluon force moderators that bind them.