From 1959 to 2007, the DESY accelerators were primarily used for particle physics, initially with the eponymous DESY electron synchrotron (1964–present), followed by DORIS (Double Ring Storage Facility, 1974–1992), PETRA (Positron–Electron Tandem Ring Facility, 1978–present) and HERA (Hadron–Electron Ring Accelerator, 1992–2007). In 1987, the ARGUS experiment at DORIS was the first to observe a large mixing of B mesons and thus a process in which matter and antimatter behave differently. The most important discovery of the experiments TASSO, JADE, MARK-J and PLUTO at PETRA was the detection of the gluon, the messenger particle of the strong force, in 1979. From 1990, PETRA served as a pre-accelerator for the even larger storage ring HERA with its four experiments H1, ZEUS, HERMES and HERA-B. HERA was the only storage ring facility in the world in which protons collided with electrons or positrons. In these collisions, the point-like electron acted like a probe, scanning the inner structure of the proton and making it visible with high resolution. HERA's precise insights into the interior of the proton formed the basis for numerous other particle physics experiments, especially at the Large Hadron Collider (LHC) at the research centre CERN and for numerous developments in theoretical particle physics.