On June 30, 2007, data taking at the electron-proton storage ring HERA at DESY will come to an end. For 15 years, deep in the earth beneath Hamburg, electrons and protons have smashed into one another. “The particle physics experiments at HERA have provided a unique and detailed picture of the proton and the interacting forces that will not be surpassed by any accelerator in the world for many years,” said Professor Rolf-Dieter Heuer, research director for particle physics at DESY. “On the one hand, the precision measurements have a great influence on particle physics theory; on the other hand they are a good basis for interpreting the exciting new physics that is expected from experiments at the Terascale in the coming years.” The analysis of data measured at the 6.3 kilometer-long electron-proton storage ring will last far into the next decade. Some results have already found their way into physics textbooks.

“Mountaineers who finally reach the top of a mountain after a long climb are rewarded with a view of the world that they have never seen before. We have seen something completely new with HERA. At the same time, we have successfully entered new territories in technology, politics and the sociology of large research teams,” said Professor Albrecht Wagner, chairman of the DESY Board of Directors. Dr. Dieter Trines, DESY director for accelerator physics, said: “We have witnessed how much heart, soul and effort was invested in the design, construction and operation of this unique facility, so 15 successful years of accelerator run on the one hand fill us with pride; on the other hand the shutdown makes us a little nostalgic.” After the shutdown of HERA operation, the storage ring will be decommissioned, the particle detectors will be dismantled and HERA’s pre-accelerator PETRA will be converted into one of the most brilliant storage-ring X-ray sources worldwide.

In the 6.3 kilometer-long electron-proton storage ring HERA, electrons and protons, hydrogen nuclei that are nearly 2000 times heavier than electrons, were accelerated to and stored at almost the speed of light. The particles circled the ring 48,000 times per second and delivered collision data to up to four experiments at the same time.

In the H1 and ZEUS experiments, electrons and quark-filled protons were brought to collision. The electron acts as a pointlike probe that scans the proton. With a resolution of 10-18 meters, corresponding to a thousandth of the proton diameter, HERA is the best electron microscope in the world. With these electron-proton collisions, it is possible to measure the structure of the proton and the strong force acting within it very precisely. One special feature of HERA collisions is the fact that the electrons do not influence the measurements because they react to other forces than the quarks in the proton. The HERA measurements for the first time confirmed the nature of the strong force, as it was predicted 20 years ago by the physicists Davis Gross, David Politzer and Frank Wilczek. For this discovery they were awarded with the Nobel Prize in 2004.

However, other forces were also investigated with the HERA experiments. H1 and ZEUS were able to prove that the electromagnetic force and the weak force have the same characteristics at high energies. Both forces can be unified to the electroweak force – the first step towards the grand unification of all fundamental forces of nature into one elementary force.

The HERMES and HERA-B experiments only used one of the stored HERA particle beams: HERA-B the protons and HERMES the electrons. At HERMES, the electron beam was directed towards a target containing protons and neutrons in a gas. This made it possible to make exact measurements, showing how the so-called spin of the proton is made up of the spins of its building blocks. HERA-B investigated the special properties of heavy quarks.