The Hadron-Electron Ring Accelerator HERA was the largest particle accelerator at DESY and Germany’s largest research instrument: a gigantic super electron microscope that provided physicists with the world’s sharpest view of the proton. The accelerator itself has been switched off, but the data analysis at the HERA experiments H1, ZEUS and HERMES is still in full swing.

Particle physics at HERA

For 15 years, electrons and protons collided at extremely high energies inside the particle accelerator ring, which lies deep in the earth beneath Hamburg. Research operation was concluded in summer 2007 and HERA was switched off. The evaluation of the recorded data, however, is continuing at full speed. The prospects are exciting, for the HERA physicists are now perfecting a comprehensive overall picture of the proton and the forces acting within it – with a precision that won’t be matched by any other particle accelerator in the world for years to come.

HERA: a precision machine

The HERA storage ring facility at DESY was the only one in the world in which two different types of particles were accelerated separately and then brought to collision. Here, in a 6.3-kilometer-long tunnel located deep below Hamburg, lightweight electrons – or their antiparticles, the positrons – collided with hydrogen nuclei, i.e. protons from the hadron family, which are nearly 2000 times heavier. In these electron-proton collisions, the point-like electron acts like a tiny probe that scans the inside of the proton and reveals its inner structure. The higher the energy of the particle collision, the deeper physicists are able to gaze into the proton, and the more details are revealed. That’s why HERA was dubbed a “super electron microscope”: Thanks to HERA’s highly precise “electron probes”, particle physicists are able to investigate the inner structure of the proton and the fundamental forces of nature in great detail.

The sharp eyes of HERA

The HERA storage ring passes through four large underground halls, one at each point of the compass. Here, seven stories below the earth, stood the detectors used by international research teams to investigate the most minute building blocks of matter. In 1992, the first two HERA experiments went into operation: H1 in the North Hall and ZEUS in the South Hall. Both experiments observed the high-energy collisions of electrons and protons in order to unravel the internal structure of the proton and the mysteries of nature’s fundamental forces. The HERMES experiment started taking data in the East Hall in 1995. It used the HERA electron beam to investigate the intrinsic angular momentum – the spin – of protons and neutrons. From 1999 to 2003, the HERA-B experiment used the proton beam from the storage ring in HERA’s West Hall to shed light on the properties of heavy quarks.

Unique insights

The huge detectors were in operation until mid-2007, and between them they recorded a gigantic amount of data. During that time, many of the insights provided by HERA into the microscosm found their way into the physics textbooks. They are now part of the basic knowledge we have of the workings of our world. The journey of discovery is far from over, however. Active data taking has been completed, but the HERA experiments are continuing: The evaluation of the recorded measurement data will provide exciting insights into the inner structure of the proton and the fundamental forces of nature well beyond 2010.

With their results, the HERA physicists are passing the baton back to the theorists – who now must refine or change their model calculations in order to explain the results gained from HERA and to incorporate them into particle theory. The HERA results have tremendously stimulated the work of the theorists right from the outset, particularly in the field of quantum chromodynamics, where a very intensive, fruitful collaboration between theory and experiments has arisen. The picture of the proton and the forces of nature gained from the HERA experiments thus represents the underlying basis not only for many future particle physics experiments but also for many current developments in the world of theoretical particle physics.