What does the inside of the proton look like? This is a question that scientists at DESY have been concerned with for a long time. For 15 years, HERA provided the world's sharpest view of the interior of the proton. After the swich-off of HERA, the new experiment OLYMPUS at the DORIS accelerator is now tackling the question again.

Insight into the proton

OLYMPUS is a comparatively small experiment. The complete detector weighs only about 50 tonnes – which makes it a lightweight compared to, for example, the 3600-tonne ZEUS experiment at HERA. Apart from its size, this is mainly due to the fact that the detector has no iron yoke. Moreover, the area surrounding the interaction point is not completely equipped with electronics, as was the case with the large HERA detectors.

In the HERA experiments, scientists measured inelastic electron-proton collisions, which allowed them to determine the structure of the proton in exquisite detail. With the help of so-called elastic scattering – i.e. collisions in which the protons remain unscathed – OLYMPUS will provide additional information about the electric and magnetic charge distributions in the proton.

Unique opportunities at DESY

The assembly of OLYMPUS began in mid-2010 in the DORIS hall. The detector is not new, however, but consists mainly of the BLAST detector which was operated from 2002 to 2005 at the BATES accelerator at the Massachusetts Institute of Technology (MIT) in the USA. But why disassemble an experiment at MIT and reassembled at DESY? This is because of the unique conditions offered by the DESY accelerators, which are nearly the only facilities in the world whose pre-accelerators can be operated with both electrons and positrons. Moreover, it is possible to switch from one kind of particle to the other within ten minutes.

Scientists at OLYMPUS require both electrons and positrons for their measurements. The particles are shot onto a hydrogen target located inside the detector. At MIT, the experiment could only be carried out with electrons. However, only with both types of particles can the researchers obtain evidence for so far undiscovered higher-order contributions. Switching between electrons and positrons at regular intervals also leads to more precise measurements.

OLYMPUS at the DORIS accelerator

A number of small alterations to the DORIS machine have to be made to enable the acceleration of both electrons and positrons as well as quick switching between the two particle types. Indeed, DORIS has been used as a synchrotron radiation source since 1993, and accelerated positrons for this purpose. All the work for and with OLYMPUS, e.g. work on DORIS and OLYMPUS research operation, will only take place when DORIS does not provide beams for synchrotron radiation users.

Since the use of DORIS for OLYMPUS is neither possible during the operation of DORIS as a light source nor the operation of PETRA III, OLYMPUS will only take data when neither accelerator is running for user operation. The first series of measurements will begin in early 2012; the second will follow at the end of the same year.