What does the inside of the proton look like? Scientists at DESY have been tackling that question for a long time. For 15 years, HERA provided the world's sharpest view of the interior of the proton. The OLYMPUS experiment at the DORIS accelerator is now taking up the challenge again.

Insight into the proton

The international OLYMPUS collaboration, which comprises 13 institutes, aims to investigate the interaction of electrons and protons with high precision. Using the OLYMPUS detector at DORIS, the scientists will clarify whether the collision of an electron and a proton involves the exchange of only one photon, or sometimes proceeds through two-photon exchange. In the latter case, differences should arise when comparing the collisions of electrons and protons with those of positrons (anti-electrons) and protons.

Unique opportunities at DESY

To this end, the team installed the OLYMPUS detector at DESY's DORIS accelerator. The detector itself is not new, 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. The reason for the move to DESY was the unique conditions offered by the DORIS accelerator, which was one of the few facilities in the world that could be operated with both electrons and positrons. Moreover, it was possible to switch from one type of particle to the other within ten minutes.

Scientists at OLYMPUS required both electrons and positrons for their measurements. The particles were shot at a hydrogen target located inside the detector, with the type of particle being switched every day. At MIT, the experiment could only be carried out with electrons. However, only with both types of particle 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.

Successful data taking at DORIS

After a first, four-week measuring period in February 2012, the scientists concluded the second, two-month data taking in January 2013, collecting a total of four inverse femtobarns of data (inverse femtobarns are a standard measure in particle physics indicating the number of particle collisions).

After the DORIS accelerator was definitively shut down in January 2013, OLYMPUS recorded tracks of cosmic muons for one month, which were used to calibrate the drift chambers and reconstruct the particle tracks more precisely. Following the data taking, the detector was optically surveyed once more and its inhomogeneous magnetic field was remapped. The scientists will use these data in their final analysis to obtain results with an uncertainty of less than one percent.