OLYMPUS starts research programme

Detector at DORIS starts the first of two measuring periods

In the past decades, the image of the proton has become ever more precise. Especially DESY's accelerator HERA has helped physicists to clearly understand the complexity of the particles they are currently smashing in the Large Hadron Collider LHC.

The OLYMPUS detector prior to installation at DORIS

Nevertheless, there are still some mysteries hidden in the proton. With the aim to reveal one of these, the OLYMPUS project was initiated at DESY: in the past two years, the rather light particle detector with about 50 tonnes dead load was installed at the DORIS storage ring to deliver the so far most precise measurements of the electric charge and magnetic momentum distributions in the proton. Therefore, at DORIS, electrons or positrons accelerated to the energy of two Giga electron volts (2 GeV) are shot onto a hydrogen gas which is conducted through the DORIS beam pipe in a target cell. Thereby these particles are scattered at the protons of the gas.

After assembly and commissioning in the past weeks and months, the first of two planned OLYMPUS measuring periods was started. One of the most important features of the experiment are the alternating collisions of protons with electrons and positrons – only this way it is possible to determine the so-called form factor of the proton with extreme accuracy. “In addition to the daily switching between electrons and protons, we change the polarization of the OLYMPUS magnets four times a day at random intervals to obtain the highest precision,” explains OLYMPUS spokesman Michael Kohl (Hampton University, USA). “Through the exact comparison of reaction rates of both particle types we will be able to calculate how often an interaction of two instead of one force particle takes place at the collisions.” At experiments at Jefferson Lab (USA), the interactions suggested the existence of so far undiscovered higher-order contributions. With the regular switching of electrons and their antiparticles as collision partners, it is possible to measure these contributions. Indeed, fast switching between both particle types at high beam intensity is only possible at DORIS.

Since the use of the detector is not compatible with the photon science programme running at DORIS, the current four-week measuring time is exclusively at the disposal of OLYMPUS. Data taking at OLYMPUS is monitored from the accelerator control room. This is ideal, because the injection frequency is considerably higher as at normal DORIS synchrotron runs: About once an hour, ten new particle bunches are injected into the storage ring, as collisions with the gas target rapidly thin out the particle beam.

Altogether, the operation team with about 20 members is quite satisfied with the start of their experiment. “Already at the DORIS start-up end of January, we were able to do some fine-tuning; now we have the detector very well under control,” explains Uwe Schneekloth who coordinated the installation of the detector. “Already after the first measuring period, we expect results with the accuracy down to a few per cent.” The second two-month measuring period including a further improvement of precision is planned for the months of October until December this year.