A definitive determination of the contribution of multi-photon exchange to elastic lepton-nucleon scattering
Final Report Abstract
The ratio between the electric and the magnetic form factors, extracted by the Rosenbluth separation, has been measured several times by different experiments, showing a constant behavior around unity. However, new measurements for the same ratio using polarization detected a completely different trend of the data points: the newly extracted values seem to be constantly dropping with increasing Q2 and are in disagreement with the previous measurements. This behavior was attributed to the multiple-photon exchange, which will contribute to the polarization measurements in contrast to the Rosenbluth separation measurements. But up to now, a confirmation of this theory is missing. The OLYMPUS experiment, performed in 2012 at the DORIS ring of the DESY research center, is focused on the measurement of the two-photon exchange by investigating the ratio between elastic positron and electron scattering. The detectors were set-up successfully and the read-out electronics was installed. One of the essential parts of the experiment is the trigger system, which was set-up by this project. The trigger decides if an event matches the previously selected criteria and initiates the storing of the data. After the first beamtime, the trigger system could be further optimized, allowing a faster data taking. Another important part of the OLYMPUS experiment is the data acquisition system, which was realized by this project. It included a hardware part, consisting of several synchronization clients and a synchronization master. The purpose of the synchronization system is to guarantee the time stability through all subdetectors of the whole experimental setup. The software part of the data acquisition system contains the readout system of each subdetector. In case of a desired event, a signal is transferred to all event builders, located at each subdetector. The readout of the detector is than initialized and the values of all ADCs, TDCs and scaler are stored on disk. For this, the essential readout of all modules were newly developed. However, the central part of the data acquisition system could be adapted from the CBELSA/TAPS experiment, which guaranteed a fast and cost-efficient solution for the OLYMPUS experiment. The data acquisition worked highly efficient and stable through both beamtimes and allowed a fast data taking and a secure storage of the received data. The fast trigger and data acquisition system allowed the OLYMPUS experiment to run smoothly and even surpass the aimed luminosity of 3.6 fm^−1 by more than 20%. The analysis of the data is still ongoing, but the desired reactions by elastic scattering could be successfully extracted from the data without major background contributions. The essential part of the precise luminosity determination is under control. Currently, the final systematics are being analyzed.
Publications
- The OLYMPUS Experiment - Nuclear Instruments and Methods, A741 (2013) 1-17
R. Milner et al.
- The OLYMPUS Internal Target - Nuclear Instruments and Methods, A755 (2014) 20-27
J.C. Bernauer et al.
(See online at https://doi.org/10.1016/j.nima.2014.04.029)
