OLYMPUS (experiment)

OLYMPUS (experiment) was a high-energy physics experiment conducted at the DESY laboratory in Hamburg, Germany, from 2010 to 2013. Its primary goal was to make a precise measurement of the proton's two-photon exchange contribution to elastic scattering by comparing cross-sections for electron-proton and positron-proton interactions. The experiment was designed to resolve a long-standing discrepancy between measurements of the proton's electric form factor and magnetic form factor obtained via the Rosenbluth separation method and those from polarization transfer techniques. Data from OLYMPUS provided critical empirical constraints on theoretical models of nucleon structure and fundamental quantum electrodynamics processes.

Overview

The OLYMPUS experiment was installed at the DORIS storage ring at DESY, utilizing a unique internal gas target system. It represented a major international collaboration involving institutions like the MIT Bates Research and Engineering Center, Mainz, and Glasgow. The project directly addressed the "proton form factor puzzle," a significant anomaly in nuclear physics that suggested potential shortcomings in the theoretical understanding of the two-photon exchange process. By alternately storing beams of electrons and positrons in DORIS and scattering them off a windowless hydrogen target, OLYMPUS sought to measure the hard-to-calculate two-photon contribution with unprecedented precision.

Scientific goals and motivation

The core motivation stemmed from conflicting results for the proton's electric form factor extracted from two experimental methods. Measurements using the traditional Rosenbluth separation, employed in experiments at SLAC and Jefferson Lab, yielded a roughly constant ratio of electric to magnetic form factor with increasing momentum transfer. In stark contrast, data from polarization transfer experiments, such as those conducted at Jefferson Lab's CEBAF, showed a dramatic linear decrease. Theoretical physicists, including John Arrington and Inna Blokhintseva, hypothesized that the neglected two-photon exchange amplitude in the Rosenbluth analysis could reconcile this discrepancy. OLYMPUS was conceived to test this hypothesis by directly measuring the difference between electron-proton and positron-proton scattering, where the two-photon effect contributes with opposite signs.

Experimental design and apparatus

The apparatus centered on a windowless, cryogenic hydrogen gas target flowing into the DORIS beam pipe, allowing for a pure proton target without contaminating materials. A sophisticated, large-acceptance recoil detector surrounded the interaction region to measure the scattered proton and the recoiling target proton in coincidence, identifying elastic events. Key subsystems included a time-of-flight barrel for particle identification, a symmetric solenoid magnet, and tracking chambers. The experiment leveraged the unique capability of DORIS to store either electron or positron beams, enabling rapid switching between the two particle types to minimize systematic uncertainties. The design was heavily informed by earlier pioneering experiments like LEP and studies at VEPP-2.

Data collection and analysis

Data collection occurred over several run periods between 2010 and 2013, accumulating roughly 4.5 fb⁻¹ of integrated luminosity. The collaboration meticulously alternated between electron and positron beam modes, often within the same day, to control for variations in beam conditions and detector performance. Analysis involved complex reconstruction of elastic event kinematics, extensive background subtraction from processes like pair production and pion electroproduction, and detailed studies of systematic effects from beam properties, target density, and detector efficiencies. The final cross-section ratio was extracted by comparing yields under identical conditions, a technique refined through previous experiments like the BABAR experiment and HERA's H1 experiment.

Results and implications

The OLYMPUS collaboration published its final result in 2016, finding the cross-section ratio of positron-proton to electron-proton scattering to be consistent with unity within an uncertainty of about 1%. This indicated that the size of the two-photon exchange effect was significantly smaller than many theoretical models had predicted to explain the full form factor discrepancy. The result ruled out large contributions from hard two-photon exchange at the experiment's momentum transfer range, challenging several theoretical frameworks and suggesting other mechanisms, such as contributions from excited nucleon states like the Δ(1232), might be more important. The findings provided crucial benchmark data for the global nuclear physics community and influenced the design of subsequent experiments at Jefferson Lab.

Collaboration and legacy

The OLYMPUS collaboration was a multinational effort led by spokespersons including Richard Milner and Douglas Hasell, bringing together teams from the United States, Germany, the United Kingdom, and Switzerland. Its legacy is profound, having delivered a definitive measurement that reshaped the theoretical landscape of nucleon structure. The experiment's techniques for handling internal gas targets and its analysis frameworks informed future projects like the DarkLight experiment and the planned MUSE experiment at the Paul Scherrer Institute. While OLYMPUS did not find a large two-photon effect, it precisely quantified one potential solution to the proton puzzle, thereby guiding ongoing research into the quark-gluon dynamics described by quantum chromodynamics. Category:Particle physics experiments Category:DESY Category:Nuclear physics experiments Category:2010 in science