Superconducting core component for international P2 experiment installed in underground hall at a depth of ten meters
25 November 2024
One of the flagship projects of the PRISMA+ Cluster of Excellence at Johannes Gutenberg University Mainz (JGU) is the construction of the new energy-recovering particle accelerator MESA (Mainz Energy-recovering Superconducting Accelerator), which will enable experiments with unprecedented precision in the future. One of the main experiments at MESA, P2, will play a key role in researching the "new physics" – physics beyond the Standard Model of particle physics – by measuring the so-called weak mixing angle. The central component of the P2 experiment, a superconducting magnet coil with a diameter of four meters and a weight of 21 tons, has now been delivered to the JGU campus and installed in the MESA particle accelerator. The magnet was manufactured in Vannes, France, and delivered to Mainz last Thursday. "We have been working together with the company SigmaPhi for almost five years to overcome the challenges and implement the state-of-the-art design for our experiment," said Professor Frank Maas, spokesperson of the P2 experiment, which is currently being set up by a collaboration of physicists from Germany, France, Canada, and the USA. "Such a solenoid magnet is being used for experiments of this kind for the first time. Its large diameter makes it possible to record particularly high particle rates. However, the size of the magnet also posed a particular challenge in terms of design and production."
At around four degrees above absolute zero, i.e., at approximately minus 269 degrees Celsius, the coil loses its electrical resistance and becomes superconducting. To reach these low temperatures, it is operated in a vacuum inside a cryostat and cooled with liquid helium. It was delivered by special transport from Vannes in Brittany and transported by a special mobile crane with a beam of more than 30 meters in length through an opening in the roof of the new MESA building to the accelerator level ten meters below ground. There it was installed in an iron yoke weighing around one hundred tons.
One magnet, two main tasks
The main aim of the P2 experiment is to determine the weak mixing angle, a measure of the relative strength of the weak and electromagnetic interactions. The supplied magnet plays a key role in determining this fundamental quantity of the Standard Model and has two crucial functions. Firstly, it is used to focus the elastically scattered electrons onto the detectors whose signals are analyzed by the P2 team. Secondly, it is used to deflect and suppress background events that are not relevant for the data analysis. A precise experimental determination of the weak mixing angle in comparison with equally precise theoretical calculations will make it possible to test the Standard Model. Deviations between theory and experiment would be an indication of new particles or forces beyond those known from the Standard Model and an experimental indication of the nature of dark matter.
A new building for MESA
Part of the underground halls was rebuilt with the Center for Fundamental Physics (CFP). The CFP forms the structural framework for central PRISMA+ research projects, the most important of which is MESA. As part of CFP I, existing underground experimental halls of the Institute of Nuclear Physics were extended by a hall covering around 600 square meters. The existing halls were modernized and partially converted. These include the halls of the Mainz Microtron Electron Accelerator (MAMI), which has been in successful operation since 1979. The new MESA hall is directly attached to it, so that the existing beam catcher is integrated into the construction project. The halls are connected via large wall openings in order to create sufficient space for the MESA accelerator and the associated scientific experiments.