LS1 Report: A brand new set-up for ASACUSA-CUSP

ASACUSA is running for the first time with a totally new set-up. Three new vital instruments have been designed, produced and installed during LS1 in addition to several other major modifications. The collaboration is now ready to perform the first high-precision measurement of the hyperfine structure of antihydrogen – a study that aims at comparing the inner properties of matter and antimatter.

The ASACUSA set-up.

The ASACUSA-CUSP collaboration comprises about 30 scientists from various institutes in Europe and Japan. Because of the Japanese contribution, the experiment is often known by its Japanese pronunciation, the experiment’s logo is in Japanese, and the logbook uses Japanese time!

This year, for the first time, the experiment is running with a completely new set-up, which now includes a new superconducting double cusp magnet, a new tracking detector and a new final antihydrogen detector. “The magnet is the heart of the ASACUSA experiment,” explains Yasunori Yamazaki. “It allows us to create a spin-polarised beam of antihydrogen, which is then studied in flight using microwave radiation. Our young colleagues have played vital roles in designing, developing and commissioning these new set-ups in parallel during LS1.”

ASACUSA aims to measure a property of antihydrogen called the “hyperfine structure” precisely and compare it to the well-known value for hydrogen in order to detect any tiny difference. Since this measurement is very sensitive to magnetic fields, ASACUSA aims to create a beam of antihydrogen atoms that can fly to a region where no disturbing fields are present.

Downstream, along the bore of the “double cusp” magnet, a new semi-cylindrical tracker has been installed to extract detailed information on the formation processes of antihydrogen atoms. “The tracker is a precision three-dimensional annihilation Micromegas detector that implements very thin flexible insulator films on which metallic patterns are printed, immersed in a gas mixture to follow the particle trajectory precisely,” explains Yasunori Yamazaki. “This is the first time a Micromegas detector has been used in a configuration with such a small curvature of ~100 mm.”

A third addition to the previous set-up is the final antihydrogen detection system comprising a BGO crystal in a vacuum and eight reed-shaped plastic scintillators surrounding it. This is needed to allow the experiment to distinguish accurately between antihydrogen particles coming from the beam (which are therefore to be measured and studied) and spurious particles coming from unshielded cosmic rays and from antiproton annihilations upstream of the antihydrogen detector primarily in the cusp magnet.

“Thanks to the new set-up, we hope to be able to measure precisely for the first time the hyperfine structure of the antihydrogen already during this run,” says Yasunori Yamazaki. The experiment has completed the commissioning phase and will be taking data for another two weeks.  The beam at the Antiproton Decelerator, which is feeding ASACUSA, will then be stopped to allow construction work in preparation for ELENA. The data-taking period for ASACUSA is expected to restart in June 2015.

by Antonella Del Rosso, Katarina Anthony