ASACUSA

ASACUSA
Name	ASACUSA
Full name	Atomic Spectroscopy And Collisions Using Slow Antiprotons
Field	Particle physics; Atomic physics; Antimatter physics
Site	CERN
Established	1990s
Collaborators	CERN, University of Tokyo, RIKEN, Stefan Meyer Institute, University of Mainz, Universität Bern
Leads	Masaki Hori, Makoto Fujiwara
Apparatus	Antiproton Decelerator, cusp trap, radiofrequency quadrupole, spectroscopy lasers

ASACUSA

ASACUSA is an experimental collaboration at CERN that studies antiprotonic and antihydrogenic systems using low-energy antiprotons from the Antiproton Decelerator to probe fundamental symmetries and atomic structure. The collaboration combines techniques from atomic spectroscopy, ion trapping, and particle detector development to measure transitions, interactions, and scattering processes involving antiprotons and exotic atoms. ASACUSA's program interfaces with broader efforts at PSI, GSI Helmholtz Centre for Heavy Ion Research, and RIKEN on antimatter precision measurements and tests of CPT symmetry.

Overview

ASACUSA operates experiments that create and analyze exotic atoms such as antiprotonic helium, antihydrogen, and antiprotonic molecular states using slow antiprotons supplied by the Antiproton Decelerator and associated beamlines like ELENA. The collaboration deploys techniques including laser and microwave spectroscopy, cusp and Paul traps, and time-of-flight mass spectrometry to extract level energies, lifetimes, and collision cross sections. ASACUSA's measurements contribute to constraints on CPT theorem violations and comparisons between matter and antimatter involving reference systems such as hydrogen and helium. The program connects to theoretical frameworks developed by groups at Max Planck Institute for Quantum Optics, Princeton University, Massachusetts Institute of Technology, and University of Tokyo.

History and Development

ASACUSA traces its roots to antiproton physics activities at CERN in the 1980s and 1990s, evolving alongside experiments such as LEAR and the establishment of the Antiproton Decelerator in 2000. Early milestones include precision spectroscopy of antiprotonic helium in collaboration with teams from University of Tokyo and RIKEN, and the development of slow antiproton trapping techniques inspired by work at University of Basel and University of Zurich. Over successive phases ASACUSA incorporated advances from groups at Stefan Meyer Institute, Moscow State University, University of Tokyo (Komaba), and University of Tokyo (Hongo), adopting new cooling, laser, and microwave sources. The collaboration expanded through the 2000s and 2010s with contributions from University of Mainz, University of Bonn, University of Sofia, and Universität Bern to refine apparatus and analysis.

Experimental Apparatus and Methods

ASACUSA's apparatus centers on the Antiproton Decelerator beamline, electrostatic and magnetic transport, and specialized traps such as the cusp trap and radiofrequency quadrupole decelerator. The cusp trap, an adaptation of magnetic bottle concepts from Penning trap research at University of Washington and Columbia University, produces polarized antihydrogen beams for in-beam spectroscopy. Laser systems for two-photon and single-photon excitation are derived from technologies used at National Institute of Standards and Technology, MPQ, and Riken, enabling precision measurements of transition frequencies. Detection relies on particle detectors and calorimeters similar to devices at CERN NA48, ALICE, and LHCb for annihilation imaging, combined with high-resolution waveform digitizers developed in collaboration with University of Tokyo electronics groups. Data analysis employs atomic theory calculations from teams at Kiel University, University of Granada, and University of Tokyo to interpret level shifts and line shapes.

Key Results and Contributions

ASACUSA produced high-precision spectroscopy of antiprotonic helium, providing stringent comparisons to three-body QED calculations and contributing to determinations of the antiproton-to-electron mass ratio that complement measurements from ALPHA and ATRAP. The collaboration demonstrated methods for producing polarized antihydrogen beams via the cusp trap, informing proposed tests of the hyperfine structure of antihydrogen relative to hydrogen measurements at MIT and Harvard University. ASACUSA's measurements constrained hypothetical CPT-violating parameters considered in analyses by the Particle Data Group and influenced theoretical work from Stanford University, Caltech, and University of Oxford on antimatter spectroscopy. Additionally, ASACUSA advanced techniques in antiprotonic atom formation and collision studies that have been adopted by experiments at PSI and GSI.

Collaborations and Institutional Context

ASACUSA is a multinational collaboration linking research groups from Europe and Asia, including institutions such as CERN, University of Tokyo, RIKEN, Stefan Meyer Institute, Universität Mainz, Universität Bern, GSI, and partner universities across Germany, France, Italy, and the United Kingdom. It coordinates with other CERN antimatter experiments including ALPHA, ATRAP, and BASE on beam-sharing, technology transfer, and complementary measurement programs. Collaborators have engaged theorists at Max Planck Institute for Quantum Optics, Harvard-Smithsonian Center for Astrophysics, and Perimeter Institute to interpret results in the context of fundamental symmetry tests and precision QED. Funding and oversight involve national agencies such as JSPS, DFG, and ERC-supported networks.

Future Plans and Upgrades

Planned upgrades focus on enhancing antihydrogen beam intensity, polarization purity, and spectroscopic resolution through improvements to the cusp trap, implementation of the ELENA low-energy antiproton source, and deployment of next-generation laser and microwave systems. Prospective goals include sub-ppb comparisons of antihydrogen hyperfine structure with hydrogen benchmarks measured at NIST and Rutherford Appleton Laboratory, and refined antiprotonic helium transition frequencies to test higher-order QED corrections computed by groups at KVI-CART and University of Groningen. Continued collaboration with ELENA operations and cross-experiment coordination with ALPHA and BASE aim to broaden constraints on CPT-violating scenarios and explore applications of low-energy antiprotons in antimatter chemistry and exotic atom research.

Category:Particle physics experiments