Atomic collision experiment using ultra-slow antiproton beams

The development of techniques to decelerate, cool and confine antiprotons in vacuo with an electromagnetic trap has opened a new research field of atomic physics of 'cold' antiprotons, including synthesis of antihydrogen atoms. At the Antiproton Decelerator (AD) facility at CERN, we the MUSASHI group of ASACUSA collaboration have so far achieved efficient confinement of millions of antiprotons in a Multi-Ring electrode Trap (MRT) installed in a superconducting magnet of 2.5 T, by a sequential combination of the AD (down to 5.3 MeV), an RFQD (Radio-Frequency Quadrupole Decelerator; down to 50–120 keV) and the MRT. Antiprotons, cooled to energies less than an electronvolt by preloaded electrons in the trap, was then extracted out of the magnetic field and transported along a 3-m beamline as a monoenergetic beam of 10–500 eV. With this unique ultra-low-energy antiproton beam, we are now planning the first atomic collision experiments under single collision conditions, to measure ionization and atomic capture cross sections of antiprotons against helium atoms. A supersonic atomic gas-jet target is prepared and crossed with the antiproton beam. Antiprotons as well as electrons emitted during the reaction will be detected by a microchannel plate (MCP) while the antiproton annihilation will be recognized by detection of annihilation products—mostly pions—by surrounding scintillation counters. When the antiproton is captured, it forms a neutral antiprotonic helium atom, some in a metastable state whose level structures have been well studied with spectroscopic methods. Severe identification of particles and atoms plays an essential role in the design of the experiment, to distinguish the small number of reaction events out of a huge pile of background events. Our strategies for our near-future experiments are discussed.