Antiprotons to hit the road

23 March 2018

A project carried out at the Technische Universität (TU) Darmstadt in Germany, funded by the European Commission, aims to build a magnetic trap that allows antiprotons to be transported from one location to another. Launched in January, the ultimate goal of the PUMA (antiProton Unstable Matter Annihilation) project is to transfer antiprotons from CERN’s Antiproton Decelerator (AD) to the nearby ISOLDE facility to study exotic nuclear phenomena.

One of PUMA’s physics goals is to explore the occurrence of neutron halos and neutron skins in very neutron-rich radioactive nuclei. By measuring pions emitted after the capture of low-energy antiprotons by nuclei, researchers will be able to determine how often the antiprotons annihilate with the constituent nucleons and therefore deduce their relative densities at the surface of the nucleus. It would be the first time that such effects were investigated in medium-mass nuclei, contributing to a better understanding of the complex nature of nuclei and related astrophysical processes. In the future, PUMA might also allow the spectroscopy of single-particle states in heavy-nuclei with atomic numbers above 100, offering new insight into the unknown shell structure at the top of the nuclear landscape.

To make such studies possible, PUMA must trap antiprotons for long enough to be transported by truck for use in nuclear experiments at the ISOLDE facility, located a few hundred metres away from the AD. Keeping the antiprotons from annihilating with ordinary matter during this process is no easy task. The idea is to develop a double-zone trap inside a one-tonne superconducting solenoid magnet and keep it under an extremely high vacuum (10–17 mbar) and at a temperature of 4 K. One region of the trap will confine the antiprotons, while a second zone will host collisions between antiprotons and radioactive nuclei that are produced at ISOLDE but decay too rapidly to be transported and studied elsewhere.

PUMA will eventually trap a record one billion antiprotons at CERN’s GBAR experiment, which is currently being hooked up to the ELENA facility at the AD (CERN Courier December 2016 p16), and keep them for several weeks to allow the measurements to be made. The team plans to build and develop the solenoid, trap and detection apparatus in the next two years, targeting 2022 for first collisions at ISOLDE.

Today, CERN is the only place in the world where low-energy antiprotons are produced, but “this project might lead to the democratisation of the use of antimatter,” says project leader Alexandre Obertelli of TU Darmstadt, who was awarded a €2.55 million five-year grant from the European Research Council. Along with researchers from RIKEN in Japan, CEA Saclay and IPN Orsay in France, Obertelli has submitted a letter of intent to CERN’s experiment committee concerning the future ELENA and ISOLDE activities. The PUMA apparatus could also, at a later stage, provide antiprotons to experiments beyond CERN. “For example, to universities or nuclear-physics laboratories where specific nuclei can be produced, such as the new SPIRAL2 facilities in Caen, France,” says Obertelli.

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