PUMA – probing the surface of atomic nuclei with antiprotons

Moritz Schlaich (TU Darmstadt, Germany)

The main goal of the PUMA (antiProton Unstable Matter Annihilation) experiment [1] is to use antiprotons as a tool to investigate the matter density of short-lived nuclei. For this, antiprotons produced at the Antiproton Decelerator (AD) facility at CERN and decelerated by the Extra Low Energy Antiproton storage ring (ELENA) will be captured, cooled and transported to the ISODLE facility at CERN where the antiprotons will be mixed with short lived isotopes. During this process, an antiproton can be captured by the nucleus and will subsequently annihilate with a neutron or a proton at the surface of the nucleus itself. The pionic fingerprint of this annihilation will be measured using a time-projection-chamber. With this knowledge, the ratio of protons to neutrons on the outermost part of the nuclei distribution can be determined and phenomena like neutron or proton halos or neutron or proton skins can be investigated.

In a first phase of the experiment, before the antiprotons are transported to ISOLDE, the experimental technique is applied to stable ions that are separated, cooled and bunched in a versatile offline ion source setup. Protons and deuterons can be used, e.g., to benchmark the pion distribution resulting from antiproton annihilations with the simplest nuclei. Furthermore, elements such as tin or xenon allow to study the evolution of the new observable along isotopic chains, while the investigation of 208Pb may provide access to neutron skins [2] already from stable nuclei.

An overview of the PUMA experiment and the offline ion source setup including a multi-reflection time-of-flight mass spectrometer [3] and a radio-frequency quadrupole cooler-buncher will be given. The current status will be discussed and some of the main physics goals will be highlighted.

[1] T. Aumann et al., Eur. Phys. J. A (2022) 58:88
[2] D. Adhikari et al., PREX, Phys. Rev. Lett. 126 (2021) 172502
[3] M. Schlaich et al. Int. J. Mass Spectrom. 495 (2024) 117166