The PUMA experiment (short for Proton‑Unaffected Matter Annihilation) is a research program proposed for the Antiproton Decelerator (AD) at CERN. Its primary scientific objective is to investigate the structure of short‑lived, neutron‑rich nuclei by employing low‑energy antiprotons as a probe. The experiment is designed to study the distribution of neutrons at the surface of exotic isotopes—often referred to as the “neutron skin”—through the measurement of antiproton annihilation signatures.
Scientific context
Understanding the neutron‑skin thickness of nuclei has implications for nuclear theory, the equation of state of nuclear matter, and astrophysical phenomena such as neutron stars. Conventional techniques for measuring neutron distributions (e.g., parity‑violating electron scattering) are limited to stable isotopes; PUMA aims to extend such studies to unstable isotopes produced at radioactive‑ion beam facilities.
Methodology
- Antiproton preparation – Antiprotons are produced by the AD, decelerated to kinetic energies of a few keV, and confined in a Penning‑Malmberg trap.
- Exotic‑nucleus delivery – Short‑lived isotopes are generated by ISOLDE (or another on‑site radioactive‑ion source) and transported as a low‑energy beam or as a trapped cloud to the interaction region.
- Antiproton–nucleus interaction – Antiprotons are released onto the exotic‑nucleus target. Annihilation predominantly occurs at the nuclear periphery, producing characteristic pions and other secondary particles.
- Detection – A set of silicon detectors, time‑projection chambers, and scintillators records the annihilation products, allowing reconstruction of the spatial distribution of annihilation points and thus inference of the neutron density profile.
Status and collaborations
The experiment has been submitted to the CERN Research Board and received provisional approval as a dedicated AD project. It involves an international collaboration of institutions from Europe, North America, and Asia, with participation from nuclear‑physics groups experienced in antiproton trapping, radioactive‑ion beam handling, and detector development. As of the latest publicly available reports (2023–2024), hardware development and integration tests are underway, with data‑taking anticipated in the mid‑2020s pending final AD schedule allocation.
Significance
If successful, PUMA will provide the first systematic antiproton‑annihilation measurements on a range of unstable isotopes, offering a novel, model‑independent probe of neutron‑skin thickness. The results are expected to constrain nuclear‑structure models and improve theoretical descriptions of neutron‑rich matter, thereby contributing to both fundamental nuclear physics and related astrophysical research.