Antiprotons as a probe to study short-lived isotopes remain unexploited despite past pioneering works with stable nuclei. In particular, low-energy antiprotons offer a unique prospect in terms of sensitivity to the neutron and proton densities at the annihilation site, i.e. in the tail of the nuclear density. Such studies with short-lived nuclei and low-energy antiprotons are the motivation of the proposed antiProton Unstable Matter Annihilation (PUMA) experiment. PUMA aims at transporting one billion antiprotons from ELENA to ISOLDE at CERN to perform the capture of low-energy antiprotons by short-lived nuclei, and probe in this way the so-far unexplored isospin composition of the nuclear-radial-density tail of radioactive nuclei.
The occurrence of neutron halos was discovered in light-mass nuclei at the limit of nuclear existence, i.e. at the neutron drip line . Historically 11Li was the first halo nucleus discovered. Its structure was soon after understood by ISOLDE physicists as a 9Li core surrounded by two neutrons with a large radial extension . Halos are truly a fascinating manifestation of quantum physics. They belong to a subset of clustering for which most of the probability to find the halo neutrons extends to a region of space that is classically forbidden. In other words, the neutrons are well beyond the range of the neutron-core potential and owe their existence to quantum tunnelling. Their properties show universal aspects of few-body systems such as scaling laws, i.e. they do not encode details of the strong force. This makes atomic nuclei, and more particularly exotic neutron-rich systems, a unique laboratory to study universal few-body phenomena. That said, universal features do not provide physicists with a true understanding of nuclear halos. This requires both a precise description of nuclear correlations and an accurate accounting of large-distance behaviour.
Proton halos are expected to exist and few candidates have been explored experimentally. For medium mass nuclei, the appearance of halos is predicted by several effective models with varying outcome from one to another. The role of deformation has been barely studied so far and no data firmly support the existence of deformed halos. A large neutron excess at the nuclear surface is not necessarily a halo and can take the form of a neutron skin. Neutron skins, corresponding to a neutron density higher than the proton density at the surface of the nucleus, have been observed in stable nuclei. Microscopic models predict the development of thick neutron skins in very neutron rich medium mass nuclei, but no experimental evidence exists so far.
Design of the transportable PUMA setup. Two penning traps are used for the storage of antiprotons and for the interaction with ions. The interaction region is surrounded by a time-projection chamber dedicated to the detection of charged pions produced by the annihilation of antiprotons at the surface of the nucleus, after the capture. Antiprotons and ions are radially confined by the 4.0 T magnetic field created by a superconducting solenoid. The transportable frame is composed of two separable units: the main frame and an annex frame dedicated to the power supply (UPS) of the experiment. (Credits: Frank Wienholtz).
Antiprotons as a probe to study short-lived isotopes remain unexploited despite past pioneering works with stable nuclei. In particular, low-energy antiprotons offer a unique prospect in terms of sensitivity to the neutron and proton densities at the annihilation site, i.e. in the tail of the nuclear density. As a matter of fact, the first mention of neutron halos comes from the pioneering antiproton-annihilation nuclear-physics experiment of Bugg et al. . Such studies with short-lived nuclei and low-energy antiprotons are the motivation of the proposed antiProton Unstable Matter Annihilation (PUMA) experiment. Today, no facility provides collisions of low-energy radioactive ions and low-energy antiprotons. The PUMA experiment aims at transporting one billion antiprotons (ten million antiprotons at a first step) from ELENA to ISOLDE, i.e. inside the CERN Meyrin site, to perform the capture of low-energy antiprotons by short-lived nuclei, as proposed by Wada and Yamazaki . The PUMA ion Penning-Malmberg trap will consist of a storage zone (S trap) dedicated to the storage of a large amount of antiprotons produced by the AD-ELENA facility, and a collision zone (C trap) dedicated to the interaction of antiprotons with unstable ions delivered at ISOLDE. The entire system, once filled with antiprotons at ELENA will be transported on a truck to ISOLDE. There, antiprotons will be transferred to the C trap from the S trap. Both zones of the trap will be located in a 4T magnetic field provided by a superconducting solenoid recently delivered at TU Darmstadt, where the experiment is being developed.
The annihilation of the antiprotons is followed by meson emission, mainly pions. Since the electric charge is conserved during the annihilation process, the reconstruction of the total charge of the emitted pions allows for the determination of the charge of the annihilated particles: 0 in the case of antiproton-proton annihilation and -1 in the case of antiproton-neutron annihilation. Thus, the distribution of protons and neutrons in the nuclear periphery can be explored through the charge distribution of the emitted pions. The pions issued from annihilations will be detected by a cylindrical time projection chamber surrounding the C trap.The time projection chamber will be built at CERN. The curvature of the charged-pion trajectories in the magnetic field of the trap will allow to identify the charge of the measured pions. After the corrections from final-state interactions, the ratio of neutron-antiproton annihilations and proton-antiproton annihilations following the antiproton capture will be determined. This is the core observable to be provided by PUMA.
To achieve the necessary long-time storage of the antiprotons and the required sensitivity for the measurement, the design and conception of PUMA requires an extremely high vacuum for the trap, to minimize annihilations from residual-gas atoms. The concept of PUMA vacuum is based on cryopumping on the 4K surface of the trap electrodes and cryostat. The extreme vacuum is maintained as long as the surface density of adsorbed molecules on the walls of the trap does not exceed a critical value given by the H2 isotherm at the temperature of the trap. The design of the PUMA trap and cryostat is in its final phase. Simulations show that an antiproton half-life better than 100 days and a rate of annihilations from residual gas molecules in the collision region of 0.3Hz can be achieved, good enough to perform the PUMA physics cases. The development of the vacuum beam lines and cryostat are being performed in close collaboration with the CERN vacuum group.
The design of the pion detectors is finished and the production should start soon. The beam line and experimental area designs of PUMA at AD-ELENA are finalized and their installation is planned in 2021. The experiment setup would be ready to be installed at the beginning of 2022. An R&D program related to ion manipulation has also been launched at TU Darmstadt. The PUMA experiment was proposed to the SPSC in January 2020  and was recommended to be accepted as an official CERN experiment at the AD . A final decision from CERN’s Research Board is now pending.
The PUMA solenoid in its transportable frame at TU Darmstadt, Germany.
After 20 years of antiprotons production at CERN’s AD facility, mostly devoted to quantify the matter-antimatter (a)symmetry, the delivery of antiprotons as a tool for nuclear physics will provide new opportunities. PUMA could pave the way for other experiments making use of slow and cooled antiprotons that make AD-ELENA a world unique facility.
 I. Tanihata et al., “Measurements of interaction cross sections and nuclear radii in the light p-shell region,” Phys. Rev. Lett. 55, 2676–2679 (1985).
 P. G. Hansen and B. Jonson, “The neutron halo of extremely neutron-rich nuclei,” EPL (Europhysics Letters) 4, 409 (1987).
 W. M. Bugg et al., “Evidence for a neutron halo in heavy nuclei from antiproton absorption,” Phys. Rev. Lett. 31, 475–478 (1973).
 M. Wada and Y. Yamazaki, « Technical developments toward antiprotons atoms for nuclear structure studies of radioactive nuclei », Nucl. Instr. Meth. Res. B 214, 196 (2004).
 “PUMA : antiprotons and radioactive nuclei,” Proposal SPSC-P-361, CERN, September 2019.
 “Minutes of the 136st Meeting of the SPSC,” January 2020. CERN-SPSC-2020-003 SPSC-136.