PUMA is a new experiment proposed at both the CERN AD and ISOLDE facilities, that would for the first time transport antiprotons trapped at AD/ELENA to ISOLDE, by carrying them in a trap loaded onto a truck. The antiprotons would then be used for nuclear physics experiments at ISOLDE. The proposed experiment has been favourably reviewed by the SPSC and INTC committees, pending for final approval after a more thorough review of the required CERN resources.
Radioactive isotopes (RI) reveal new many-body phenomena originating in their neutron-to-proton asymmetry or in their low binding energy compared to stable nuclei. Highlight examples of these phenomena are the development of neutron skins at the nuclear surface, neutron halos for weakly bound nuclei close to the so-called neutron dripline, or the nuclear shell evolution as a function of number of protons and neutrons. The investigation of RI is necessary for a global understanding of the nuclear many-body problem and the role of the underlying many-body forces.
In particular, the discovery of halo nuclei was made in 1985 . An effective matter radius of neutron-rich light nuclei was extracted from their reaction cross section when impinging heavy-ion targets. Nuclei such as 6He, 8He and 11Li showed a strong increase of their matter radius compared to lighter isotopes. This increase was later interpreted, first by P. G. Hansen and B. Jonson from ISOLDE at CERN, as a tunnelling effect of loosely bound neutrons whose wave function extends beyond the short range attraction of the rest of the nucleus . This neutron-halo phenomenon was at the origin of RI studies at a large scale, and halos have continued to be an object of fascination and exploration in nuclear physics . Interestingly enough, the term “neutron halo” was first used a decade before this discovery from the interaction of heavy nuclei with… low energy antiprotons to qualify the excess of neutrons at the nuclear surface in stable nuclei .
The development of a neutron skin on the nuclear surface along an isotopic chain, i.e. a nucleus with the same number of protons and varying number of neutrons, from stability towards the neutron drip line can be correlated with the bulk properties of the in-medium nucleon-nucleon interaction  with a strong connection to the nuclear equation of state that drives, among others, the physics of neutron stars .
Antiprotons, as a probe to study short-lived isotopes, remain unexploited despite the pioneering work with stable nuclei, in particular at CERN in the 90s . Indeed, low-energy antiprotons offer a very unique sensitivity to the neutron and proton densities at the annihilation site, in the tail of the nuclear matter density. Experimental techniques such as nucleon removal reactions and elastic scattering are other hadronic probes sensitive to the nuclear surface which are used to extract the matter radius of RI (see [8,9,10] for recent studies). They are complementary to experiments with low-energy antiprotons since they are sensitive to the nuclear surface, typically where the density is half of the saturation density, while the annihilation with low energy antiprotons is sensitive to the tail typically at 2 fm from the surface, where the nuclear density is 10% or less of the saturation density. Such studies with short-lived nuclei and low-energy antiprotons are the motivation of the proposed antiProton Unstable Matter Annihilation (PUMA) experiment .
The first objectives of the PUMA experiment are (i) to provide a new observable for radioactive nuclei that characterises the neutron- to-proton asymmetry of their matter density tail, namely the neutron-to-proton annihilation ratio, (ii) to characterise the matter density tail of known halos and neutron skins with this new method, (iii) to evidence new proton and neutron halos, (iv) to understand the development of neutron skins in medium-mass nuclei along isotopic chains.
Today, no facility provides a collider of low-energy radioactive ions and low-energy antiprotons: PUMA aims at transporting antiprotons (the long term goal is 1 billion antiprotons) from CERN/ELENA to CERN/ISOLDE to perform the capture and annihilation of low-energy antiprotons by short-lived nuclei, and probe in this way the so-far unexplored isospin composition of the radial-matter-density tail of radioactive nuclei.
Fig. 1: Itinerary of PUMA from ELENA to ISOLDE.
PUMA will consist of a fully-transportable experimental setup composed of a 28-cm-large bore 4-Tesla solenoid, with active and passive shielding, a 4-K Penning trap for storing antiprotons and ions and charged-particle trackers composed of a time-projection chamber and a plastic scintillator barrel for the detection of annihilation products. Non destructive diagnostics for the antiproton plasma will also be included in the system. The trap will be composed of two zones, a storage zone and a collision zone.
The main challenge of PUMA is related to the extreme high vacuum required for a long term storage of the antiprotons with a half-life of thirty days corresponding to about a hundred of residual gas molecules per cm3 or about 10−17 mbar at 4 K. As the objective of PUMA is to inject low-energy ions into the antiproton plasma, the antiproton trap needs to be open to the beam lines at ISOLDE. PUMA has been designed for a vacuum of 10-10 mbar at the interface of the apparatus with both ELENA and ISOLDE beam transfer lines.
Once the antiprotons are trapped, the entire system will be transported in operation from ELENA to ISOLDE. The superconducting wire of the magnet itself as well as the trap electrodes are cooled by pulsed tube cold heads. The full experiment requires 70 kW electrical power during transportation, with the main power consumers being the cold-head compressors and the associated chiller. The system relies on a switchable power source going through an uninterruptible power supply (UPS) and thus batteries. At ELENA and ISOLDE, the system will be powered by the normal electrical network. In case of power outages of a few minutes, the UPS will be able to provide the power to the setup to avoid losing the antiprotons. The full experiment will be moved by crane from the experimental zone to a truck for transportation from ELENA to ISOLDE and vice versa. During the transportation, the experiment will be powered by a generator located on the truck.
Fig. 2: Schematic side view of the PUMA setup composed of antiproton and ion traps inside a 4K cryostat and pion detection.
At ISOLDE, low-energy Radioactive Ion Beams will be introduced into the PUMA trap and mixed with antiprotons to favour the formation of antiprotonic atoms, followed by the annihilation of the antiprotons with protons or neutrons of the nucleus. The ratio of the number of annihilated neutrons to the number of annihilated protons will be evaluated by measuring the charged pions produced by the annihilation. The basic principle of PUMA relies on the electric charge conservation of the annihilation process: the total charge of the pions is −1 for a neutron annihilation and 0 for a proton annihilation. The determination of this ratio requires a correction of final state interactions, acceptance and detection efficiency. The accuracy of the final state interaction is key to control systematic uncertainties. A precision better than 10% is targeted based on simulations and reference measurements to be performed at CERN/ELENA. Complementary X-ray measurements from the decay of the formed antiprotonic atoms are also considered for future plans.
Today, the PUMA apparatus is under development. The transportable solenoid is being built and should be delivered at TU Darmstadt in 2020. A trap prototype is being assembled and will be soon operated in a test solenoid. The parameters for the trapping, rotating wall technique and sympathetic cooling will be optimised during the test phase in 2020.
Simulations for the vacuum inside the trap were performed in collaboration with CERN TE-VSC. A program of measurements of hydrogen isotherms at low pressure, down to 10-13 mbar, is foreseen. The cryostat of the PUMA trap will be conceived in collaboration with CERN TE-VSC. In addition, R&D for a fast cold-gate valve to limit the amount of residual gas molecules entering the trap has been initiated at TU Darmstadt. The cryostat will be built in 2020 and the full experiment assembled in 2021.
Moreover, the time-projection chamber for the pion detection is being designed and will be built at CERN EP/ESE. while the electronics has been developed at CEA. The goal is to assemble the full detector in 2021.
The PUMA experiment requires a new experimental zone at ELENA. If the experiment is accepted, the ELENA LNE51 beam line will be completed and PUMA will be installed between the existing GBAR and ASACUSA experiments. At ISOLDE, the location of the experiment is still under discussion while beam optics calculations for both ELENA and ISOLDE are ongoing.
The PUMA technical design report (TDR) should be issued at the end of 2020. A first installation at ELENA is foreseen in 2021, while first measurements at ISOLDE in 2022.
The proposed method is indeed first an unambiguous discovery tool for halos: annihilation from a neutron halo nucleus should lead to a neutron-to-proton annihilation ratio exceeding by an order of magnitude the N/Z ratio of the nucleus, annihilation from a proton halo, on the contrary, should lead to a neutron-to-proton annihilation ratio significantly smaller than unity. Neutron skins could be characterised by a neutron- to-proton annihilation ratios larger than N/Z.
The physics cases to be proposed to ISOLDE range from a re-visit of the historical halos 6He and 11Li, the search for proton halos in neutron-deficient nuclei, the search for thick neutron skins or halos in neutron rich Ne and Mg isotopes and a quantitative study of the development of the neutron skin in oxygen and tin isotopes.
Beyond its Nuclear Physics objectives, PUMA will allow to transport a significant amount of antiprotons and to deliver them to experiments outside the AD and, eventually, outside CERN. PUMA will contribute to democratise antiprotons for basic research and, maybe, trigger new ideas.
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