Short-Range Correlations and Isospin at R3B – COCOTIER

For the study of these correlations, we use the mechanism of the quasi free scattering in inverse kinematics: the exotic ion beam is sent on at a proton target, and all the reaction products are detected. It is then possible to count the pair numbers, and to deduce the momentum impulse of the nucleons forming the pair. For this purpose a liquid Hydrogen target has been built at IRFU and will be coupled to the R3B detection system. This system combines a tracking detector (Silicon tracker), the CALIFA calorimeter, the NeuLAND neutron detector, and the analysis system of the fragment residue and of the charged particles, which are emitted at forward angles and deviated by the magnetic field of GLAD.

LH2 target construction has been completed. In the meanwhile, we took the opportunity to join the first measurement of SRC in 12C nucleus in inverse kinematics at Dubna, and we are now contributing to data analysis.

The LH2 target will be sent in October 2019 at GSI, to be commissioned in beam in November-December 2019 and be used in 2020 for first physics experiments. The first measurement of SRC in exotic nuclei is expected from 2022.

A paper on the R3B setup and program is being prepared by the collaboration. Several publications on international reviews based on the results of experiments are expected are expected.

Understanding how the nuclear force emerges from the basic constituents of matter is one of the challenges of contemporary physics. It is empirically established that the nucleon-nucleon (NN) interaction has an intermediate-range attractive part (for NN distances between 1 and 1.5 fm) and a short-range repulsive part (for NN distances below 1 fm), which is poorly known.
The presence of a high momentum tail (k > kF, with kF ~ 250 MeV/c) in the nucleon momentum distribution, and the back-to-back emission of nucleons with such high momenta have been observed in proton and electron induced quasi-free scattering and interpreted in terms of short-range correlations (SRC).
SRC, i.e. the combination of the attraction at intermediate range and repulsion at short range of NN potentials give rise to a spatially compact configuration (~1-1.5 fm) of nucleons with high relative momentum. SRC offer a unique laboratory benchmark for the short-range part of NN interaction that plays an important role in calculations involving nuclear matter at high density and momentum, such as neutron stars.
Nucleons (protons and neutrons) are commonly considered as the relevant degrees of freedom to describe nuclear properties. Nevertheless, some phenomena fail to be accounted for in this approach, like the European Muon Collaboration effect, which shows that the partonic structure of bound nucleons is modified by the nuclear medium. The striking linear correlation between SRC and EMC effect strength shows that nucleons affected by SRC are susceptible to undergo such modifications.
About 20% of nucleons are paired in the SRC regime. Among them, 90% are paired in neutron-proton pairs (isospin T=0). The prevalence of isospin T=0 pairs is associated with the dominance of the tensor interaction, mainly active in the T=0 channel, for NN distances ~1-1.5 fm and implies that the minority species (protons in the case of stable nuclei with mass A>20) has on average higher momentum and kinetic energy than the majority species. The isospin content of SRC has been determined up to now only for stable nuclei with N/Z asymmetry close to 1 (N/Z ~ 1-1.5), and data are integrated over very large intrinsic nucleon momentum windows due to the limited statistics available. Exploring the N/Z and momentum dependence of the isospin content of SRC will allow testing our understanding of SRC. Is the current description of SRC as short-distance and high-relative momentum nucleon pairs correct? What is the role of the different terms of the nuclear interaction (central, tensor) at different relative momenta (and therefore distances)? Are nucleons and mesons the right degrees of freedom to describe nuclear structure when high-momentum probes are involved?
The goal of the COCOTIER project is to address these questions bypassing both limitations in statistics and N/Z asymmetry by performing a high-luminosity measurement with radioactive beams in inverse kinematics. The study of the evolution of SRC along the Oxygen isotopic chain from 14O to 24O will be the subject of the first COCOTIER experiment.
The experimental method that will be used to probe SRC is the well known Quasi-Free Scattering reaction. In inverse kinematics this demands the use of a proton target, that for luminosity reasons has to be a thick cryogenic liquid hydrogen target. Such a target with a thickness of 20 cm along the beam axis (corresponding of 1.5 g.cm-2 of hydrogen) will be developed at IRFU within the COCOTIER project, and combined with the R3B detection system at the GSI accelerator facility in Germany, a world-unique laboratory capable to deliver radioactive ion beams at energies above 1 GeV/u, key for the extraction of SRC observables. The combination of GSI radioactive ion-beams, the R3B detection system and the liquid hydrogen target from IRFU is a unique asset of the COCOTIER project.