Nucleon-nucleon correlations in exotic nuclei

Exploring the Structure of Exotic Nuclei

Our research investigates the properties of exotic nuclei—those far from stability with a significant proton-to-neutron imbalance. These systems offer unique insights into the fundamental nuclear forces that govern atomic structure and are key to understanding how nuclear matter behaves under extreme conditions.

One of our central goals is to explore the role of nucleon-nucleon (NN) correlations, which influence nuclear structure and dynamics, especially in systems with large isospin asymmetry. These correlations modify the effective single-particle energies and drive the evolution of nuclear shapes and properties—from spherical to deformed to superfluid—as strikingly illustrated in the N = 20 Island of Inversion. While traditional shell-model calculations with effective two-body interactions have provided a strong foundation, recent ab-initio approaches have revealed the crucial role of three-nucleon (3N) forces; these forces are essential for accurately reproducing experimental results when starting from realistic two-body interactions. As we probe nuclei closer to the dripline, additional phenomena—such as weak binding and the coupling to continuum states—become increasingly important.

Furthermore, experimental evidence suggests the presence of short-range neutron-proton correlations (SRCs), where nucleons form strongly correlated pairs. A full understanding of nuclear SRCs and their isospin dependence is pivotal for studies of compact objects like neutron stars and the nuclear equation of state. Indeed, SRC nucleon-nucleon pairs are formed as temporary high-density fluctuations with 2-5 times the nuclear saturation density, limited by the nucleon-nucleon (NN) interaction that becomes highly repulsive for NN distances smaller than about 1 fm.

Our Experimental Programme

We lead cutting-edge experiments at major radioactive-ion beam facilities, including:
· GSI/FAIR (Germany)
· RIBF/RIKEN (Japan)
· NSCL/FRIB (USA)

These large-scale efforts are complemented by targeted experiments at stable ion-beam laboratories.
Our work contributes directly to the UK Science and Technology Facilities Council (STFC) Grand Challenge C6: “What is the nature of nuclear matter?”
Specifically, we address:
· How do short-range correlations manifest in low-energy observables, and how do they evolve in isospin-asymmetric systems?
· How do single-particle and collective degrees of freedom evolve in exotic nuclei?
· What role do weak binding and continuum effects play near the dripline?
· Are current first-principles theories sufficient to capture all relevant NN and 3N correlation effects?
· How do isoscalar (spin-singlet) neutron-proton pairing correlations influence nuclear structure?

Building the Future of Experimental Nuclear Physics
Innovation in detector technologies and experimental methods is central to our mission. We aim to both exploit existing STFC-funded infrastructure and lead the development of new experimental devices.
We currently coordinate two major international initiatives:
· HYPATIA – a novel detector array for experiments at RIBF (Japan)
· TRT@R3B – a cutting-edge tracking system to be deployed at FAIR (Germany)
These efforts ensure that we remain at the forefront of global nuclear physics research, while training the next generation of experimentalists and building lasting international partnerships.

Picture from [Protons and neutrons cosy up in nuclei and neutron stars; Short-range correlations can give rise to dense nuclear systems; Douglas Higinbotham, Eli Piasetzky and Mark Strikman, Feature in Astrophysics and Cosmology 2009]

Influence of short-range correlations in massive objects like neutron stars. On the left, a large object, such as a neutron star, without SRCs could be approximated as a neutron Fermi gas with a small fraction of protons acting as a separate Fermi gas. With SRCs the protons and neutrons interact, leading to changes in the physical properties of the system. Indeed, the isospin dependency of SRCs allows us also to speculate about the nature of a quasi-proton (nuclear polaron) in neutron matter, with some studies suggesting a proton kinetic energy of approximately 3 times that of a proton in a Fermi Gas, an important quantity for the properties of neutron stars. By studying SRCs in the laboratory we can infer the properties of neutron stars.

Picture from [Protons and neutrons cosy up in nuclei and neutron stars; Short-range correlations can give rise to dense nuclear systems; Douglas Higinbotham, Eli Piasetzky and Mark Strikman, Feature in Astrophysics and Cosmology 2009]