Exploring exotic nuclear landscapes and their cosmic implications

Researchers at Peking University in China have successfully observed the elusive 0₂⁺ state of ⁸It revealed a new cluster structure with two strongly correlated neutron pairs. This discovery provides insight into exotic nuclear structures and their potential implications for understanding neutron stars. The results are published in Physical Examination Letters.

The conventional nuclear model in physics posits a single-particle picture in which nucleons, protons, and neutrons move independently within a nucleus, forming a well-defined shell structure. Governed by an average potential created by nuclear forces, nucleons fill distinct energy levels or shells, leading to the increased stability associated with magic numbers.

This model, rooted in quantum mechanics, successfully explains nuclear structure and stability, but faces limitations when dealing with exotic nuclei, particularly those that are neutron-rich and unstable.

The study’s first author, Professor Zaihong Yang, explained the team’s motivation to Phys.org: “As a nuclear physicist, one of our main goals is to understand what the structure of the nucleus is and how it results from complex nuclear interactions. between the constituent nucleons.

Of particular interest is the condensate-like cluster structure in the neutron-rich core. ⁸He.

“A condensate-like cluster state composed of one alpha cluster and two dineutron clusters has been theoretically predicted in the neutron-rich core. ⁸Him, but its experimental observation has remained elusive due to the difficulty of both producing and identifying this exotic cluster state,” said Professor Yang.

Cluster states and resonant states of ⁸He

The mentioned cluster state refers to a specific nuclear configuration in the neutron-rich nucleus. ⁸He.

In this state, two highly correlated pairs of neutrons, called dineutron clusters, combine with an alpha cluster (four helium nuclei), forming what the researchers describe as a “condensate-like cluster structure.”

The term “condensate-like” draws an analogy to Bose-Einstein condensates (BEC), a state of matter formed at extremely low temperatures.

In BECs, particles such as atoms occupy the same quantum state, exhibiting collective behavior. Likewise, within the framework of ⁸The cluster state term suggests that the two dineutron clusters and the alpha cluster collectively contribute to the nuclear structure.

Dineutron clusters are rated 0₂⁺with “0” indicating spin parity (in this case spin 0), the “2” being the energy state and the “+” being (positive) parity.

To observe the theorized state, the research team performed a nuclear diffusion experiment at the RIKEN Nishina Center in Japan. This experimental enterprise was designed to probe and scrutinize the theorized state of the cluster within ⁸He.

Emphasis has been placed on distinctive features including the elusive spin parity of the cluster state, an unusually considerable isoscalar monopole transition force, and the emission of a highly correlated neutron pair.

“Together with state-of-the-art theoretical calculations, our results provide strong evidence that the four valence neutrons in the 0₂⁺ excited state of ⁸It can form two highly correlated neutron pairs (dineutron clusters) and further form an exotic condensate-like cluster structure,” explained Professor Yanlin Ye, second author of the study.

This achievement not only validates theoretical predictions, but also highlights the ingenuity required in experimental design to navigate the intricacies of nuclear physics.

Speaking about this, Professor Yang said: “An immediate implication of our findings is that unstable nuclei at the limit of stability may exhibit exotic structures that are distinct from conventional images of single particles or shell models. , which calls for the improvement of nuclear systems. structural theories.

“Moreover, while the nucleus is essentially made up of fermionic nucleons (protons and neutrons), our results show that the structure of this 0₂⁺ The state is nevertheless bosonic – a cluster state analogous to the BEC – composed of two dineutron clusters and an alpha cluster.

Neutron stars and pulsars

The 0 observed₂⁺ state of ⁸Its implications go beyond nuclear and quantum physics. This is of profound importance for our understanding of astrophysical phenomena, particularly the cooling process of neutron stars and pulsar glitches.

Dr. Yang elucidated the potential link between 0₂⁺ state and neutron stars. The observed condensate-like cluster structure is consistent with the suggested occurrence of neutron superfluidity inside neutron stars. This phenomenon is similar to the condensation of Cooper electron pairs in superconductors.

“Although we cannot visit a real neutron star to acquire its dense neutron-rich matter, its properties can be inferred from experiments with finite cores in the laboratory.”

“The 0₂⁺ This state, characterized by its unique cluster configuration, offers valuable insights into the formation of a neutron pair condensation state. Importantly, it could be a precursor state to a macroscopic condensate of neutron pairs in neutron-rich systems, including neutron stars,” he explained.

This link between nuclear physics and astrophysics not only improves our understanding of exotic nuclear structures, but also helps unravel the mysteries of cosmic phenomena, shedding light on the complex interplay between the microscopic world of nuclei and the macroscopic domains of stars neutrons and pulsars.

Considering the path forward, the researchers plan to extend the measurements to other neutron-rich nuclei located around the neutron drip line (the limit of existence on the nuclear map).

“We are particularly interested in how the structure of condensate-like clusters evolves with more dineutron clusters. Exploring systems consisting of only neutrons, such as tetraneutrons and hexaneutrons, adds intrigue.”

“Producing and identifying such states is a challenge, but building global radioactive ion beam facilities and new detection systems provides good opportunities,” Professor Ye concluded.

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