First observation of ²⁸O.

Area of Science:

• Nuclear Physics
• Quantum Chromodynamics

Background:

• Investigating atomic nuclei under extreme conditions, specifically those with asymmetric neutron-to-proton (N/Z) ratios, is crucial for understanding nuclear organization.
• Neutron-rich isotopes beyond binding limits, existing as short-lived resonances, offer stringent tests for nuclear structure theories.

Purpose of the Study:

• To report the first observation of the neutron-rich oxygen isotopes 28O and 27O.
• To compare the decay energies of 27O and 28O with theoretical models, including large-scale shell-model calculations and statistical approaches.
• To investigate the N=20 shell structure in 28O.

Main Methods:

• Observation of 28O and 27O through their decay products (24O and neutrons).
• Comparison of experimental decay energies with theoretical predictions derived from effective field theories of quantum chromodynamics.
• Analysis of the production cross-section of 28O from a 29F beam.

Main Results:

• The first observation of 28O and 27O is reported, decaying into 24O plus neutrons.
• Both isotopes were identified as narrow, low-lying resonances.
• Experimental data suggest that 28O may not exhibit a closed N=20 shell structure, contrary to some theoretical expectations.

Conclusions:

• The study provides crucial data on the properties of extremely neutron-rich oxygen isotopes.
• The findings challenge and refine current nuclear structure theories, particularly concerning shell closures in exotic nuclei.
• The results highlight the importance of studying nuclei at the limits of stability to test fundamental physics principles.