Observation of a correlated free four-neutron system
Dr Stefanos Paschalis and researchers from Technical University of Darmstadt publish report in Nature.
Their report on the first unambiguous experimental observation of resonance-like correlations between four free neutrons, the so called tetraneutron system, which has evaded detection for six decades and has sparked numerous debates within the nuclear physics community. The pioneering experiment was carried out at the SAMURAI apparatus at RIBF/RIKEN, Japan, in which the alpha core from a high intensity radioactive-ion beam of 8He was suddenly removed in a quasi-elastic collision with a liquid hydrogen target leaving the 4 neutrons as spectators and observing their correlations.
Nuclei form the core of every atom around us and are held together tightly under the strong nuclear force in various combinations of neutron and proton numbers. In nuclear scales, the Coulomb repulsion between positively charged protons is weaker than the strong nuclear force; yet nuclear physicists have long wondered if a chargeless nucleus - composed purely neutrons and hence with no Coulomb repulsion - could ever exist, even as an elusive resonant structure. This question has proved rather challenging to answer both experimentally and theoretically and constituted one of the holy grails of nuclear physics research.
The system of four neutrons provides a stringent test for nuclear theories and offers unique access to the detailed nature of two- and three-body forces. Its properties can also help us to model and understand the interior of unique astrophysical sites such neutron stars, where such system could possibly condensate.