Existence of a short-lived tetraneutron predicted

A member of the Lomonosov Moscow State University together with his colleagues, using new interaction between neutrons, have theoretically justified the low-energy tertaneutron resonance obtained recently experimentally. This proves the existence for a very short period of time of a particle consisting of four neutrons. According to the supercomputer simulations, the tetraneutron lifetime is 5×10^-22 sec. The research results are published in a top-ranked journal Physical Review Letters.

A team, consisting of Russian, German and American scientists, and among them Andrey Shirokov, Senior Researcher at the Skobeltsyn Institute of Nuclear Physics, has calculated the energy of the resonant tetraneutron state. Their theoretical computations, based on a new approach and new interaction between neutrons, correlate with the results of the experiment in which the tetraneutron has been produced.

Searching for neutron stability

A neutron lives about 15 min before it decays producing a proton, electron and antineutrino. There is also another known stable system consisting of a huge number of neutrons – a neutron star. Scientists have aimed to find out whether there are other systems, even short-lived, composed purely of neutrons.

A system made up of two neutrons doesn't form even a short-lived state. Due to multi-year experimental and fundamental researches, scientists conclude that there are no such states in a system made up of three neutrons. Searches for a tetraneutron, a cluster of four neutrons, have been conducted for more than 50 years. These searches were fruitless until 2002 when a group of French researchers in an experiment at the Large Heavy Ion National Accelerator (Grand accélérateur national d'ions lourds – GANIL) in Caen has found 6 events which could be interpreted as the tetraneutron production. However, the reproduction of this experiment failed, and some scientists suppose that at least a part of the original data analysis was incorrect.

A new phase of the tetraneutron searches takes place at the Radioactive Ion Beam Factoryin the RIKEN Institute, Japan, where a high-quality beam of ⁸He nuclei is available. The ⁸He nucleus consists of an α-particle (the ⁴He nuclei) and four neutrons. A few research teams from different countries have proposed the tetraneutron searches in RIKEN. In the first of these experimental searches, the ⁸He nuclei were bombarding the ⁴He target. As a result of the collision, the α-particle was knocked out from ⁸He leaving the system of 4 neutrons. Four events interpreted as the short-lived tetraneutron resonant statehave been detected. This experiment of the Japanese group has been published at the beginning of this year, and it will be continued.

How long could a tetraneutron live?

The scientist from Lomonosov Moscow State University and his collaborators have published in their article theoretical evaluations of the tetraneutron resonant state energy and its lifetime. They have contributed to the preparation of one of the proposed experimental searches for the tetraneutron when a group of experimentalists from Germany asked for the assistance.

Andrey Shirokov, the first author of the article, says: “Such evaluations were made by us in different models, and the obtained results were used to support the experiment application. Afterwards, we thoroughly elaborated thetheoretical approach and performed numerous simulations on supercomputers. The results have been published in our paper in Physical Review Letters“.

The theoretical results for the energy of tetraneutron resonance of 0.84 MeV correlate well with the Japanese experimental findingof 0.83 MeV which is however characterized by a large uncertainty (about ±2 MeV). The calculated width of the resonant tetraneutron state is 1.4 MeV which corresponds to the lifetime of about 5×10^-22 sec.

Andey Shirokov continues: “It's worth noting that none of theoretical papers up to now has predicted the existence of the resonant tetraneutron state at such low energies of about 1 MeV”.

The new theoretical result probably stems from a new theoretical approach to the studies of resonant states in nuclear systems developed by the scientists. This approach has been carefully tested on model problems and in less complicated systems and only afterwards applied to the tetraneutron studies accounting for the specifics of the four-particle decay of this system.

Andrey Shirokov however indicates an alternative possibility: “Another possible reason is the fact that we've used a new interaction between neutrons elaborated by our team. Our tetraneutron studies will be continued, we'll perform simulations with other more traditional interactions. At the same time, our French colleagues are going to study thetetraneutron with our interaction within their approach. Of course, all of us are looking forward for the results of new experimental tetraneutron searches”.

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The research has been conducted by a large international team of theorists with Russia been represented by scientists not only from the Lomonosov Moscow State University, but also from the Pacific National University (Khabarovsk). This team includes also collaborators from USA and Germany. Researchers from South Korea are joining the group for future studies. The Russian side has been at the forefront of this research leading the elaboration of the theoretical approach to the resonant states and the design of the new interaction between particles in atomic nuclei.