Physicists observe elusive four-neutron system: tetraneutron |

Physicists using the Superconducting Analyzer for Multi-particles from Radio Isotope Beams (SAMURAI) in Japan have experimentally observed a resonance-like structure consistent with a tetraneutron state after 60 years of experimental attempts to clarify its existence.

Schematic representation of the quasi-elastic reaction investigated by Duer et al† Above: quasi-elastic scattering of the helium-4 (4He) nucleus of a helium-8 (8He) projectile from a proton target in the lab frame. The length of the arrows indicates the momentum per nucleon (the velocity) of the incoming and outgoing particles. zRay is the radius axis. Soil: the equivalent p-4The elastic scattering in their center of gravity frame, where we consider reactions with backward angles close to 180°. In this frame, the momentum of the proton balances that of 4He, Pp=−P4He, that is, the proton is four times faster than 4He. Image credit: Due et al., doi: 10.1038/s41586-022-04827-6.

A long-standing question in nuclear physics is whether chargeless nuclear systems can exist.

As far as we know, only neutron stars represent nearly pure neutron systems, where neutrons are compressed to very high densities by gravity.

The free neutron has has a lifetime of just under 15 minutes and decays into a proton, electron and antineutrino.

The system made of two neutrons, the dineutronwas unequivocally observed in the decay of beryllium-16 in 2012 and is known to be unbound for only about 100 keV.

The next simplest system of three neutrons is less likely due to the odd number of nucleons and therefore weaker bonding; yet a recent calculation has suggested its existence.

In light of these considerations, the four-neutron system, the tetraneutron, is a suitable candidate to answer this question.

Numerous attempts have been made to find a clue to its existence as a bound or resonant state.

Most of these experiments were performed with stable nuclei. By the 21st century, with the development of radioactive ion beam facilities, it became possible to use extremely neutron-rich nuclei in which one can expect improved formation of a tetraneutron system.

“Our experimental breakthrough provides a benchmark to test nuclear force with a pure system composed only of neutrons,” said Dr. Meytal Duer, a physicist at the Institute of Nuclear Physics at the Technische Universität Darmstadt.

“The nuclear interaction between more than two neutrons could not be tested until now, and theoretical predictions yield a wide spread regarding the energy and width of a possible tetraneutron state.”

dr. Duer and colleagues conducted the experimental study using the Multi-particle radioisotope beam superconducting analyzer (SAMURAI) at the Radioactive Ion Beam Factory administered by the RIKEN Nishina Center and the Center for Nuclear Study, University of Tokyo.

To produce a tetraneutron state, they used the knockout of an alpha particle (helium-4 nucleus) from a high-energy helium-8 projectile induced by a proton target.

“Key to the successful observation of the tetraneutron was the chosen reaction, which isolates the four neutrons in a fast – compared to the nuclear scale – process, and the chosen kinematics of large momentum transfer, which separates the neutrons from the charged particles in momentum space. ”, says Professor Thomas Aumann, a physicist at the Institute of Nuclear Physics at the Technische Universität Darmstadt.

“The extreme kinematics resulted in an almost background-free measurement.”

“We now plan to use the same reaction to get an accurate measurement of the low-energy neutron-neutron interaction. A special neutron detector for this experiment is currently being built.”

A paper about the findings appears in the journal Nature

M. Duero et al† 2022. Observation of a correlated free four-neutron system. Nature 606, 678-682; doi: 10.1038/s41586-022-04827-6

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