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 illustration of the quasi-elastic reaction investigation by Duer et al. Top: quasi-elastic scattering of the helium-4 (4He) core from a helium-8 (8He) projectile off a proton target in the laboratory frame. The length of the arrows represents the momentum per nucleon (the velocity) of the incoming and outgoing particles. Zbeam is the beam axis. Bottom: the equivalent p-4He elastic scattering in their center-of-mass frame, where we consider reactions at 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: Duer et al., doi: 10.1038/s41586-022-04827-6.

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

To the best of current knowledge, only neutron stars represent near-pure neutron systems, where neutrons are squeezed together by the gravitational force to very high densities.

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

The system made of two neutrons, the dineutron, was unambiguously observed in 2012 in the decay of beryllium-16 and is known to be unbound by only about 100 keV.

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

Following these considerations, the four-neutron system, the tetraneutron, is an appropriate candidate to address this question.

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

Most of these experiments were performed with stable nuclei. Towards 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 an enhanced formation of a tetraneutron system.

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

“The nuclear interaction among more than two neutrons could not be tested so far, and theoretical predictions yield a wide scatter concerning the energy and width of a possible tetraneutron state.”

Dr. Duer and colleagues carried out the experimental study using the Superconducting Analyzer for Multi-particles from Radio Isotope Beams (SAMURAI) at the Radioactive Ion Beam Factory operated 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 for 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,” said Professor Thomas Aumann, a physicist in the Institute for Nuclear Physics at the Technische Universität Darmstadt.

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

“We now plan to employ the same reaction to make a precision measurement of the low-energy neutron-neutron interaction. A dedicated neutron detector for this experiment is currently being built.”

A paper on the findings appears in the journal Nature.

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M. Duer 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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