Long sought-after particle consisting of four neutrons discovered

Research team for the first time observed a neutral nucleus — the tetraneutron


This news is based on a press release of the Technical University Darmstadt.

An international research team with participation by the GSI Helmholtzzentrum für Schwerionenforschung succeeded for the first time to create an isolated four-neutron system with low relative energy in a volume corresponding to that of an atomic nucleus. The scientists have overcome the experimental challenge by employing a new method.

The experiment has been carried out at the Radioactive Ion Beam Factory RIBF at RIKEN (Japan) by a large international research team led by Technical University Darmstadt. Significantly involved besides GSI were scientists from TU Munich and the RIKEN Nishina Center. The experiment yielded an unambiguous signal for the first observation of the tetraneutron. The result has been published in the current issue of “Nature”.

The building blocks of atomic nuclei are nucleons, which exist as two kinds, the neutral neutrons and the charged protons, representing the two isospin states of the nucleon. To our present knowledge, nuclei made of neutrons only are not existing as bound nuclei. The only bound systems known made of almost only neutrons are neutron stars, which are very compact high-density objects in the universe bound by the gravitational force with typical radii of around 10 kilometers. Atomic nuclei are bound by the nuclear strong force with a preference to balance neutrons and protons, as known for the light stable nuclei we find on earth.

Better understanding of neutron-star properties

The study of pure neutron systems is of particular importance since they provide the only means to extract experimental information on the interaction among several neutrons and thereby on the nuclear force. If multi-neutron systems do exist as resonances or even bound states has been a long-standing quest in nuclear physics. The exploration of the so far hypothetical particles might furthermore provide information helping for a better understanding of neutron-star properties. If multi-neutron systems do exist as unbound resonant states or even bound states has been a long-standing quest in nuclear physics. The research team set out to undertake a new attempt by using a different experimental technique as compared to previous attempts. This work was supported by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) via the SFB 1245.

“This experimental break-through provides a benchmark to test the nuclear force with a pure system made of neutrons only", says Dr. Meytal Duer from Institute for Nuclear Physics at the TU 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. We are currently planning to a next-generation experiment at R3B at FAIR, which will detect directly the correlations among the four neutrons with the R3B NeuLAND detector, which will give deeper insight to the nature of this four-neutron system”.

The experimental study of pure neutron systems is challenging because targets — which are the matter samples subject to the particle beam — solely made of neutrons do not exist. In order to create multi-neutron systems in a volume where the neutrons can interact via the short-range nuclear force (few femto-meter, 10-15 meter), nuclear reactions have to be used. Here, the interaction of the neutrons with other particles involved in the reaction process poses a major problem, which can mask the properties of the pure neutron interaction. The scientists have overcome this problem by using a high-energy 8He beam. The 8He consists of a compact alpha particle (4He) which is surrounded by the additional four neutrons in a cloud of lower density. The alpha particle is removed from 8He in a high-energy reaction instantaneously, induced by a proton of the liquid hydrogen target. The remaining four neutrons are suddenly free and can form a four-neutron state.

“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”, says Professor Dr. Thomas Aumann, head of the research department “Nuclear Reactions” at GSI/FAIR and a professor at the Institute for Nuclear Physics of TU Darmstadt. “The extreme kinematics resulted in an almost background-free measurement. We now plan to employ the same reaction in an 6He experiment at the RIBF to make a precision measurement of the low-energy neutron-neutron interaction. A dedicated neutron detector for this experiment is currently being built at our university”. (TUDa/CP)

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