New evidence of the tetraneutron experiment could have proven a long-sought particle made up of four neutrons –

Searched for 50 years: For a long time it was disputed whether there could be a particle made up of four neutrons – now physicists could have detected this tetraneutron. This was indicated by the collision of lithium-7 isotopes, in which a telltale “hump” in the measured values ​​indicated the formation of tetraneutrons. Together with evidence from two other experiments, this makes the existence of the “impossible” particle more and more likely.

Together with the positively charged protons, neutrons form the basic building blocks of all matter – the atomic nucleus consists of them. Both core components are held together by the strong nuclear force and can transform into one another. For example, neutrons released by radioactive elements decay into protons after just under 15 minutes, whereas under the extreme conditions inside a neutron star, according to current theory, protons are supposed to be converted into neutrons.

For around 50 years, however, it has been disputed whether there are also particles that only consist of neutrons. According to the theory, they should not be stable because this would contradict current models of strong nuclear force and also the Pauli exclusion principle, according to which fermions cannot occur in the same state in the same place. On the other hand, the existence of neutron stars suggests that objects must very well only exist from closely packed neutrons.

Schematic representation of the nuclear reaction from lithium-7 to carbon-10 and the tetraneutron. © Mahmoud Mahgoub / TUM

Lithium isotope as a generation aid

Thomas Faestermann from the Technical University of Munich and his colleagues have now tried to produce and prove the controversial tetraneutron in an experiment. To do this, they shot a beam of highly accelerated lithium atomic nuclei at a target made of lithium oxide and carbon. The highlight: In both cases, the lithium atomic nuclei were the isotope lithium-7, which consists of three protons and four neutrons.

“Lithium-7 is the closest stable atomic nucleus to the tetra-neutron,” the researchers explain. In order to produce a particle made up of four neutrons, only three protons have to be extracted from the lithium-7. This is exactly what the collision in the tandem Van de Graaff accelerator at the Technical University of Munich was supposed to do. If the tetraneutron existed, both would have to be used when bombarding the target
Carbon-10 isotopes as well as tetraneutrons are formed.

Telltale hump in the curve

In fact, the physicists discovered a striking signature in the energies and masses of the particles generated by the collisions. It was shown in the form of a “hump” in the curve at 20.8 megaelectron volts. More detailed analyzes showed that this peak cannot be explained by any of the common particles, but can be explained by the formation of tetra neutrons and excited carbon-10 isotopes.

“For us, this is the only physically plausible explanation of the measured values ​​in all points,” explains Faestermann. According to the measurements, the tetraneutron has a binding energy of 0.42 megaelectron volts, which agrees well with theoretically determined values. According to this, such a particle of four neutrons would remain stable for around 450 seconds before it decays into protons and other elementary particles in beta decay.

Evidence for the tetraneutron is hardening

The significance of the results with 99.7 percent or three sigma is still not quite sufficient for an official discovery – five sigma would be necessary for this. Therefore, the measurements have to be further refined and checked. On the other hand, this is not the first indication of the existence of tetra neutrons: French physicists had already obtained evidence of a particle made up of four neutrons around 20 years ago when they bombarded carbon with beryllium-14 nuclei – but could not prove that these neutrons are connected to one another was.

In 2016, however, an experiment in Japan also provided evidence of the tetra-neutron. Researchers shot at Helium-4 with Helium-8 nuclei. A tiny pause lasting only ten trillionths of a second in the measurement data indicated that four bound neutrons instead of individual neutrons could have been released. Taken together, these experiments corroborate the assumption that the long sought-after tetraneutron actually exists. (Physics Letters B, 2021; doi: 10.1016/j.physletb.2021.136799)

Source: Technical University of Munich

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