For six decades, researchers have hunted for clusters of four neutrons
called tetraneutrons. But evidence for their existence has been shaky. Now,
scientists say they have observed neutron clusters that appear to be
tetraneutrons. The result strengthens the case that the fab four is more
than a figment of physicists’ imaginations. But some scientists doubt that
the claimed tetraneutrons are really what they seem.
Unlike an atomic nucleus, in which protons and neutrons are solidly bound
together, the purported tetraneutrons seem to be quasi-bound, or resonant,
states. That means that the clumps last only for fleeting instants — in this
case, less than a billionth of a trillionth of a second, the researchers
report in the June 23 Nature.
Tetraneutrons fascinate physicists because, if confirmed, the clusters would
help scientists isolate and probe mysterious neutron-neutron forces and the
inner workings of atomic nuclei. All atomic nuclei contain one or more
protons, so scientists don’t have a complete understanding of the forces at
play within groups composed only of neutrons.
Conclusively spotting the four-neutron assemblage would be a first. “Up to
now, there was no real observation of … such a system that is composed only
from neutrons,” says nuclear physicist Meytal Duer of the Technical
University of Darmstadt in Germany.
To create the neutron quartets, Duer and colleagues started with a beam of a
radioactive, neutron-rich type of helium called helium-8, created at RIKEN
in Wako, Japan. The team then slammed that beam into a target containing
protons. When a helium-8 nucleus and proton collided, the proton knocked out
a group of two protons and two neutrons, also known as an alpha particle.
Because each initial helium-8 nucleus had two protons and six neutrons, that
left four neutrons alone.
By measuring the momenta of the alpha particle and the ricocheting proton,
the researchers determined the energy of the four neutrons. The measurement
revealed a bump on a plot of the neutrons’ energy across multiple collisions
— the signature of a resonance.
Particle smashup
Physicists collided a helium-8 nucleus with a target proton and measured the
momenta of the ricocheting proton and an escaping alpha particle — a clump
of two neutrons and two protons. Those measurements revealed signs that the
four neutrons released in the smashup formed a long-sought cluster called a
tetraneutron.
In the past, “there were indications, but it was never very clear” whether
tetraneutrons existed, says nuclear physicist Marlène Assié of Laboratoire
de Physique des 2 Infinis Irène Joliot-Curie in Orsay, France. In 2016,
Assié and colleagues reported hints of only a few tetraneutrons. In the new
study, the researchers report observing around 30 clusters. The bump on the
new plot is much clearer, she says. “I have no doubts on this measurement.”
But theoretical calculations of what happens when four neutrons collide have
raised skepticism as to whether a tetraneutron resonance can exist. If the
forces between neutrons were strong enough to create a tetraneutron
resonance, certain types of atomic nuclei should exist that are known not
to, says theoretical nuclear physicist Natalia Timofeyuk of the University
of Surrey in Guildford, England.
Because of that contradiction, she thinks that the researchers have not
observed a true resonance, but another effect that is not yet understood.
For example, she says, the bump could result from a “memory” that the
neutrons retain of how they were arranged inside the helium-8 nucleus.
Other types of theoretical calculations are a closer match with the new
results. “Indeed, theoretical results are very controversial, as they either
predict a tetraneutron resonance in good agreement with the results
presented in this paper, or they don’t predict any resonance at all,” says
theoretical nuclear physicist Stefano Gandolfi of Los Alamos National
Laboratory in New Mexico. Further calculations will be needed to understand
the results of the experiment.
New experiments could help too. Because detecting neutrons, which have no
electric charge, is more difficult than detecting charged particles, the
researchers didn’t directly observe the four neutrons. In future
experiments, Duer and colleagues hope to spot the neutrons and better pin
down the tetraneutrons’ properties.
Future work may reveal once and for all whether tetraneutrons are the real
deal.
Reference:
M. Duer et al. Observation of a correlated free four-neutron system. Nature.
Vol. 606, June 23, 2022, p. 678.
DOI: 10.1038/s41586-022-04827-6.
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Physics