While all atomic nuclei except hydrogen are composed of protons and
neutrons, physicists have been searching for a particle consisting of two,
three or four neutrons for over half a century. Experiments by a team of
physicists of the Technical University of Munich (TUM) at the accelerator
laboratory on the Garching research campus now indicate that a particle
comprising four bound neutrons may well exist.
While nuclear physicists agree that there are no systems in the universe
made of only protons, they have been searching for particles comprising two,
three or four neutrons for more than 50 years.
Should such a particle exist, parts of the theory of the strong interaction
would need to be rethought. In addition, studying these particles in more
detail could help us better understand the properties of neutron stars.
"The strong interaction is literally the force that holds the world together
at its core. Atoms heavier than hydrogen would be unthinkable without it,"
says Dr. Thomas Faestermann, who directed the experiments.
Everything now points to the fact that precisely these kinds of particles
were created in one of the last experiments carried out at the now
decommissioned tandem Van de Graaff particle accelerator on the Garching
research campus.
The long search for the tetra-neutron
As early as 20 years ago, a French research group published measurements
that they interpreted as the signature of the sought-after tetra-neutron.
However, later work by other groups showed that the methodology used could
not prove the existence of a tetra-neutron.
In 2016, a group in Japan attempted to produce tetra-neutrons from helium-4
by bombarding it with a beam of radioactive helium-8 particles. This
reaction should produce beryllium-8. In fact, they were able to detect four
such atoms. From their measurement results, the researchers concluded that
the tetra-neutron was unbound and quickly decayed back into four neutrons.
In their experiments, Faestermann and his team bombarded a lithium-7 target
with lithium-7 particles accelerated to about 12 percent of the speed of
light. In addition to the tetra-neutron, this should produce carbon-10. And
indeed, the physicists succeeded in detecting this species. A repetition
confirmed the result.
Circumstantial evidence
The team's measurement results matched the signature that would be expected
from carbon-10 in its first excited state and a tetra-neutron bound by 0.42
megaelectronvolts (MeV). According to the measurements the tetra-neutron
would be roughly as stable as the neutron itself. It would then decay by
beta-decay with a half-life of 450 seconds. "For us, this is the only
physically plausible explanation of the measured values in all respects,"
explains Dr. Thomas Faestermann.
With their measurements, the team achieves a certainty of well over 99.7
percent, or 3 sigma. But in physics, the existence a particle is only
considered conclusively proven once a certainty of 5 sigma is achieved.
Thus, the researchers are now eagerly awaiting independent confirmation.
Reference:
Thomas Faestermann et al, Indications for a bound tetraneutron, Physics
Letters B (2021).
DOI: 10.1016/j.physletb.2021.136799
Tags:
Physics