The atomic nucleus, which accounts for the majority of the mass of matter in the natural world, usually consists of a combination of protons and neutrons. Further, the number of protons and neutrons in stable nuclei that exist in nature are usually in balance. How far unbalanced atomic nuclei can be is closely related to the nature of the nuclear force that binds protons and neutrons. Among the various possible combinations, the question of whether the most extreme case of a nucleus composed of only neutrons exists is one of the most important issues in nuclear research both experimentally and theoretically.
What is a Tetraneutron?
A Tetraneutron is a hypothetical stable cluster of four neutrons. The existence of this cluster is not supported by current models of nuclear forces. However, there is some evidence that this particle does exist, based on some experiments by Francisco Marques and coworkers at the Ganil accelerator in Caen. But subsequent attempts to replicate this observation have failed. In February 2016, scientists in Japan found most convincing signs of a tetraneutron adding weight to the possibility that hypothetical particles do exist. According to theory, this highly elusive particle cluster is impossible, because lone neutrons are very unstable, but scientists say, they have spotted its signature during their recent experiments.
While the result needs to be replicated independently before we can truly say the fabled particles exists, if other teams confirm its existence, we are going to have some serious changes to our current understanding of the nuclear forces. Scientists have been searching for the tetraneutron for decades, and while some papers claim that no evidence could be found, four separate papers have since reported experimental observation of the particle. In 2001 Francisco Marques and his team used a new technique to observe the particle by watching the disintegration of beryllium and lithium nuclei. In 2001 – 2002 , they were smashing beryllium-14 particles into carbon particles in an attempt to blow apart beryllium’s cluster of four neutrons. When it happened, they should have observed four little flashes, but instead, they got one big flash, signaling that these neutrons broke off in a cluster.
So why the idea of four neutrons teaming up is so impossible?
Well, the Pauli’s exclusion principle specifies that particles in the same system cannot be in the same quantum state. As a consequence of this, even two neutrons should not be able to stick together, let alone four. However four neutrons smashing at high speed into a carbon atom, and then reaching a detector at exactly the same place and time is nearly impossible as the idea that a basic tenet of physics needs to be modified.
In 2004, his team found similar evidence but no one has been able to replicate their results, making true confirmation impossible until now. A team from University of Tokyo has also worked with Beryllium particles to produce tetraneutron states. They did this by firing a beam of helium nuclei (which have two protons and six neutrons), and when the particles collided, four neutrons were missing. Their conspicuous absence lasted around 1 billionth of a trillionth of a second before reappearing as a particle decay. Whenever a pair of alpha particles was detected from these second collisions, simple counting dictated that four neutrons must have been left behind- but were they bound to each other or they simply flew off in different directions as debris?
To confirm which was more likely scenario, Shimoura and his team measured the energy given off by the particles in the reaction, and figured out that there wouldn’t have been enough to propel each of the missing neutrons away independently. This indeed confirmed that they flew as a bound four-neutron particle.
Separate teams of researchers are now needed to follow this team’s methodology and come up with results to prove or disprove their inferences. If they can do that, we would have to nut out more accurate guidelines for our laws of quantum chemistry and nuclear forces. And that’s a really exciting thing. It’s not every day that our fundamental understanding of the universe needs to be officially re-established, and not only could we watch that happen, it could also lead us to unveil more exciting paths to undertand the truth of reality.
“Both very large atomic nuclei (where neutrons outnumber protons about three or two on average) and neutron stars containing large lumps of neutrons whose behavior remains poorly understood could be stood upon from here” as Shimoura says.