Last updated on July 14th, 2020 at 04:08 am
The atomic nucleus, responsible for the majority of matter’s mass in the natural world, typically comprises a mixture of protons and neutrons. In stable nuclei found in nature, the numbers of protons and neutrons are usually balanced. The extent to which atomic nuclei can be unbalanced is closely tied to the characteristics of the nuclear force that binds protons and neutrons together. Within the realm of various potential combinations, the existence of an extremely unbalanced nucleus composed solely of neutrons poses a significant question in both experimental and theoretical nuclear research.
What is a Tetraneutron?
A tetraneutron is hypothetically defined as a 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 reproduce this experimental result 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?
That’s because Pauli’s exclusion principle specifies that particles in the same system cannot be coexistent in the same quantum state. As a consequence of this, even two neutrons should not be able to stick together, let alone four. Nevertheless, the scenario of four neutrons colliding with a carbon atom at high velocity and subsequently arriving at a detector simultaneously and at the same location is highly improbable, as it would necessitate the modification of a fundamental principle in physics.
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. Upon the detection of a pair of alpha particles resulting from these subsequent collisions, a straightforward enumeration indicated the presence of four remaining neutrons. However, the question arose as to whether these neutrons were bound together or if they dispersed in various 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.
What now?
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 understand 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.
References
K. Kisamori et al, “Candidate Resonant Tetraneutron State Populated by the 4He(8He,8Be)
Reaction,” Phys. Rev. Lett. 116, 052501 (2016), American Physical Society, doi: 10.1103/PhysRevLett.116.052501,
URL: https://link.aps.org/doi/10.1103/PhysRevLett.116.052501
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Vishwas Singhhttps://www.scilynk.in/author/vishwas/
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