Four neutrons form a transient isolated entity - a tetraneutron?

[Astronuc]

I was reading another article when this headline from June 22 caught my attention.

Collisions hint that four neutrons form a transient isolated entity​

https://www.nature.com/articles/d41586-022-01634-x

An experiment firing helium-8 nuclei at a proton target has generated evidence that four neutrons can exist transiently without any other matter. But doubts remain, because the existence of such systems is at odds with theory.

Unfortunately, one has to subscribe or otherwise have access through one's institution. I haven't read the paper yet. Helium-8 is already pretty exotic.

Meanwhile, there is an apparently related paper with open access.

Observation of a correlated free four-neutron system​

https://www.nature.com/articles/s41586-022-04827-6

I've noticed relate threads on PF. In 2010, there was apparently no evidence of a tetraneutron system, but by 2016, there was some possible indications. Now apparently, there are claims of such as system.

Edit/update: I was reading a post in Nuclear Engineering which included a Nature article.
https://www.physicsforums.com/threads/what-is-the-ignition-cliff.1017073/

 

[Vanadium 50]

I'm not sure what - if anything - this means.

The tetraneutron is unbound. They don't seem to dispute that. Is there some unbound state where the 4 neutrons are briefly close together? If you roll a ball onto the top of the hill, so it has just barely enough energy to go over the top and down the other side, it will move slowly at the top and spend a lot of time there. I don't see anything beyond the analogy here.

That doesn't mean there's no interest in it. Just that I don't know what the point is to say "unbound but..."