It’s tempting to call the tetraneutron a theoretical particle, as its existence has yet to be confirmed. But that would imply that it’s a consequence of some existing theoretical model, that it’s predicted by some theory. The tetraneutron, however, contradicts the relevant theories—it should be impossible.

And yet, amidst all the (deserved) excitement for the detection of gravitational waves last week, an experiment quietly turned up the strongest evidence for a tetraneutron thus far. It’s not full confirmation yet, but if the new study’s conclusions are borne out, things are going to get weird.

The story so far

The troublesome particle may have first appeared in 2001 after decades of speculation and a few doubtful experiments. Researchers fired beryllium-14 atoms at a carbon target to observe the resulting chaos of particles, a relatively common practice.

Beryllium-14 has what’s called a halo nucleus, unlike many simpler atoms. Essentially, it has an “inner nucleus” wrapped in a wider “outer nucleus” that orbits it. Beryllium-14’s halo is made of four neutrons, so the researchers expected to see that halo break apart and become four individual neutrons. That would have been detected as four distinct signals.

Instead, what they actually observed was one big signal, implying the neutrons had somehow stuck together as a single particle, a tetraneutron (rendered as 4n ), which should be impossible.

The problem is the Pauli exclusion principle, a characteristic of quantum mechanics that says that two identical fermions can’t share the same quantum state (fermions are a class of fundamental particles to which both neutrons and protons belong). Protons and neutrons can join together, as they have different quantum states from each other. But without any protons, a group of neutrons shouldn’t be able to form a nucleus due to the exclusion principle.

So the apparent detection of one left researchers scratching their heads. Might it be time to go back to the drawing board and re-think some of our most fundamental physics?

That physics revolution was staved off when further studies failed to reproduce the results. It began to look like the original detection had been a fluke. Other researchers found that at least part of the original study design had been flawed. The tetraneutron seemed dead in the water.

That didn’t stop researchers from exploring the theoretical implications of tetraneutrons, however, with some proposing ways they might exist. But other theoretical work argued the tetraneutron simply wasn’t possible within our current theories.

Discovery?

That brings us to the present day and another experiment that seems to have produced the tetraneutron. A team of researchers at RIKEN in Wako, Japan, was actively looking for the particle. To that end, they fired a beam of helium nuclei at a liquid form of helium.

The helium atoms in the beam were a heavy isotope with two protons and six neutrons. The liquid, meanwhile, had only two of each (the most common form of helium). This particular combination was chosen by the researchers because the collision is essentially recoil-free. In other reactions, recoil might send a shock back into the newly formed tetraneutron, disrupting it. But this specific arrangement might allow it to persist for a brief time.

When they were smashed together, in some instances beryllium was produced, with four protons and four neutrons. That leaves four neutrons conspicuously missing from the final product. Perhaps fittingly, it was apparently produced four times. The researchers estimate each tetraneutron lasted a billionth of a trillionth of a second before decaying into other particles.

This constitutes an indirect detection. The tetraneutron itself was not detected but was inferred from the missing mass from the final product. While this is not a full confirmation of the tetraneutron’s existence, the researchers consider it the best evidence yet, with a 4.9 sigma significance level. (Scientists often consider five sigma to be the standard for confirmation).

That sounds pretty compelling, but don’t sound the death knell for the end of the exclusion principle just yet. Further work is needed to reproduce the result, and a direct detection would be much more satisfying. The same team of researchers is looking to set up an improved study that will increase the confidence in this result by a full order of magnitude (if indeed they detect the tetraneutron again). Other experimentalists are hoping to create the particle by other means.

If it’s confirmed, it’ll be back to the drawing board for the theoretical side of things. But in physics, being forced back to the drawing board is often the most exciting outcome. One way or another, interesting things are to come.

Physics Review Letters, 2015. DOI: doi:10.1103/PhysRevLett.116.052501 (About DOIs)