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What Are Some Mysteries and Challenges That Neutrons Bring to Physics?

Leonard Kelley holds a bachelor's in physics with a minor in mathematics. He loves the academic world and strives to constantly explore it.

Neutrons are the atomic particle that carry no charge, but that doesn’t mean they don’t have any intrigue. Quite the contrary, they have plenty that we do not understand and it is through these mysteries that maybe new physics may be discovered. So, let’s take a look at some of the mysteries and challenges of the neutron and see what possible solutions there are.

The Tetraneutron

While this isn't a conclusive finding, decades of research and findings point to the potential existence of the tetraneutron or four neutrons that briefly bind together. The first signs of it were spotted in 2002 after colliding beryllium and carbon atoms seem to give these tetraneutrons as a brief product of their impact. However, potential errors in the results cast doubt as to the authenticity of the tetraneutron. Decades later, researchers instead used a heavy version of helium with four extra neutrons and collided these with protons. This was done so that after the collision those four could hopefully separate as a tetraneutron, and by comparing the energy of the collision to what models point to it was possible to see signs of the elusive neutron grouping. But with a lifespan of 10-22 seconds, its potential uses seem limited (Padavic-Callaghan).

Decay Rate Conundrum

Everything in nature breaks down, including lone atomic particles because of the uncertainties in quantum mechanics. Scientists have a general idea for the rate of decay of most of them, but neutrons? Not yet. You see, two different methods of detecting the rate give different values, and not even their standard deviations can explain it fully. On the average, it seems to take about 15 minutes for a lone neutron to decay, and it turns into a proton, an electron, and an electron antineutrino. The spin is conserved (two – ½ and one ½ for a net – ½) and also the charge (+1, -1, 0 for a net of 0). But depending on the method used to arrive at that 15 minutes, you get some different values when no discrepancy should exist. What is going on? (Greene 38)

Beam method.

Beam method.

Bottle method.

Bottle method.

Comparing the results.

Comparing the results.

To help us see the problem, let’s take a look at those two different methods. One is the bottle method, where we have a known number inside a set volume and count how many we have left after a certain point. Normally this is hard to achieve, for neutrons like to pass through normal matter with ease. So, Yuri Zel’dovich developed a very cold supply of neutrons (which have low kinetic energy) inside a smooth (atomically) bottle where collisions would be kept at a minimum. Also, by increasing the bottle size further error was eliminated. The beam method is a bit more complex but simply fires neutrons through a chamber where the neutrons enter, decay occurs, and the number of protons released from the decay process is measured. A magnetic field ensures that outside charged particles (protons, electrons) won’t interfere with the number of neutrons present (38-9).

Geltenbort used the bottle method while Greene used the beam and arrived at close, but statistically different answers. The bottle method resulted in an average decay rate of878.5 seconds per particle with a systematic error of 0.7 seconds and a statistical error of 0.3 seconds so a grand total error of ± 0.8 seconds per particle. The beam method yielded a decay rate of 887.7 seconds per particle with a systematic error of 1.2 seconds and a statistical error of 1.9 seconds for a grand total error of 2.2 seconds per particle. This gives a difference in values of around 9 seconds, way too big to likely be from error, with only a 1/10,000 chance it is…so what’s going on? (Greene 39-40, Moskowitz)

Likely some unforeseen errors in one or more of the experiments. For example, the bottles in the first experiment were coated with copper that had oil over it to reduce interactions via neutron collision, but nothing makes it perfect. But some are looking into using a magnetic bottle, a similar principle used to store antimatter, that would contain the neutrons because of their magnetic moments (Moskowitz).

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Why Does It Matter?

Knowing this decay rate is crucial for early cosmologists as it can change how the early Universe operated. Protons and neutrons floated around freely in that era until about 20 minutes post Big Bang, when they started to combine to make helium nuclei. A difference of 9 seconds would have implications for how much helium nuclei were formed and so have impacts on our models of universal growth. It could open the door for dark matter models or pave the way for alternate explanations for the weak nuclear force. One dark matter model has neutrons decaying into dark matter, which would give a result consistent with the bottle method - and that makes sense since the bottle is at rest and all we are doing is witnessing the natural decay of the neutrons, but a gamma ray coming from a 937.9-938.8 MeV mass should have been seen. An experiment by the UCNtau team found no sign of the gamma ray to within 99% accuracy. Neutron stars have also shown a lack of evidence for the dark matter model with neutron decay, for they would be a great collection of colliding particles to create the decay pattern we expect to see, but nothing has been seen (Moskowitz, Wolchover, Lee, Choi).

The rate could even imply the existence of other universes! Work by Michael Sarrazin (University of Namur) and others have shown that neutrons can sometimes hop over to another realm via superposition of states. If such a mechanism is possible, then the odds of a free neutron doing it are less than one in a million. The math hints at a magnetic potential difference as being the potential cause of the transition, and if the bottle experiment were to be run over a year then fluctuations in gravity form orbiting the Sun should lead to experimental verification of the process. The current plan to test if neutrons do indeed Universe hop is to place a heavily shielded detector near a nuclear reactor and catch neutrons that do not fit the profile of those leaving the reactor. By having the extra shielding, external sources such as cosmic rays shouldn't impact the readings. Plus, by moving the proximity of the detector they can compare their theoretical findings to what is seen. Stay tuned, because the physics are just getting interesting (Dillow, Xb).

Works Cited

Choi, Charles. "What Can the Death of a Neutron Tell Us About Dark Matter." American Institute of Physics, 18 May 2018. Web. 12 Oct. 2018.

Dillow, Clay. “Physicists Hope to Catch Neutrons in the Act of Jumping from Our Universe to Another.” Popular Science, 23 Jan. 2012. Web. 31 Jan. 2017.

Greene, Geoffrey L. and Peter Geltenbort. “The Neutron Enigma.” Scientific American Apr. 2016: 38-40. Print.

Lee, Chris. "Dark matter not at the core of neutron stars." Conte Nast., 09 Aug. 2018. Web. 27 Sept. 2018.

Moskowitz, Clara. “Neutron Decay Mystery Baffles Physicists.” Huffington Post, 13 May 2014. Web. 31 Jan. 2017.

Padavic-Callaghan, Karmela. "New sign of strange neutron material." New Scientist. New Scientist, 02 Jul. 2022. Print. 22.

Wolchover, Natalie. "Neutron Lifetime Puzzle Deepens, but No Dark Matter Seen." Quanta, 13 Feb. 2018. Web. 03 Apr. 2018.

Xb. "The Search for Neutrons That Leak into Our World From Other Universes." Physics arXiv Blog, 05 Feb. 2015. Web. 19 Oct. 2017.

© 2017 Leonard Kelley

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