Scientists Shine a Spotlight on Photons Produced in Neutron Decay

Studying decay might seem like a job for the biologists, but not so when it comes to particles. The strange, but common process through which particles decay, or change from one type into two or more other types, is fundamental to the way the universe works. After a year-long experiment and analyzing terabytes of data, a team of scientists has just published in Physical Review Letters the first precise measurements of one of the byproducts of the decay of a neutron—light.

This photo of the RDK II experiment was taken looking in the direction of the neutron beam source. The three rectangles in the center are detectors for the lowest-energy photons.
Image Credit: Herbert Breuer.

For the most part, the stuff in the universe seems reliable and stable. Matter, made of protons, electrons, and neutrons doesn’t just go “poof” and turn into something else. However, many particles are around for only a tiny fraction of a second before they decay. In fact, the Higgs boson was discovered through its signature decay pattern, like some of the other elementary particles. Just as a paleontologist or anthropologist might examine biological decay patterns in order to reconstruct what happened in the past, physicists use particle decay patterns to reconstruct the past.

For example, the decay of a free neutron (a neutron not inside of a nucleus) plays an important role in our understanding of how elements formed, according to Matthew Bales, one of the researchers on the neutron decay project. “Element formation in the early universe is directly influenced by how long it takes neutrons to decay. We can observe the amounts of hydrogen, helium, etc., and then match these values very closely to what our theoretical models predict based on what we measured from neutron decay.”

Studying neutron decay in detail is important for another reason. Over the last 40 years or so, scientists have developed and refined a mathematical framework called the standard model that describes particles and their interactions. As scientists carry out new experiments with higher-powered equipment, they can compare their results to predictions from the standard model and see how well the model matches reality. Neutron decay makes for an intriguing test of the standard model.

On average, a free neutron is stable for only minutes before decaying into other particles—a proton, electron, and an antineutrino. The standard model predicts that decaying neutrons can also give off light, or photons, across a range of energies. Other aspects of neutron decay have been well-studied, but scientists have only recently been able to detect this light. Like a faint sound in a noisy restaurant, the light signal is difficult to see because there is so much background light produced by other activities that occur in a neutron beam, where scientists study neutron decays.

Using a beam of neutrons produced at the National Institute for Standards and Technology’s Center for Neutron Research, in 2006 scientists working on experiment called RDK I confirmed that neutron decays sometimes produce photons. With a well-designed experiment that moved some of the background light to another location and that only looked for photons when the other particles produced by neutron decay were detected, the scientists made the first measurements of these photons across a limited energy range.

Ten years later, an expanded and upgraded RDK II experiment has given us a much more detailed picture of this kind of decay. The final data set includes 22 million electron-proton detections, about 20,000 of them accompanied by these photons. The photons span a range of energies that cover three orders of magnitude, from x-rays to gamma rays. Their distribution matches the theory well.

The researchers say that insights from this experiment can be used to design even higher-precision neutron decay experiments that could help us not just test the standard model, but explore physics beyond it. That’s something they definitely won’t leave to the biologists.

Kendra Redmond

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