Sat. Jan 28th, 2023
The neutron source at Oak Ridge National Lab.
Enlarge / The neutron source at Oak Ridge National Lab.

Neutrinos are known to be extremely reluctant to interact with other matter. While it is possible to build hardware that will detect them, these detectors are usually huge to provide enough material for the neutrinos to interact with. Those interactions also take the form of energetic events that transform particle identities (for example, converting protons into neutrons).

Given the neutrino’s low mass and tendency not to interact, the idea of ​​detecting a neutrino simply colliding with another particle seems almost ridiculous. But that’s what Oak Ridge National Lab scientists report today. They’ve seen brief flashes as atoms are excited by a neutrino, which imparts some of its tiny momentum to the atomic nucleus.

Oak Ridge National Lab is home to hardware called the Spallation Neutron Source. This accelerates a jet of protons and slams them into a tank of mercury. This creates debris with lots of neutrons, which are used for various scientific purposes. But the debris also contains some neutrinos that are otherwise lost in the nebula of particles emerging from the collisions.

When they got the idea to start using the neutrinos, the facility’s researchers started placing detectors to find all parts of the building that weren’t receiving as many neutrons. They envisioned an area with lots of concrete and gravel between the mercury tank and the mercury tank: “A basement corridor, now called the ‘neutrino alley.'” The corridor was equipped with a large water tank to collect more of the neutrons and other particles, as well as cosmic rays from the atmosphere.

The neutrino interactions we detected earlier all involve neutrinos hitting the target material, triggering some kind of particle transformation. In this case, we’re watching the neutrino collide with the nucleus and give it a little push. More technically, we’re watching the neutrino impart some of its momentum to the nucleus by exchanging a Z boson with one of the nucleus’ quarks, then the nucleus loses that energy by emitting a photon. This interaction was first proposed on theoretical grounds 43 years ago, but has gone unnoticed.

A number of things make this kind of interaction more likely. One has a relatively low energy neutrino, something the Oak Ridge facility offers. A second has a relatively large core that it can collide with. The research team solved this by making a detector with cesium iodide, two relatively heavy elements. Because these elements are similar in mass, the photons they emit after an interaction must be similar in energy for a single detector to pick up all the interactions.

So 14.6 kg of cesium iodide was set up in the basement below the mercury tank and flanked by photo detectors to pick up the light emitted by the interactions. After that, the team took data for just over a year.

The neutron source provided great experimental control: protons hit the mercury in pulses, allowing the researchers to collect data on times between the pulses, as well as times just after. Comparing the two allowed them to see what differed between them. And what was different was a slight excess of events — about nine a month — right after a pulse of protons hit the mercury tank. This is consistent with predictions from the Standard Model of particles. And it’s nearly seven standard deviations from what you’d expect if you weren’t seeing neutrino interactions.

So, 43 years after all this was predicted, we finally have experimental support that neutrinos can collide with an atom. However, the authors seem just as enthusiastic about the rest of the work they do. They’ve put in several additional detectors to track the neutrinos in the basement, and they’ve upgraded their source. In addition, the researchers believe it may be possible to use their expertise in cesium iodide detectors to create a small, portable neutrino detector. Eliminating the need for building-sized hardware to detect neutrinos could open up some previously impossible experimental possibilities.

Science2017. DOI: 10.1126/science.aao0990 (About DOIs).

By akfire1

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