Neutrinos are noted for being extremely reluctant to interact with other matter. While it's possible to build hardware that will detect them, these detectors tend to be enormous in order to provide sufficient material for the neutrinos to interact with. Those interactions also take the form of energetic events that transform the identity of particles (for example, converting protons to neutrons).

Given the neutrino's low mass and tendency not to interact, the idea of detecting one simply bumping into another particle seems almost ludicrous. But that's what scientists from Oak Ridge National Lab are reporting today. They've seen brief flashes as atoms get nudged by a neutrino, which imparts a tiny bit of its tiny momentum to the atom's nucleus.

Oak Ridge National Lab is home to some hardware called the Spallation Neutron Source. This accelerates a beam of protons and smashes them into a tank of mercury. This creates debris that includes lots of neutrons, which are used for a variety of scientific purposes. But the debris also includes some neutrinos that are otherwise lost in the spray of particles that comes flowing out of the collisions.

When they got the idea to start using the neutrinos, the researchers at the facility started placing detectors to find any areas of the building that didn't receive as many neutrons. They came up with an area that had a lot of concrete and gravel between it and the mercury tank: "A basement corridor, now dubbed the 'neutrino alley.'" The corridor was fitted with a large water tank to block out more of the neutrons and other particles, as well as cosmic rays from the atmosphere.

The neutrino interactions we've detected previously all involve neutrinos slamming into the target material and triggering some sort of particle transformation. In this case, we're looking to see the neutrino bump into the nucleus and give it a slight nudge. More technically, we're looking to see the neutrino give some of its momentum to the nucleus by exchanging a Z boson with one of the nucleus' quarks, after which the nucleus loses that energy by emitting a photon. This interaction was first proposed on theoretical grounds 43 years ago but has remained undetected.

A number of things make this sort of interaction more likely. One is having a relatively low-energy neutrino, something that the Oak Ridge facility provides. A second is having a relatively large nucleus for it to bump into. The research team handled 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 should be similar in energy, ensuring that a single detector can pick up any interactions.

So 14.6kg of cesium iodide was set up in the basement beneath the mercury tank and flanked by photodetectors to pick up the light emitted by the interactions. Then, the team took data for a bit over a year.

The neutron source provided a great experimental control: protons slammed into the mercury in pulses, which allowed the researchers to take data from times in between the pulses, as well as times just after them. By comparing the two, they could see what was different 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 away from what you'd expect if you weren't seeing neutrino interactions.

So, 43 years after this was all predicted, we finally have experimental support that neutrinos can bump into an atom. The authors, however, seem just as excited about the rest of the work they're doing. They've set up several additional detectors to track the neutrinos down in the basement, and they have upgraded their source. In addition, the researchers think it might be possible to use their cesium iodide-detector expertise to make a small, portable neutrino detector. Getting rid of the need for building-sized hardware to detect neutrinos could open up some previously impossible experimental opportunities.

Science, 2017. DOI: 10.1126/science.aao0990 (About DOIs).