Neutrinos are nearly elusive particles that pass through Earth without noticing it. But sometimes one slams into an atomic nucleus in water, and a whole shower of particles is born: muons, protons, pions. Catching that moment is like photographing a bullet hitting an apple from a kilometer away.
At Japan's J-PARC laboratory, a neutrino beam was aimed at a water tank with a stack of photographic film, nuclear emulsion, inside. When a particle flew through the film, it left a trail of tiny silver grains, like a skier leaving a groove in the snow. After developing the film, scientists saw 82 clear tracks from neutrino collisions with water. That's only 5% of the total water volume in the tank.
Each track was measured under a microscope: length, angle, curvature. This reconstructed the speed and direction of the protons and other particles. It turned out that protons tend to fly out at a wider angle than predicted by the computer model used in the T2K experiment. This discrepancy could simply be a fluke due to the small number of events, but it's like a hairline crack in glass: you don't know if it will spread until you look closer.
The main value of the work is that for the first time, protons with very low energy, just 200 MeV, were spotted. For comparison, the proton's rest energy is about 938 MeV, so the captured protons were moving slowly, like pedestrians after a shove. Previously, such slow protons drowned in noise, but now scientists have a method to catch them. When ten times more data are collected in the future, this technique will help fine-tune models of neutrino interactions with matter — which is crucial for understanding why the Universe has so much matter and so little antimatter.
Nuclear emulsion is essentially the same photographic film used in old cameras, but with a thicker silver layer, and it can record the track of a single particle.