A giant sphere filled with liquid flashes when an elusive particle, a neutrino, flies through it. Inside the sphere, like fireflies on a Christmas tree, hang thousands of tubes that catch these flashes. To pinpoint exactly where the flash occurred and how much energy arrived, you need to know the position of each tube to within a millimeter.
During the assembly of the JUNO detector, the steel structure holding the tubes sagged slightly under its own weight. The tubes ended up not where they were drawn in the blueprint. Measuring the position of every tube directly is too expensive and time-consuming, so the scientists did what cartographers do: they measured only a few points and then reconstructed the entire displacement map from those. It turned out that the sag has almost no effect on measuring the particle's energy — the count remains accurate. But the point where the particle hit the liquid shifts by as much as 4 centimeters without correction.
When the scientists incorporated the real, slightly tilted geometry into the signal processing software, the error disappeared. Now each flash of light lands on the map precisely, like a pin in the right spot on a globe. This matters because JUNO aims to answer a long-standing question: which of the three neutrino flavors is the lightest, and which is the heaviest. The answer determines how the hidden part of the universe is structured.
Neutrinos are the lightest known particles of matter, and they are so elusive that trillions of them pass through your body every second without hitting a single atom.