I assume any particle like that can only be detected by a single sensor in one location. How do you verify that this isn't some sort of noise artifact in your system?
It creates a cascade of secondary particles that are detected over an array of sensors. In this case, the shower triggered 23 detectors, which are part of 507 stations arranged in a square grid. The stations are spaced by 1.2 km, giving a total effective area of 700km^2
Right, guess I just don't know enough on what their experiment looks like.
While traveling through the atmosphere, they create particle showers over large areas, which can be reliably detected. There's even the citizen science project called CREDO [1] that relies on volunteers using their smartphone camera sensors as particle detectors.