Neutrinos are incredibly difficult to detect. What makes them effective messengers of the most violent places in the universe makes it very difficult to capture them when they arrive at Earth. When a neutrino comes into existence, for example, in the jet of a black hole, it travels to us in a straight line undisturbed of what else is going on in its surrounding. That lack of interaction lets us look deep inside otherwise obscured sources. But when it comes to capturing a neutrino on Earth, their elusiveness poses a big challenge to the experimentalist.
Trinity uses the Earth-skimming technique to detect neutrinos shown in the figure above. When an ultrahigh-energy neutrino hits the Earth and penetrates it, the neutrino interacts within tens to hundreds of kilometers and produces a tau lepton. The tau is a short-lived particle, but if the remaining distance between the neutrino interaction point and the Earth’s surface is just right, the tau emerges from the ground. The tau then decays in the atmosphere kicking off the development of a cascade of charged particles, or as we call it, an air shower. The mostly electrons and positrons in the shower travel faster than the speed of light and, in doing so, generate Cherenkov light. Trinity‘s telescopes capture some of the light, project it onto their cameras, and thus form an image of the air shower.
The recorded image holds the key to figure out where the neutrino was coming from and what energy it had. These are the only two parameters needed to reconstruct the event. But the reconstruction requires a massive computer simulation that replicates the experiment. After simulating many million neutrino events, the recorded image is compared with all of the simulated ones. After finding the simulated neutrino with the best matching image, one adopts the direction and energy of the simulated neutrino for the observed one.