Arequipa, Peru
1.5 km deep · 4.5 km wall to wall
No lens, no mirror, no tank of water. TAMBO's detector is a canyon a kilometre and a half deep and four and a half kilometres across, in Arequipa, Peru. Five thousand particle detectors sit on the near slope, watching the empty air. The far wall has nothing on it at all — its job is simply to be four kilometres of solid rock. Now watch what arrives.
Eight things that can happen when a neutrino arrives at the canyon. Only one of them is a detection. Step through them and the design of the whole observatory explains itself.
An electron showers away inside the rock. A muon escapes but starts nothing. Only the tau is short-lived enough to decay just after leaving the mountain, and long-lived enough to have got out at all. TAMBO is a tau detector by construction, and that is what makes its sample so clean.
At these grazing angles the far wall puts more than four kilometres of rock in the way. That rock is the filter: it stops every atmospheric muon and every cosmic-ray shower, so essentially the only thing that can appear in the open valley is a neutrino that came through the mountain.
An air shower needs kilometres of atmosphere to multiply. Decay too close to the array and only a thin spray arrives. The 4.5 kilometres of open air is not a coincidence — it is the shower's runway.
Too low and the tau decays before it can escape the rock. Too high and it crosses the entire canyon without decaying. In between, roughly 1 to 100 PeV, sits the window TAMBO was designed around.
Every number here is computed from the physics, not scripted: cross sections follow a power-law fit to CSMS, tau transport uses continuous energy loss with decay sampled along the path, and showers develop on the Greisen profile. Built from arXiv:2507.08070 and arXiv:2606.11230.
