Panorama across the Colca Canyon from its rim

An entire canyon as a detector

TAMBO is a next-generation neutrino observatory designed to detect ultra-high-energy tau neutrinos from deep space.

TAMBO's design differs from those of the first-generation neutrino observatories IceCube, KM3NeT and Baikal-GVD. Those observatories are three-dimensional grids of light detectors buried in a huge volume of ice or water, and they see neutrinos by looking for the blue Cherenkov light produced by the charged products of neutrino interactions. Large volumes and full-sky coverage make such designs sensitive to many kinds of medium- to high-energy neutrino.

At the highest energies, though, neutrinos become both much rarer and less likely to travel straight through the Earth. The tau leptons produced by tau-neutrino interactions also travel further at these energies, which makes them hard to identify in an IceCube-style detector. Since tau neutrinos are the most interesting flavour for new fundamental physics, that loss of discrimination is unfortunate.

To reach this ultra-high-energy frontier, TAMBO uses a different strategy: it leverages the natural topography of a deep mountain valley to turn an entire canyon into a detector.

Illustrated schematic of a tau neutrino entering the left canyon wall, producing a tau lepton that decays in the valley and fans into an air shower over the detector array on the right wall — Schematic of neutrino detection at TAMBO. An Earth-skimming tau neutrino undergoes a charged-current interaction within the left canyon face, producing a tau lepton. The tau lepton emerges into the valley and decays, creating an air shower that is detected by TAMBO on the opposite canyon face. Credit: TAMBO collaboration.

How it works

A high-energy tau neutrino from deep space travels through the Earth's crust towards TAMBO. If we are lucky, it interacts with the rock of the canyon wall and produces a short-lived tau lepton. The tau is just long-lived enough to punch out of the rock, but then decays in air, producing a shower of particles that can extend up to ten kilometres. That extensive air shower is what TAMBO detects — an array of roughly 5,000 particle detectors spaced 150 metres apart on the opposing canyon face. By comparing the timing and size of the signals across different units, TAMBO reconstructs the energy and direction of the neutrino.

Step through a detection

Why a valley?

The canyon is more than a backdrop — it is part of the detector. The surrounding rock provides several kilometres of natural shielding against the background of atmospheric muons. And a canyon detector can observe tau neutrinos arriving from two directions: down-going neutrinos that interact in the mountains above, and Earth-skimming, up-going neutrinos that interact in the canyon face itself. A detector on a flat plain is sensitive only to one or the other.

Three diagrams comparing a flat-plain detector, sensitive only to down-going or only to up-going tau neutrinos, with a canyon detector sensitive to both — A detector on a flat plain is sensitive only to down-going or up-going tau neutrinos. A canyon detector is sensitive to both. Credit: TAMBO collaboration.

Why tau neutrinos?

All three neutrino flavours interact with rock with roughly the same probability — but what happens next is very different. The electrons produced by electron neutrinos are absorbed by the canyon rock. The muons produced by muon neutrinos are very good at retaining their energy and punch through the valley without creating a shower. Only the tau lepton decays quickly enough to create an extensive air shower inside the valley, which is exactly what TAMBO detects.

Diagram of a mountain with three neutrino tracks: the electron neutrino is absorbed, the muon neutrino passes through, and only the tau neutrino produces an extensive air shower — All flavours have the same interaction probability, but only the tau lepton efficiently creates a detectable air shower in the valley. Credit: TAMBO collaboration.

What a detection looks like

In its nominal configuration, TAMBO comprises 5,000 plastic scintillator detection units, each about a square metre, spaced 150 metres apart on a triangular grid along one face of the valley. From the number of particles incident on each unit and their arrival times, TAMBO infers the neutrino's energy and direction with sub-degree angular resolution.

Three-dimensional rendering of the Colca Canyon terrain with the detector array on one slope and a cluster of triggered units coloured by arrival time — Simulation of a tau neutrino event in TAMBO. The neutrino interacts inside the opposite valley face and produces a tau lepton whose decay initiates a particle shower detected by the array. Credit: TAMBO collaboration.

Animated event display showing detector units in the array lighting up as shower particles arrive — Simulated event display in Peru's Colca Canyon, a candidate site. Coloured circles are triggered detector units; size shows particle count, colour shows arrival time. Credit: TAMBO collaboration.

Quick facts

01TAMBO stands for Tau Air-shower Mountain-Based Observatory.
02It will detect tau neutrinos with energies between 1 PeV and 1 EeV.
03The nominal array is 5,000 plastic scintillator units, each about 1 m², spaced 150 m apart on a triangular grid.
04The candidate site is the Colca Valley in the Peruvian Andes — about 1.5 km deep, with sides about 4.5 km apart.
05The surrounding rock gives over 4 km of natural shielding against atmospheric muons.
06Every neutrino TAMBO detects will likely be of cosmic origin.
07About 6 extragalactic neutrinos are expected per decade conservatively, and more than 20 in optimistic scenarios.
08Directions are reconstructed with sub-degree angular resolution.
09The detection units use well-understood, low-cost scintillator technology that scales with the size of the array.
10The collaboration includes 28 institutions in 11 countries, with a strong Peruvian core.

TAMBITO, and what comes next

TAMBITO — “little tambo” — is TAMBO's prototype and first stage, currently under design and construction. It will validate the detector technology and serve as a stepping stone towards the full 5,000-unit array, expected to be completed in 2028.