Why a Peruvian mountain is becoming an ‘impossible’ particle detector

Neutrinos live in a lonely universe. Every second, millions of them pass through our planet, but they so rarely interact with other matter that they don’t leave much of a trace. The highest-energy of these mysterious particles are cosmic neutrinos, which descend from space with thousands of times the energy of those whipped up at particle colliders like the one at CERN. They are thought to come from violent cosmic accelerators, like supermassive black holes, or from exotic objects that we have yet to discover.

The trouble is that cosmic neutrinos are incredibly difficult to spot. So far, we have found only a handful of them, with each observation opening a treasure trove of information about the extreme reaches and deepest layers of reality. Not least, last year, the Cubic Kilometre Neutrino Telescope (KM3NeT) took astronomers by surprise when it found a seemingly “impossible” particle, the most energetic neutrino ever recorded, leaving them impatient to find more.

Carlos Argüelles-Delgado has been hunting for these particles for over a decade, largely using the IceCube Neutrino Observatory at the South Pole. Now, his wish to reveal the enigmatic ways of neutrinos is taking him back to his home country of Peru and into the heights of the Andes mountains.

Here Argüelles-Delgado is leading the effort to build a new telescope called the Tau Air-shower Mountain-Based Observatory (TAMBO), which is planned to comprise thousands of detectors installed across several square kilometres of a near-vertical rock face. Assuming his team can navigate the prospect of landslides and nesting condors, TAMBO will soon act as a viewfinder across the sky for the most energetic cosmic neutrinos as they skim across the edges of Earth.

Thomas Lewton: When did we first discover these ultra-high-energy cosmic neutrinos?

Carlos Argüelles-Delgado: The first ones were discovered by the IceCube neutrino observatory at the South Pole in 2013. We think many of these are produced around black holes at the centre of galaxies. When these behemoths accumulate matter, they can accelerate particles to very large energies. These then collide with material around the black hole to produce other particles, which go on to disintegrate into cosmic neutrinos.

What was your reaction when you heard about the “impossible” cosmic neutrino announced by KM3NeT last year?

I couldn’t go to the meeting where the finding was unexpectedly announced. One of my postdoctoral students came back, telling me about this weird event, but the energy was so crazily high that I couldn’t believe it – even after he told me many, many times. My mind couldn’t process the news; it was like somebody telling me about the existence of a new colour.

Why was it so unexpected?

IceCube, a much larger experiment, had been operating for more than 10 years and had never seen neutrinos at these energies. So, it was surprising that a newcomer experiment found it. It was also such a high energy that it could have come from a cosmic process that had never been observed before – it could be the first “cosmogenic neutrino”.

What does cosmogenic mean here?

The origin of cosmic rays is a long-standing mystery in physics. It’s been 100 years since we first saw these charged particles, which travel from deep space, but we don’t have a very good understanding of how they are produced. Deep space isn’t completely empty: there is the cosmic microwave background, made up of lots of photons that are a relic of the big bang. Every now and then, a cosmic ray is thought to interact with the cosmic microwave background and produce a cosmogenic neutrino. This effect was predicted back in the 1960s, but never seen. Ultra-high-energy neutrinos, such as cosmogenic neutrinos, are very, very, very rare. So to catch them, one needs huge detectors, much larger than IceCube.

Neutrino telescopes can tell us more about the origin of cosmic rays, what they are made of and how they are distributed across our universe. In this way, the whole evolution of the universe is encoded in the neutrinos that we expect to see in these detectors.

Do we know for sure that KM3NeT saw a cosmogenic neutrino?

The detection is still in a grey area. It could also have been produced around a black hole or in another violent process. To figure out where it came from, we need to find more of these particles and compare their energies and study their points of origin. Cosmogenic neutrinos won’t point back to specific sources. Rather, they would be evenly distributed in the sky, and they will have a characteristic set of energies.

How are neutrino astronomers like you planning to do that?

There’s been a renewed effort to build neutrino telescopes. There are several experiments around the world, such as IceCube and KM3NeT, that look for neutrinos using natural mediums – usually water, ice or rock. You need a very large amount of material to stop a neutrino, so you need, essentially, an entire lake or sea or mountain full of detectors. But these only have spotty sky coverage, and we need continuous coverage.

So why build your telescope in a canyon?

We were looking for a very specific kind of valley about 4 kilometres deep and 3 to 5 kilometres wide. This is deep enough to shield us from background signals and provide a large area for neutrino detection, and wide enough to contain the long-lived, high-energy particles made from neutrino interactions. Using Google Maps, we found only about 10 locations like this on the planet, mostly in the Himalayas and the Andes mountains. Then we carried out expeditions to the Andes to scout potential locations, which are about 5 kilometres above sea level.

Why are these steep canyons ideal for finding ultra-high-energy neutrinos?

The mountain plays two important roles. If you were standing near the top of the mountain and holding a detector, you would see many cosmic rays and gamma rays hitting the atmosphere and creating background noise. The mountain blocks out almost all of these background particles. At the same time, it also converts the ultra-high-energy cosmic neutrinos that we want to study into other particles that we can detect. Neutrinos are often known as “ghost” particles because they pass through material very easily. That’s definitely true for most neutrinos, but for these ultra-high-energy neutrinos, the interaction with matter becomes stronger, and they can’t traverse whole planets without interacting. Instead, they typically pass through only a sliver of the planet – such as a mountain range – before interacting.

TAMBO will look for these Earth-skimming neutrinos. When one of them travels through the mountain face opposite the detector, it may interact inside the mountain and produce particles that are relatively long-lived, which exit the mountain. These then disintegrate into a shower of millions and millions of lighter particles inside the canyon that spread themselves across a big area.

To catch these, we will spread flat detectors, each about the size of a dining table, across the opposite surface of the canyon. TAMBO plans to have about 5000 of these detectors, but in our pilot project we’ll start with 100. If all goes well, by the early 2030s, we’ll have a full-scale working telescope.

Why do you need so many detectors?

It’s important that we have a big collection area because these are such rare events. These detectors also allow TAMBO to act as a viewfinder that looks across the sky and can pinpoint where the neutrino is coming from. So, we can then ask our partner experiments, like IceCube and KM3NeT, which see more neutrinos at lower energies: “Hey, in this particular direction, do you see something weird at a similar time?”

It must be tricky to build a telescope on the slope of a near-vertical canyon…

There are so many challenges. How will we get the detectors into one of those valleys? Do we use cables and lower them down, or do we use helicopters? The steeper the valley, the harder it is to deploy the detectors, and the greater the likelihood of landslides. Other things happen when you’re out in the wild. There can be intense sun and rain.

We recently came back from a trip to the Colca Canyon, one possible site in Peru, where condors nest in the valley, so we even have to think about animals building nests in the detectors.

Why go to these extreme efforts?

I care about neutrinos because they are very mysterious. They are one of the least-understood particles in the standard model of particle physics – we still don’t know how neutrinos get their masses, which is what causes them to strangely oscillate between different kinds or “flavours” of neutrino.

Cosmic neutrinos are particularly interesting, as they come from some of the most violent processes in the universe. This means they have the highest energies – between 1000 and 1,000,000 times more energetic than the ones we make on Earth using particle accelerators – and they travel extremely long distances. The ratio of distance to energy is what determines how neutrinos oscillate, and we have never explored this region of their oscillation before, so this makes cosmic neutrinos perfect for looking for new phenomena in physics.

The second thing that cosmic neutrinos could do is find evidence for quantum gravity. Quantum gravity should result in tiny fluctuations in space, which would affect neutrinos as they oscillate across space between their three different flavours. Neutrinos would feel the presence of quantum gravity effects as they travel from distant galaxies, changing the neutrino flavours that we observe here on Earth in strange ways.

Why is the experiment called TAMBO?

Tambo is a Quechua word that means “inn” or “resting place”. We wanted to recognise the land where our data is collected and the communities that live there. During the Inca Empire, inns were used by messengers called Chasquis, who ran around the empire relaying messages. So, I thought, the name was appropriate because neutrinos are cosmic messengers that will have their resting place here.

How do locals feel about the project?

This is a very important question. We haven’t decided on any site yet, but one central goal of our collaboration is to build good local relationships and have the locals benefit from TAMBO in many ways. Jaco de Swart, a historian and anthropologist at the University of Cambridge, is leading the collaboration’s effort in “responsible siting”: understanding the important local contexts, developing local collaborations and working out the most sustainable approaches.

There are colonial histories when it comes to telescope construction around the world that we don’t want to repeat. On Mauna Kea in Hawaii, for instance, researchers wanted to build another large telescope called the Thirty Meter Telescope, but that place is also a sacred mountain for the people who live there. The local community’s perspective and interest weren’t properly taken into account and there were large protests, resulting in the telescope construction being put on hold.

In the area we are thinking about, there are small towns where people are either farmers or work in the tourist industry. We don’t just want the community to be OK with the project; we want them to be enthusiastic. So, we are thinking about how to involve them, collaborate with them and take into account their interests and ways of seeing the world and connecting to the universe. For example, the position of the Milky Way mirrors one of the edges of the Colca valley, and there’s a Quechua story in which the Majes river runs along this valley and then flows directly up into the Milky Way.

Sometimes, astronomers think they are coming to a place and bringing the knowledge with them. But our “Western science” is just one way of attending to the universe. You have to respect local knowledge and different ways of doing things.

What does it feel like to stand in the canyon and look up at the universe and know that we’re about to find out some of its secrets?

The Colca valley is very impressive; awe-inspiring. It feels incomprehensibly big – and somehow full of hope. You’re in this canyon looking up, and you realise you’re not just staring at the universe, you’re standing inside a kind of instrument that we’re building together.

And I’m genuinely excited, because physics has this pattern: when we learn how to look somewhere new, surprises show up. So part of me is standing there like a kid waiting for Christmas morning – knowing something is coming, not knowing what it is, and loving that.

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