Neutrinos inhabit a secluded corner of the cosmos. Each second, countless numbers stream through our planet, yet their infrequent interactions with ordinary matter leave minimal traces. The highest-energy among these enigmatic particles are cosmic neutrinos, originating from space with energies thousands of times greater than those achievable in particle accelerators like CERN. Their presumed sources are powerful cosmic phenomena, such as supermassive black holes, or as-yet-undiscovered exotic celestial objects.

Detecting these cosmic neutrinos presents a formidable challenge. To date, only a scant few have been observed, each discovery unveiling a wealth of information about the universe’s most extreme environments and fundamental layers of reality. Last year, the Cubic Kilometre Neutrino Telescope (KM3NeT) notably detected a particle of astonishing energy, setting a new record and fueling anticipation for further discoveries.

Carlos Argüelles-Delgado has dedicated over a decade to the pursuit of these particles, primarily utilizing the IceCube Neutrino Observatory located at the South Pole. His long-standing ambition to elucidate the mysterious behavior of neutrinos now leads him back to his native Peru, specifically to the high altitudes of the Andes mountains.

In the Andes, Argüelles-Delgado is spearheading the development of a novel telescope named the Tau Air-shower Mountain-Based Observatory (TAMBO). This facility is envisioned to incorporate thousands of detectors spread across several square kilometers of a nearly vertical rock face. Provided his team can successfully manage potential risks like landslides and the presence of nesting condors, TAMBO is poised to become a vital instrument for observing the most energetic cosmic neutrinos as they traverse the Earth’s periphery.

The Genesis of Ultra-High-Energy Cosmic Neutrino Detection

Thomas Lewton: When were these ultra-high-energy cosmic neutrinos first identified?

Carlos Argüelles-Delgado: The initial observations of these particles were made by the IceCube neutrino observatory at the South Pole in 2013. Our current understanding is that many of them are generated in the vicinity of black holes at galactic centers. As these massive objects accrete matter, they can accelerate particles to immense energies. These high-energy particles then interact with surrounding material, leading to their decay and the subsequent production of cosmic neutrinos.

Responding to the “Impossible” Discovery

Thomas Lewton: What was your initial reaction upon learning about the “impossible” cosmic neutrino detected by KM3NeT last year?

Carlos Argüelles-Delgado: I was unable to attend the meeting where this unexpected finding was announced. One of my postdoctoral students returned and informed me about this peculiar event. The energy levels were so extraordinarily high that I found it difficult to accept the information, even after repeated explanations. It was a sensation akin to being told about the existence of a new color; my mind struggled to process the news.

The Significance of Unexpected Findings

Thomas Lewton: Why was this detection considered so surprising?

Carlos Argüelles-Delgado: IceCube, a substantially larger experiment, had been operational for over a decade without detecting any neutrinos at comparable energies. Therefore, it was unexpected that a newer experiment would achieve such a detection. Furthermore, the energy level was so exceptionally high that it suggested a potential origin from a cosmic process previously unobserved, possibly marking the first instance of a “cosmogenic neutrino.”

Understanding Cosmogenic Neutrinos

Thomas Lewton: What is the meaning of “cosmogenic” in this context?

Carlos Argüelles-Delgado: The origin of cosmic rays remains a persistent enigma in physics. Although these charged particles from deep space were first observed a century ago, their production mechanisms are not well understood. Interstellar space is not entirely devoid of matter; it contains the cosmic microwave background, a collection of photons that are remnants of the Big Bang. It is theorized that, occasionally, a cosmic ray interacts with this background radiation, giving rise to a cosmogenic neutrino. This theoretical prediction emerged in the 1960s but had not been experimentally confirmed until now. Ultra-high-energy neutrinos, such as cosmogenic ones, are exceedingly rare. Consequently, their detection necessitates detectors of immense scale, significantly larger than IceCube.

Neutrino telescopes offer profound insights into the origins of cosmic rays, their composition, and their distribution throughout the universe. In this manner, the entire evolutionary trajectory of the cosmos is encoded within the neutrinos that we anticipate detecting with these instruments.

The Ambiguity of the KM3NeT Detection

Thomas Lewton: Are we certain that KM3NeT detected a cosmogenic neutrino?

Carlos Argüelles-Delgado: The classification of this detection remains uncertain. It is also plausible that the particle originated from interactions around a black hole or another violent cosmic event. To ascertain its origin, we need to identify additional such particles, analyze their energy spectra, and study their points of origin. Cosmogenic neutrinos, by their nature, will not originate from specific, localized sources. Instead, they are expected to be uniformly distributed across the sky and possess a discernible spectrum of energies.

Strategies for Future Detection

Thomas Lewton: How are neutrino astronomers like yourself planning to achieve this?

Carlos Argüelles-Delgado: There has been a resurgence of effort in constructing neutrino telescopes. Several global experiments, including IceCube and KM3NeT, employ natural media such as water, ice, or rock for neutrino detection. Neutrinos require a substantial quantity of material to be stopped, necessitating instruments that are essentially equivalent to an entire lake, sea, or mountain filled with detectors. However, these facilities provide only intermittent coverage of the sky, and continuous observation is crucial.

The Rationale Behind TAMBO’s Location

Thomas Lewton: Why choose to build your telescope in a canyon?

Carlos Argüelles-Delgado: Our search focused on identifying valleys of a specific geological formation, approximately 4 kilometers deep and spanning 3 to 5 kilometers in width. This depth ensures sufficient shielding from background radiation while offering a broad area for neutrino detection. Its width is adequate to accommodate the long-lived, high-energy particles generated by neutrino interactions. Utilizing satellite imagery, we identified only about ten such locations globally, predominantly in the Himalayas and the Andes mountains. Subsequently, expeditions to the Andes were undertaken to scout potential sites situated at an altitude of approximately 5 kilometers above sea level.

Advantages of Canyon Environments for Neutrino Astronomy

Thomas Lewton: Why are these steep canyons ideal for detecting ultra-high-energy neutrinos?

Carlos Argüelles-Delgado: The mountainous terrain serves two critical functions. If a detector were positioned near the mountain’s summit, it would be bombarded by numerous cosmic rays and gamma rays interacting with the atmosphere, creating significant background noise. The mountain itself effectively shields against the majority of these background particles. Concurrently, it facilitates the conversion of the ultra-high-energy cosmic neutrinos we aim to study into other detectable particles. Neutrinos are often called “ghost” particles due to their ability to pass through matter with ease. While this is certainly true for most neutrinos, at ultra-high energies, their interaction with matter becomes significant enough that they cannot traverse entire planets without interaction. Instead, they typically pass through only a segment of the planet, such as a mountain range, before interacting.

TAMBO is designed to detect these “Earth-skimming” neutrinos. When such a particle travels through the mountain face opposite the detector, it may interact within the mountain and produce particles with a relatively long lifespan, which then emerge from the mountain. These emergent particles subsequently decay into an immense cascade of millions of lighter particles within the canyon, radiating across a wide area.

To capture this phenomenon, flat detectors, each roughly the size of a dining table, will be deployed across the canyon’s opposing surface. TAMBO is planned to house approximately 5,000 such detectors, with an initial pilot project deploying 100. If development proceeds as anticipated, a fully operational telescope is expected by the early 2030s.

The Necessity of Dense Detector Arrays

Thomas Lewton: Why is such a large number of detectors required?

Carlos Argüelles-Delgado: The scale of the detection area is paramount due to the extreme rarity of these events. These detectors also enable TAMBO to function as a sky-viewing instrument, capable of pinpointing the direction from which a neutrino originates. This directional information allows us to query our partner experiments, such as IceCube and KM3NeT, which detect lower-energy neutrinos, and ask: “Is there any unusual activity detected in this specific direction around the same time?”

Challenges in Field Deployment

Thomas Lewton: Deploying a telescope on the slope of a nearly vertical canyon must present considerable difficulties.

Carlos Argüelles-Delgado: The challenges are numerous. How will the detectors be transported into these valleys? Will they be lowered using cables, or will helicopters be employed? The steeper the canyon, the more complex the deployment process and the higher the risk of landslides. Furthermore, operations in remote natural environments present other obstacles. Intense sunlight and rainfall can occur.

We recently completed a trip to the Colca Canyon, a potential site in Peru, where condors nest within the valley. This necessitates considerations for the impact of wildlife, such as birds building nests among the detectors.

Motivation for Extreme Endeavors

Thomas Lewton: Why undertake such arduous efforts?

Carlos Argüelles-Delgado: My fascination with neutrinos stems from their profound mystery. They are among the least understood particles within the standard model of particle physics. We still lack clarity on how neutrinos acquire their mass, the phenomenon that drives their peculiar oscillation between different “flavors.”

Cosmic neutrinos hold particular intrigue as they emanate from some of the universe’s most violent processes. This implies they possess exceptionally high energies—ranging from 1,000 to 1,000,000 times greater than those generated by particle accelerators on Earth—and travel vast distances. The ratio of distance to energy dictates the dynamics of neutrino oscillation, a region of oscillation that has remained unexplored. This makes cosmic neutrinos ideal probes for discovering novel phenomena in physics.

Furthermore, cosmic neutrinos could provide evidence for quantum gravity. This theoretical framework predicts minute fluctuations in spacetime, which would influence neutrinos as they propagate across vast distances and oscillate between their three distinct flavors. As neutrinos travel from distant galaxies, they would encounter these quantum gravity effects, leading to observable deviations in their flavor compositions upon reaching Earth.

The Naming of TAMBO

Thomas Lewton: Why is the experiment named TAMBO?

Carlos Argüelles-Delgado: “Tambo” is a Quechua word that signifies an “inn” or “resting place.” We sought to acknowledge the land from which our data will be collected and the communities residing there. During the Inca Empire, inns known as Tambos served as waystations for messengers called Chasquis, who traversed the empire relaying vital information. Consequently, I deemed this name appropriate, as neutrinos function as cosmic messengers, and this facility will serve as their resting place.

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Community Engagement and Responsible Siting

Thomas Lewton: How do local residents perceive the project?

Carlos Argüelles-Delgado: This is a critical consideration. While a specific site has not yet been finalized, a central objective of our collaboration is to cultivate strong local relationships and ensure that local communities derive tangible benefits from TAMBO. Jaco de Swart, a historian and anthropologist at the University of Cambridge, is leading the collaboration’s efforts in “responsible siting.” This involves understanding the significant local contexts, fostering local partnerships, and developing sustainable operational approaches.

The history of telescope construction globally is marked by practices that we are committed to avoiding. For example, on Mauna Kea in Hawaii, plans for constructing the Thirty Meter Telescope encountered strong opposition from the local community, as the mountain holds sacred significance for them. The perspectives and interests of the local populace were not adequately respected, leading to large-scale protests and a halt in construction.

In the regions we are considering, small towns are primarily populated by individuals engaged in agriculture or the tourism industry. Our aim is not merely for the community to tolerate the project but to embrace it enthusiastically. Therefore, we are actively exploring avenues for their involvement, collaboration, and the integration of their perspectives, cultural understanding, and unique ways of connecting with the universe. For instance, the orientation of the Milky Way aligns with one of the edges of the Colca Valley, and a Quechua legend describes the Majes River flowing along this valley before ascending directly into the Milky Way.

At times, astronomers may approach a new location with a predetermined notion of bringing knowledge. However, our “Western science” represents but one method of understanding the universe. It is imperative to respect indigenous knowledge systems and diverse approaches to observation and interpretation.

Contemplating the Universe from the Canyon

Thomas Lewton: What is the feeling of standing in the canyon, gazing at the universe, and knowing that we are on the verge of uncovering some of its secrets?

Carlos Argüelles-Delgado: The Colca Valley is an extraordinarily impressive and awe-inspiring location. It evokes a sense of incomprehensible vastness, coupled with an underlying feeling of hope. Standing within this canyon, one realizes that you are not merely observing the universe but are situated within an instrument being collaboratively constructed.

I am genuinely filled with excitement, as physics consistently demonstrates a pattern: whenever we develop new ways of observing, unexpected discoveries emerge. Therefore, a part of me experiences the anticipation of a child on Christmas morning—knowing something wonderful is coming, yet uncertain of its exact form, and cherishing that uncertainty.

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