[Paper Review] An Andean Deep-Valley Detector for High-Energy Tau Neutrinos

A. Romero‐Wolf, Jaime Álvarez-Muñiz+12 more|arXiv (Cornell University)|Feb 16, 2020

Astrophysics and Cosmic PhenomenaPhysics and Astronomy40 references31 citations

TL;DR

Proposes TAMBO, a deep-valley tau neutrino detector in Colca Canyon, Peru, to detect 1–100 PeV tau neutrinos via tau-induced air showers using a large array of water-Cherenkov tanks.

ABSTRACT

High-energy astrophysical neutrinos, recently discovered by IceCube up to energies of several PeV, opened a new window to the high-energy Universe. Yet much remains to be known. IceCube has excellent muon flavor identification, but tau flavor identification is challenging. This limits its ability to probe neutrino physics and astrophysics. To address this limitation, we present a concept for a large-scale observatory of astrophysical tau neutrinos in the 1-100 PeV range, where a flux is guaranteed to exist. Its detection would allow us to characterize the neutrino sources observed by IceCube, to discover new ones, and test neutrino physics at high energies. The deep-valley air-shower array concept that we present provides highly background-suppressed neutrino detection with pointing resolution <1 degree, allowing us to begin the era of high-energy tau-neutrino astronomy.

Motivation & Objective

• Assess the scientific potential of detecting high-energy tau neutrinos to constrain production mechanisms and new neutrino physics.
• Develop a concept for a deep-valley detector optimized for tau-induced air showers in the 1–100 PeV range.
• Quantify diffuse flux acceptance, energy and angular resolutions, and background suppression necessary for 5σ sensitivity.
• Explore site suitability, especially Colca Valley, and outline a roadmap for prototype and full-scale deployment.

Proposed method

• Outline detector concept: tau neutrinos interact in rock to produce taus that exit into air and decay to showers detectable by water-Cherenkov tanks on valley slopes.
• Optimized array: ~22,000 tanks, 150 m spacing, in a triangular grid to achieve ⟨AΩ⟩ ≥ 400 m^2 sr at 1 PeV for ντ sensitivity.
• Estimate energy resolution impacts from ντ–τ decay, with ~80% energy transfer to tau and ~14% tau exiting energy spread; target shower energy resolution ΔE/E ≤ 80%.
• Define pointing resolution goal σθ ≤ 1°, achievable with PMT timing (~ns) and detector spacing.
• Assess backgrounds (random coincidences, directional reconstruction, high-energy muons, prompt νμ) and propose cuts to maintain ≤1 background event per 3 years with ≈20 signal events per 3 years.
• Evaluate detector performance such as diffuse flux acceptance, evolutionary energy dependence ⟨AΩ⟩ ∝ E^1.5, and compare to IceCube benchmarks.

Experimental results

Research questions

• RQ1 Can a deep-valley detector like TAMBO achieve ≥400 m^2 sr ντ diffuse acceptance at 1 PeV and > IceCube sensitivity in 10–100 PeV?
• RQ2 What is the expected τ-induced air shower signal rate for a Colca Valley array under extrapolated IceCube and cosmogenic neutrino flux models?
• RQ3 What are the achievable energy and angular resolutions, and how effectively can backgrounds be suppressed to reach 5σ sensitivity?
• RQ4 How does the detector's sky coverage and point-source effective area translate to multi-messenger observations of transient sources?
• RQ5 What is the practical feasibility, timeline, and cost framework for deploying a 22,000-tank array in a deep valley?

Key findings

• Projected diffuse flux acceptance reaches ~400 m^2 sr at 1 PeV, rising with energy as E^1.5, with an expected rate of ~21 events per 3 years using extrapolated IceCube flux.
• Mean energy transfer to the tau in CC interactions exiting rock is ~80%, with exiting tau energy spread ≈14%; shower energy from tau decay averages ~55% with ~60% uncertainty.
• Estimated point-source effective area is ⟨A⟩ ≳ 300 m^2 × (E/PeV)^1.5, aiming to match IceCube at 1 PeV and exceed it at 100 PeV.
• Energy resolution challenges arise from τ decay and shower development, but realistic goals include ΔE/E ≤ 100% (dominant uncertainties ~62% when combined in quadrature).
• Backgrounds considered include random coincidences, boundary reconstruction uncertainties, high-energy muons, and prompt νμ; background suppression strategies center on multi-tank triggers, shower profile fits, and timing constraints.
• Sky coverage can exceed 0.5 sr over a full rotation, with instantaneous field of view >0.1 sr for the fiducial layout, enabling multi-messenger alerts and potential for point-source sensitivity enhancements.