[Paper Review] A multi-cubic-kilometre neutrino telescope in the western Pacific Ocean

Ziping Ye, Fei-Long Hu+56 more|arXiv (Cornell University)|Jul 10, 2022

Astrophysics and Cosmic Phenomena25 citations

TL;DR

This paper proposes the TRIDENT next-generation deep-sea neutrino telescope in the South China Sea, reports an expedition and site characterization (optical properties, radioactivity, currents), and outlines a preliminary design using advanced sensors (hDOM, SiPMs) for improved angular resolution and sensitivity.

ABSTRACT

Next-generation neutrino telescopes with significantly improved sensitivity are required to pinpoint the sources of the diffuse astrophysical neutrino flux detected by IceCube and uncover the century-old puzzle of cosmic ray origins. A detector near the equator will provide a unique viewpoint of the neutrino sky, complementing IceCube and other neutrino telescopes in the Northern Hemisphere. Here we present results from an expedition to the north-eastern region of the South China Sea, in the western Pacific Ocean. A favorable neutrino telescope site was found on an abyssal plain at a depth of $\sim$ 3.5km. At depths below 3km, the sea current speed, water absorption and scattering lengths for Cherenkov light, were measured to be $v_{\mathrm{c}}<$10cm/s, $λ_{\mathrm{abs} }\simeq$ 27m and $λ_{\mathrm{sca} }\simeq$ 63m, respectively. Accounting for these measurements, we present the design and expected performance of a next-generation neutrino telescope, TRopIcal DEep-sea Neutrino Telescope (TRIDENT). With its advanced photon-detection technology and large dimensions, TRIDENT expects to observe the IceCube steady source candidate NGC 1068 with 5$σ$ significance within 1 year of operation. This level of sensitivity will open a new arena for diagnosing the origin of cosmic rays and probing fundamental physics over astronomical baselines.

Motivation & Objective

• Motivate the construction of a next-generation neutrino telescope near the equator to complement IceCube and northern-hemisphere detectors.
• Identify a deep-sea site with suitable depth, flat seabed, low biological activity, and reachable power/data infrastructure.
• Characterize optical properties, water currents, radioactivity, and biological activity to inform detector design.
• Propose a preliminary TRIDENT design utilizing advanced photon-detection technologies to enhance angular resolution and sky coverage.

Proposed method

• Conduct a scouting expedition (T-REX) at ~3.4 km depth to measure sea-water absorption and scattering lengths, scattering phase function, and refractive index.
• Deploy a three-module T-REX apparatus with a central light emitter (LEDs at 405, 450, 460, 525 nm) and two receiver modules (PMT and camera) to perform near-far measurements.
• Analyze PMT data by reconstructing photon arrival time distributions and fitting with a Geant4-based propagation model to extract lambda_abs, lambda_Ray, lambda_Mie, cos(theta_Mie), and refractive index n; validate with MCMC (emcee) cross-check.
• Analyze camera data via the I_center method and chi-squared fitting of angular radiance distributions to separate absorption and scattering effects and estimate lambda_att and lambda_eff_att.
• Assess radioactivity (40K) via in-situ water sampling and Geant4-based trigger-rate estimates; monitor bio-activity with cameras during deployment.

Experimental results

Research questions

• RQ1 What optical properties of deep-sea water at the chosen site (absorption, Rayleigh and Mie scattering lengths, scattering anisotropy, and refractive index) govern Cherenkov photon propagation for a neutrino telescope?
• RQ2 Can a near-equator deep-sea site support a large-scale neutrino detector with acceptable backgrounds and mechanical stability for long-term operation?
• RQ3 What detector technologies and geometry yield improved angular resolution and sensitivity for TeV–EeV neutrinos in this environment?
• RQ4 What is the expected performance (attenuation lengths, trigger rates, and background levels) for a TRIDENT-like telescope given the measured site properties?

Key findings

• Measured optical properties at 3420 m depth: lambda_abs ≈ 26–27 m, lambda_Ray ≈ 200–203 m, lambda_Mie ≈ 64–84 m, cos(theta_Mie) ≈ 0.97 (Mie forward-scattering) in PMT data; effective attenuation length lambda_att_eff ≈ 17–25 m depending on method.
• Camera-based analysis yields lambda_abs ≈ 26.5 m, lambda_sca ≈ 62.9 m, lambda_att ≈ 18.7 m, and lambda_att_eff ≈ 26.8 m, with consistency to PMT-derived results.
• Background radioactivity from 40K corresponds to a ~4 kHz PMT trigger rate, deemed acceptable for operation.
• Bio-activity was not observed below ~3000 m during the two-hour camera data-taking window, indicating low bio-background at the site.
• TRIDENT concept proposes a hybrid digital optical module (hDOM) with PMTs and SiPMs to improve angular resolution by ~40% relative to PMT-only DOMs, leveraging White Rabbit timing and high-bandwidth data links.
• The preliminary telescope layout uses a Penrose-tiling-like geometry with 1211 strings, each with 20 hDOMs, aiming to cover sub-TeV to EeV neutrinos.