POLARIS: A Sparse Radial Neutrino Telescope Design for the Pacific Ocean

Abstract: The cubic-kilometer neutrino telescopes have opened neutrino astronomy as an observational discipline. The recent detection of KM3-230213A, the highest-energy neutrino ever observed at ~220 PeV, as a near-horizontal muon track underscores that the ultra-high-energy frontier is accessed through horizontal directions where the Earth's opacity above ~100 TeV confines the observable sky to a narrow band around and above the horizon. Yet extending general-purpose detector architectures into this regime requires disproportionate increases in instrumentation, cost, and logistical complexity. A compelling alternative is to deploy specialized detectors that target this natural geometry. POLARIS (Pacific Ocean Large Area Radial Instrumented Sparse array) is a sparse planar deep-water Cherenkov array optimized for neutrino-induced muon tracks from horizontal directions in the multi-TeV to PeV regime. By rotating the conventional vertical string layout into a radial planar configuration, the detector presents maximal cross-section to horizontal tracks while naturally suppressing the down-going atmospheric background. With only 1100 optical modules, the five-arm design reaches point source and diffuse flux sensitivities at PeV energies competitive with detectors deploying several times more instrumentation. As a dedicated $ν_μ$ track detector, POLARIS provides the muon-flavor channel that tau-optimized experiments such as TAMBO and Trinity do not cover, enabling full flavor composition measurements from astrophysical sources. Using the Prometheus simulation framework, this study demonstrates that targeted sparse geometries can open new discovery space at the high-energy frontier at a fraction of the cost of general-purpose arrays.

• The paper introduces a sparse radial neutrino telescope design featuring a five-arm configuration with 1100 optical modules to optimize muon track detection.
• The design enhances sensitivity to horizontal muon tracks by retaining 40% of signal events while effectively reducing atmospheric background noise.
• Simulations demonstrate improved point source sensitivity at multi-TeV to PeV energies, offering a scalable and cost-efficient alternative to traditional detectors.

POLARIS: A Novel Sparse Radial Neutrino Telescope Design

The POLARIS project introduces a sparse radial neutrino telescope tailored for deployment in the Pacific Ocean, specifically engineered to optimize detection of neutrino-induced muon tracks in the multi-TeV to PeV energy regime. This configuration diverges from conventional cubic-kilometer architectures, offering significant advantages in terms of cost-efficiency and sensitivity to horizontal neutrino tracks. POLARIS utilizes a unique radial planar setup that maximizes cross-sectional exposure to these tracks, simultaneously attenuating atmospheric background noise.

Introduction to Neutrino Astronomy

Neutrino astronomy has been propelled into the forefront of observational astrophysics through instruments like IceCube and its successors, which have pioneered large-scale detection capabilities from the South Pole to the Northern Hemisphere. However, these detectors face challenges in expanding their architecture to address the ultra-high-energy regime (above ~100 TeV), necessitating disproportionate infrastructural expansion. The paper argues that detecting near-horizontal muon tracks is vital for exploring the ultra-high-energy frontier due to the Earth's opacity confining observations to narrow horizon bands.

Figure 1: Illustration of the five-arm POLARIS detector geometry with perpendicular string pairs.

Detector Design

POLARIS employs a sophisticated geometry composed of five radial arms extending from a central hub, each equipped with optical modules arranged in gates. This design includes a radial planar configuration, strategically aimed at maximizing sensitivity to horizontal muon tracks, with each arm instrumented with 220 optical modules resulting in a total of 1100 modules. The configuration (Figure 1) minimizes background noise, leveraging vertical spacing to discourage down-going cosmic-ray secondaries.

Event Selection and Classification

The telescope's architecture inherently categorizes event topologies, distinguishing between muon tracks that produce sequential hits along radial strings and cascade events occupying localized regions. The design suppresses background through geometric selection criteria, retaining 40% of signal tracks while eliminating cascade-induced noise efficiently across a vast energy spectrum.

Astronomy Potential

POLARIS exhibits promising point source sensitivity; simulations illustrate superior detection prospects compared to existing detectors. The instrument demonstrates effectiveness by achieving statistically significant detection results across the energy spectrum, with marked performance gains at PeV energies. This capability is mirrored in the diffuse flux sensitivity analysis, showcasing POLARIS's adeptness at capturing astrophysical signals with fewer modules than its predecessors.

Figure 2: Sensitivity curves for point source detection and comparison across instruments.

Discussion and Implications

POLARIS's hierarchical sparse geometry exemplifies a practical solution targeting ultra-high-energy neutrinos while circumventing the broad-band limitations inherent in denser configurations. This specialization is underscored by the ability to provide fundamental coverage of the muon-flavor channel, thereby complementing tau-optimized experiments like TAMBO and Trinity. The potential scalability of the radial arm design, coupled with its favorable modular architecture, positions POLARIS as a compelling addition to the global neutrino observational network.

Future research will focus on optimizing site-specific optical assessments and implementing advanced reconstruction techniques. Additionally, refining geometric spacing and event selection through machine learning paradigms could bolster detections, propelling POLARIS towards groundbreaking contributions within neutrino astronomy.

Figure 3: Effective area for CC muon and anti muon neutrinos for the POLARIS geometry.

Conclusion

Through its innovative radial arm configuration, POLARIS demonstrates the strategic efficacy of specialized detector arrays in neutrino astronomy, positioning itself as a cost-effective and potent tool for high-energy astrophysical exploration. This design not only complements existing instruments but expands observational capabilities across the Northern Hemisphere, promising substantial contributions to both theoretical and practical advancements in high-energy particle astrophysics.