[Paper Review] A Possible Future Long Baseline Neutrino and Nucleon Decay Experiment with a 100 kton Liquid Argon TPC at Okinoshima using the J-PARC Neutrino Facility

A. Badertscher, T. Hasegawa+6 more|ArXiv.org|Apr 14, 2008

Neutrino Physics Research21 references16 citations

TL;DR

This paper proposes a 100 kton liquid argon time projection chamber (LAr TPC) at Okinoshima, Japan, as a next-generation long-baseline neutrino and nucleon decay experiment using the upgraded J-PARC neutrino beam at 1.66 MW. The detector, located at a 658 km baseline with an off-axis angle of ~1°, leverages the LAr TPC’s superior energy resolution and background suppression to precisely measure the neutrino mixing angle θ₁₃ and CP-violating phase δCP, while also enabling high-sensitivity proton decay searches in the 10³⁴–10³⁵ year range.

ABSTRACT

In this paper, we consider the physics performance of a single far detector composed of a 100 kton next generation Liquid Argon Time Projection Chamber (LAr TPC) possibly located at shallow depth, coupled to the J-PARC neutrino beam facility with a realistic 1.66 MW operation of the Main Ring. The new far detector could be located in the region of Okinoshima islands (baseline $L\sim 658$ km). Our emphasis is based on the measurement of the $θ_{13}$ and $δ_{CP}$ parameters, possibly following indications for a non-vanishing $θ_{13}$ in T2K, and relies on the opportunity offered by the LAr TPC to reconstruct the incoming neutrino energy with high precision compared to other large detector technologies. We mention other possible baselines like for example J-PARC-Kamioka (baseline $L\sim 295$ km), or J-PARC-Eastern Korean coast (baseline $L\sim 1025$ km). Such a detector would also further explore the existence of proton decays.

Motivation & Objective

• To explore the physics potential of a 100 kton liquid argon TPC at Okinoshima as a next-generation long-baseline neutrino and nucleon decay experiment.
• To improve sensitivity to the neutrino mixing angle θ₁₃ and CP-violating phase δCP, especially following possible indications of non-zero θ₁₃ from T2K.
• To evaluate the feasibility of determining the neutrino mass hierarchy and resolving parameter degeneracies through precise energy spectrum measurements.
• To assess the detector’s capability for proton decay searches with sensitivity in the 10³⁴–10³⁵ year range, even at shallow depth.
• To compare performance across different baselines, including J-PARC–Kamioka (295 km) and J-PARC–Eastern Korean coast (1025 km), using the same detector concept.

Proposed method

• The experiment uses a 100 kton liquid argon TPC located at Okinoshima, with a baseline of ~658 km and an off-axis angle of ~1°, to observe neutrino oscillations from the J-PARC neutrino beam.
• The detector leverages the high energy resolution and fine-granularity charge imaging of the LAr TPC to reconstruct incoming neutrino energy with precision, enabling clear separation of the first and second oscillation maxima.
• The beam is assumed to operate at a realistic 1.66 MW power, derived from an upgraded J-PARC Main Ring, to provide sufficient statistics for oscillation measurements.
• Background suppression is achieved primarily through the LAr TPC’s excellent particle identification and rejection of π⁰ backgrounds, with the main irreducible background being the intrinsic νₑ component of the beam.
• Performance is compared across multiple baselines (295 km, 658 km, 1025 km) to evaluate optimal configurations for measuring θ₁₃ and δCP.
• The study relies on simulation-based performance assessment, focusing on energy spectrum features and statistical sensitivity, with future work to include systematic error modeling.

Experimental results

Research questions

• RQ1 Can a 100 kton LAr TPC at Okinoshima with a 658 km baseline achieve high-precision measurements of the neutrino mixing angle θ₁₃ and CP-violating phase δCP?
• RQ2 How does the LAr TPC’s energy resolution and background suppression compare to other detector technologies in resolving the first and second oscillation maxima?
• RQ3 What is the sensitivity of such a detector to proton decay in the 10³⁴–10³⁵ year range, particularly for multi-prong and kaon-containing decay modes?
• RQ4 How do different baselines (295 km, 658 km, 1025 km) affect the sensitivity to θ₁₃ and δCP, and which offers optimal performance?
• RQ5 Can a LAr TPC achieve high proton decay sensitivity even at shallow depth, given its self-shielding and 3D imaging capabilities?

Key findings

• The 100 kton LAr TPC at Okinoshima (658 km baseline) can resolve both the first and second oscillation maxima with good statistical significance due to the beam’s off-axis configuration (~1°) and the detector’s high energy resolution.
• The LAr TPC’s fine-granularity readout enables effective suppression of π⁰ backgrounds, reducing systematic uncertainties and making the intrinsic νₑ component the dominant irreducible background.
• The detector achieves high sensitivity to proton decay in the 10³⁴–10³⁵ year range, with signal-to-background ratios favorable for discovery at the few-event level, especially for multi-prong and kaon-containing decay modes.
• The study shows that the LAr TPC offers an order of magnitude improvement in detection efficiency for many proton decay modes compared to water Cherenkov detectors, due to superior imaging and background discrimination.
• The configuration at 658 km baseline provides a favorable balance between baseline length and beam energy, enabling effective measurement of the energy dependence of oscillation features to resolve parameter degeneracies.
• The paper concludes that a 100 kton LAr TPC at Okinoshima is a viable and competitive option for future long-baseline neutrino and nucleon decay physics, provided further R&D and prototype testing are pursued.