A Long Baseline Neutrino Oscillation Experiment Using J-PARC Neutrino Beam and Hyper-Kamiokande (1412.4673v2)

Abstract: Hyper-Kamiokande will be a next generation underground water Cherenkov detector with a total (fiducial) mass of 0.99 (0.56) million metric tons, approximately 20 (25) times larger than that of Super-Kamiokande. One of the main goals of Hyper-Kamiokande is the study of $CP$ asymmetry in the lepton sector using accelerator neutrino and anti-neutrino beams. In this document, the physics potential of a long baseline neutrino experiment using the Hyper-Kamiokande detector and a neutrino beam from the J-PARC proton synchrotron is presented. The analysis has been updated from the previous Letter of Intent [K. Abe et al., arXiv:1109.3262 [hep-ex]], based on the experience gained from the ongoing T2K experiment. With a total exposure of 7.5 MW $\times$ 10$^7$ sec integrated proton beam power (corresponding to $1.56\times10^{22}$ protons on target with a 30 GeV proton beam) to a $2.5$-degree off-axis neutrino beam produced by the J-PARC proton synchrotron, it is expected that the $CP$ phase $\delta_{CP}$ can be determined to better than 19 degrees for all possible values of $\delta_{CP}$, and $CP$ violation can be established with a statistical significance of more than $3\,\sigma$ ($5\,\sigma$) for $76%$ ($58%$) of the $\delta_{CP}$ parameter space.

• The paper introduces an experiment that probes neutrino CP violation using J-PARC’s high-intensity beam and the next-generation Hyper-Kamiokande detector.
• The study details enhancements in detector design such as increased mass, improved water purity, and advanced photosensors to boost Cherenkov detection sensitivity.
• The experiment aims to achieve a CP phase determination within 19° and confirm CP violation with over 3σ significance, offering new insights into lepton physics.

Overview of the Proposed Long Baseline Neutrino Oscillation Experiment Using J-PARC and Hyper-Kamiokande

The document outlines the proposal for a long baseline neutrino oscillation experiment aimed at investigating $CP$ asymmetry in the lepton sector utilizing the J-PARC neutrino beam and the Hyper-Kamiokande detector. Hyper-Kamiokande is posited as a next-generation water Cherenkov detector whose design supersedes that of the Super-Kamiokande, promising significant advancements in both mass and capabilities for detector performance and physics investigations.

Detector and Experimental Setup

The Hyper-Kamiokande detector is designed to have a total mass of 0.99 million metric tons, markedly enhancing the sensitivity for neutrino detection through its considerable increase over Super-Kamiokande's design. The document details engineering developments and strategies for refining detector calibration and improving water purity protocols—essential for Cherenkov radiation detection. Moreover, advancements in photosensor technology and data acquisition systems are crucial for enhancing signal processing speed and accuracy.

The experiment is to be conducted using the J-PARC facility's high-intensity proton beam, with plans detailed for a baseline energy level of 30~GeV and an integrated proton beam power of 7.5~MW$\times$10$^7$ sec. Key infrastructure modifications and technology upgrades are described to facilitate these operations, aiming to boost the proton beam power and accommodate future multi-MW capabilities.

Physics Goals and Methodology

A primary objective of the paper is the exploration of neutrino $CP$ violation, utilizing data from the high-statistics Hyper-Kamiokande alongside the J-PARC neutrino beam. The document outlines anticipated physics goals and paths for determining the $CP$-violating phase with improved precision. By employing both neutrino and anti-neutrino beams, and thanks to the planned design improvements, it's projected that the $CP$ phase can be determined to within 19 degrees, and $CP$ violation can be confirmed with a greater than 3$\sigma$ statistical significance for a substantial parameter space.

Innovations in analyses expected from this experiment bring notable enhancements over prior experiments like T2K. Updated statistical tools, along with refined systematic error handling, promise more robust and comprehensive results. These include likelihood methods that use reconstructed neutrino energy spectra and embrace a full parameter space fit to accurately determine key oscillation parameters.

Projected Impact and Future Directives

The proposed experiment is of notable importance for the field of particle physics, given its potential to yield insights into one of the most compelling areas of lepton physics—understanding $CP$ asymmetry. Discovering or constraining $CP$ violation in the neutrino sector presents implications for theories beyond the Standard Model, namely, for understanding the mass hierarchies and potential matter-antimatter asymmetries.

Practical outcomes could include more precise constraints on the existing neutrino oscillation parameters, and possibly, avenues for novel physics given sufficient deviation from Standard Model predictions. Importantly, the combination of long baseline and atmospheric neutrino measurements will enhance the capacity to resolve mass hierarchy ambiguities.

In conclusion, the outlined document depicts a promising vision for the future exploration of neutrino physics, detailing the necessary infrastructural and analytical advancements required to achieve significant scientific milestones. The synergetic use of two experimental avenues—accelerator and atmospheric neutrinos—within the Hyper-Kamiokande framework appears to be a productive strategy for unraveling questions related to $CP$ symmetry in the lepton sector.