Hyper-Kamiokande Design Report (1805.04163v2)

Abstract: On the strength of a double Nobel prize winning experiment (Super)Kamiokande and an extremely successful long baseline neutrino programme, the third generation Water Cherenkov detector, Hyper-Kamiokande, is being developed by an international collaboration as a leading worldwide experiment based in Japan. The Hyper-Kamiokande detector will be hosted in the Tochibora mine, about 295 km away from the J-PARC proton accelerator research complex in Tokai, Japan. The currently existing accelerator will be steadily upgraded to reach a MW beam by the start of the experiment. A suite of near detectors will be vital to constrain the beam for neutrino oscillation measurements. A new cavern will be excavated at the Tochibora mine to host the detector. The experiment will be the largest underground water Cherenkov detector in the world and will be instrumented with new technology photosensors, faster and with higher quantum efficiency than the ones in Super-Kamiokande. The science that will be developed will be able to shape the future theoretical framework and generations of experiments. Hyper-Kamiokande will be able to measure with the highest precision the leptonic CP violation that could explain the baryon asymmetry in the Universe. The experiment also has a demonstrated excellent capability to search for proton decay, providing a significant improvement in discovery sensitivity over current searches for the proton lifetime. The atmospheric neutrinos will allow to determine the neutrino mass ordering and, together with the beam, able to precisely test the three-flavour neutrino oscillation paradigm and search for new phenomena. A strong astrophysical programme will be carried out at the experiment that will detect supernova neutrinos and will measure precisely solar neutrino oscillation.

• The paper presents the design of a large-scale water Cherenkov detector enabling precise neutrino oscillation measurements and CP violation studies.
• It details the use of approximately 40,000 high-efficiency 50 cm PMTs within a 258-kiloton water tank to achieve 40% photocoverage and enhanced event detection.
• The report outlines strategies for observing astrophysical neutrinos and rare proton decay events, pushing the boundaries of detector technology and particle physics.

Overview of the Hyper-Kamiokande Design Report

The Hyper-Kamiokande design report outlines the proposed construction and capabilities of the Hyper-Kamiokande (Hyper-K) experiment, a next-generation, large-scale water Cherenkov detector. Building upon the success of its predecessors, Kamiokande and Super-Kamiokande, Hyper-K aims to significantly advance the paper of neutrinos, offering insights into some of the most critical unsolved questions in physics.

Detector Specifications

The Hyper-K detector is planned to be hosted in the Tochibora mine in Japan, situated 295 km from the J-PARC proton accelerator. The site was selected to match the off-axis angle of the neutrino beam provided by J-PARC. The detector will reside in a cavern with an overburden of 650 meters of rock. The design includes a main cylindrical water tank measuring 74 meters in diameter and 60 meters in height, containing 258 kilotons of ultra-pure water.

The inner detector (ID) will employ approximately 40,000 state-of-the-art 50 cm photomultiplier tubes (PMTs) developed for high quantum efficiency to detect Cherenkov light produced by neutrino interactions. The ID PMT array aims for 40% photocoverage, ensuring a high detection efficiency. The outer detector (OD) will be equipped with smaller PMTs to help differentiate between internal and external events. The overall design strives for enhanced sensitivity, leveraging improved PMT technology and high water purity standards.

Scientific Goals

Hyper-K will address several key areas in particle physics and astrophysics:

1. CP Violation in Neutrinos: One primary goal is to measure CP violation in the lepton sector, which could provide insights into the matter-antimatter asymmetry in the Universe. The experiment aims to precisely measure neutrino oscillation parameters using a well-characterized neutrino beam from J-PARC.
2. Neutrino Mass Ordering: Determining the neutrino mass hierarchy is crucial for understanding neutrino properties and for formulating a comprehensive theory of particle masses and mixing. Hyper-K is designed to exploit the matter effects in neutrino oscillations for this purpose.
3. Astrophysical Neutrinos: Hyper-K will be sensitive to neutrinos from supernovae, the Sun, and other extraterrestrial sources, providing data essential for understanding astrophysical phenomena and processes.
4. Proton Decay: A fundamental test of Grand Unified Theories, Hyper-K will search for proton decay with unprecedented sensitivity, potentially setting new limits or detecting these rare events.

Technical Challenges and Innovations

The project incorporates several technical advancements to realize its scientific goals. The use of new high-efficiency PMTs ensures better sensitivity and resolution, crucial for distinguishing subtle oscillation effects. Additionally, the detector's large volume and water transparency requirements push the boundaries of water purification and handling technology. The design also accounts for potential geological challenges at the chosen site, ensuring structural stability and safety of the detector over long operational periods.

Future Prospects

Hyper-Kamiokande represents a significant leap forward in neutrino physics and related fields. With the anticipated start of data-taking in the next decade, it promises to deliver valuable insights into fundamental physics questions, complementing other international experiments like DUNE. Furthermore, it sets a precedent for large-scale experiments, demonstrating the feasibility and the frontier-level advancements in detector technology and collaboration.

In summary, the Hyper-K design report showcases a comprehensive plan to build a world-leading neutrino observatory that will expand the boundaries of our current understanding of neutrinos and their role in the cosmos. It emphasizes technological innovations and strategic planning to achieve its ambitious scientific goals while ensuring practical feasibility and safety.