Hyper-Kamiokande at Sheffield
Hyper-Kamiokande is the proposed successor to Super-Kamiokande. It will act both as the far detector for an upgraded T2K beam (with the same energy and the same off-axis geometry, but higher intensity) and as a detector for solar, supernova and atmospheric neutrinos and other non-accelerator physics such as proton decay.
The current Hyper-Kamiokande design calls for two water tanks, each with diameter 74 m and height 60 m (compared to 39 m diameter and 42 m height for Super-K). Because of surface-area-to-volume scaling, although the total water mass of one Hyper-K tank is only 5 times that of Super-K, the fiducial mass is over 8 times larger. Improved photomultiplier tubes with better resolution and higher quantum efficiency should lead to improved performance compared to Super-K, especially for faint signals.
Hyper-Kamiokande construction will be staged, with one tank currently scheduled to start operation in 2026 and the other a few years later. It is not necessary that the two tanks be located in the same place, and recently there has been an interesting proposal to locate the second tank in Korea, where it would have a longer baseline and sit on the second oscillation peak for the T2K beam. This has the potential to reduce systematic errors in oscillation measurements.
The physics goals of Hyper-Kamiokande are as follows:
- Neutrino oscillation measurements: Using a combination of accelerator neutrinos from the upgraded T2K beam and atmospheric neutrinos, Hyper-K aims to determine the mass hierarchy (is mass state ν3 the lightest or the heaviest eigenstate?), to measure CP violation in the lepton sector—a possible explanation for the matter-antimatter asymmetry of our universe—and to make high-precision measurements of oscillation parameters (Δm2 values and mixing angles). In addition, neutrino oscillation measurements provide sensitivity to exotic phenomena such as sterile neutrinos and violation of Lorentz invariance.
- Solar neutrino measurements: There is currently some tension in the θ12 sector between solar and reactor neutrino measurements. With a greater fiducial mass and higher-efficiency PMTs than Super-K, Hyper-K could investigate this by measuring day-night asymmetry in solar neutrino flux caused by matter-enhanced oscillations in the Earth. Other new avenues that could be explored in solar neutrino physics include monitoring the Sun's fusion reactions by observing 8B neutrinos, and a first observation of the higher-energy hep neutrino flux.
- Nucleon decays: The original Kamiokande experiment was designed to search for proton decays (the acronym originally stood for Kamioka Nucleon Decay Experiment not Neutrino Detection Experiment!), and this has continued to be a physics topic of interest for Super-K. Hyper-K should be able to improve significantly on Super-K limits, especially as the increased efficiency of Hyper-K PMTs should lead to improved performance, especially in detecting the 2.2 MeV γ ray from neutron capture on hydrogen (rejecting events containing neutrons is an important method of reducing neutrino backgrounds in proton decay searches).
- Supernova burst neutrinos: 30 years ago, Kamiokande-II was one of the two experiments to detect a significant neutrino signal from Supernova 1987A in the Large Magellanic Cloud. The estimated rate of core-collapse supernovae in our Galaxy is of the order of one every 50 years, so we have been slightly unfortunate not to have a Galactic supernova occur in the 20 years that Super-Kamiokande has been active. Hyper-Kamiokande would detect approximately 60000 neutrinos per tank from a supernova at a typical Galactic distance of 10 kpc (about 30000 light years). This would permit detailed study of the explosion. It is worth noting here that Hyper-K and DUNE are almost perfectly complementary in their sensitivity to supernova neutrinos, with DUNE detecting νe (via scattering off argon) and Hyper-K ν̅e (via inverse beta decay).
- Supernova relic neutrinos: Detecting neutrinos from a core-collapse supernova in our Galaxy is easy (60000 neutrinos arriving in a ten-second window would be hard to miss), but relies on cooperation from the Galactic population of massive stars: no supernova, no neutrino burst. However, neutrinos are stable particles, so the neutrinos from every core-collapse supernova that has exploded in the history of the universe are still travelling through space. Super-Kamiokande has searched for these supernova relic neutrinos without success, because of a background from below-Cherenkov-threshold muons decaying in the detector (producing an apparently isolated Michel electron whose energy is unfortunately comparable to that expected from a supernova neutrino signal). This is one of the motivations for introducing gadolinium into Super-K, as the supernova relic neutrino interactions should produce a neutron (they are inverse beta decay, ν̅e + p → e+ + n) while the muon decays obviously do not. However, Hyper-K could complement the Super-K measurement by concentrating on the higher energy region, 20–30 MeV (the Super-K measurement will be most sensitive to 10–20 MeV neutrinos).
- Dark matter searches: If the dark matter is made up of supersymmetric neutralinos, regions of high dark matter density (the core of the Sun, the Galactic centre) should experience significant rates of neutralino annihilation. Many of the possible final states, e.g. WW, ZZ, bb̅, have decay modes that include neutrinos, making neutrino telescopes a possible means of searching for dark matter. The signal should be detectable over the large atmospheric neutrino background because of its directionality. Hyper-K can improve on limits set by Super-K by a factor of a few, and has sensitivity to low-mass neutralinos which are difficult to detect in conventional direct-detection experiments (and where there are a number of controversial claims of potential signals).
Hyper-Kamiokande in the UK
The HyperK-UK consortium consists of the Universities of Edinburgh, Imperial College, Lancaster, Liverpool, Oxford, Queen Mary, Royal Holloway, Sheffield and Warwick together with RAL and the Institute of Particle Physics Phenomenology (IPPP) at Durham. The intended UK contributions to Hyper-K include:
- sensitivity studies and software infrastructure;
- detector R&D;
- a gadolinium-loaded water Cherenkov near detector;
- an improved DAQ system;
- development of a calibration system;
- secondary beamline R&D.
These build directly on the UK's contribution to the T2K experiment and will give the UK a high profile in Hyper-K.
The Sheffield group is contributing primarily to calibration, software infrastructure, and sensitivity studies.
For more information about Sheffield's work on Hyper-K, consult the links on the right. For a detailed overview of the Hyper-Kamiokande experiment, download the Hyper-K design report (warning! 282 pages!!). A short movie about Hyper-K, suitable for non-experts, is available on YouTube.