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Super-Kamiokande plus

Periodic Reporting for period 2 - SKPLUS (Super-Kamiokande plus)

Reporting period: 2016-12-01 to 2018-11-30

Super-Kamiokande Plus (SKPLUS, SK+) is part of the ongoing global efforts to understand the most fundamental elements of matter and their interactions. We aim to investigate neutrino interactions using the existing experimental facility in Japan, the Super-Kamiokande (SK) detector located in the Kamioka Observatory (Gifu Prefecture) and owned by the Partner of this Project: the Institute for Cosmic Ray Research of the University of Tokyo. Furthermore, we are working on the design and construction of new neutrino detectors planned at the Kamioka Observatory. They are SuperK-Gd (with its prototype EGADS), an upgrade of SK that will allow the identification of neutrons in the final state, and Hyper-Kamiokande (HK), its next-generation successor, along its accompanying intermediate detector E61.

SKPLUS addresses several fundamental problems in Physics at different stages: by analyzing the data from SK, by building and commissioning SuperK-Gd, and by designing HK and E61. These subjects are: neutrino oscillations, the search for dark matter, the physics of nearby galactic supernova, the diffuse supernova neutrino background and Grand Unification.

This collaborative work with the leaders of the field provides invaluable experience for these studies that cover neutrino physics, cosmology, astrophysics, technical design, construction and operation of Water-Cherenkov detectors, data analysis techniques, hardware and software development for the next generation detectors.
"In order to assure a smooth and continuous running of the SK neutrino telescope, we supervise its operation through various types of shifts: 1) regular SK data-taking in the SK control room and, 2) Data-Acquisition quality (High and Low energy samples) done remotely. In addition, the performance of the EGADS prototype is also constantly monitored remotely by 8-hours, round the clock, shifts.

In order to reduce the experimental uncertainties of SK, we are working on two calibration programs: the continuous 0.01 Hz flash of a Xenon light and the monthly study of response to ∼9 MeV γ’s produced at the capture of neutrons by a Nickel ball. With them we monitor the properties of the SK water and the gains of the 11400 photomultipliers (PMTs) that form SK.

We have upgraded the real-time monitor (SKHist) that, using data streamed by the data acquisition system, is running permanently in the SK control room. We are working towards a web service with similar monitoring capabilities as SKHist.

We are searching for Dark Matter with SK in an indirect way by measuring excess of neutrino fluxes from the Earth, the Sun and in the Galactic Center and halo. We have not observed any excess above the expected background level related to atmospheric neutrinos. Due to quality of our data and our elaborated analysis procedures, we are significantly narrowing down the properties that hypothetical Dark Matter candidates could have.

We have thoroughly included neutron tagging with Hydrogen in the global oscillation analysis of atmospheric neutrinos. With it we have improved the previous constrains by SK on the neutrino mass hierarchy and the leptonic charge-parity violation.

For the SuperK-Gd upgrade program we are thoroughly measuring the radioactive contamination of Gd-salts. It is crucial to minimize it. Based on our physics goals, we have set the radioactive contamination levels that the Gd salt has to fulfill to be dissolved in the SK; they are extremely stringent. However, mostly with our ICRR colleagues, we have achieved a successful R&D programs for their realization in mass production.

We have developed new ideas and made new tools for maximizing the impact of neutron tagging by Gd in the physics program of Water-Cherenkov detectors. Remarkably, we have developed neural-network based algorithms for discriminating neutrino versus anti-neutrino interactions in the water and to discriminate Neutral Current from Charged Current interactions.

We have developed a method to successfully use the Gd-tagged neutrons from those produced in the final state to estimate a large fraction of the otherwise non-measurable energy transferred to the nuclei or emitted as neutrino particles in the reaction.

We are continuously working towards the final design of the key elements of the HK and E61 detectors: front-end electronics and photo-sensors. We have performed a thorough study of properties of digitization based on direct sampling approach and developed of decent models of full electronics chain. We are also studying 3"" PMTs intended for the multi-PMT modules (for both HK and E61) as well as the novel Hybrid-Photo-Detector option (HPD).

We have designed a novel PMT protection cover to prevent the transmission of the shock wave formed after a hypothetical implosion of the PMT that would destroy all the other PMTs immersed in the water in the same tank. A thorough test program was realized, and the several hydrostatic pressure tests made in Japan and Spain and implosion tests made in Japan validated the approach. It has significant advantages in terms of cost and weight, compared to other designs considered for HK.

We have signed 24 publications related to SKPLUS in peer-reviewed journals (Phys. Rev. Lett., Phys. Rev. D, others). We have presented results of our work at more than 20 international conferences during the period of the project. There are also 2 PhD theses resulting directly from SKPLUS works.

Large disseminatio"
We have progressed in the construction of new generation of neutrino detectors, positively impacting the careers of the engaged researchers and communicating to the community throughout obtained achievements.

On June 2015, the SK collaboration approved and endorsed the SuperK-Gd project. The refurbishment of the tank has finished in January 2019, when regular data taken has resumed. Gd-loading will occur most probably in October.

On the July 2016 J-PARC PAC meeting, the nuPRISM experiment has been given a stage-I approval; now is known as J-PARC Nuclear and Particle Experiment E61.

The Japanese Ministry of Education, Culture, Sports, Science and Technology (MEXT) has chosen the HK project to be included in its Large Project Roadmap. MEXT has provided what is called “seed funding” for JFY 2019 to prepare the main funding request for starting in JFY 2020. Moreover, U. Tokyo has committed to provide any necessary funds to ensure start of construction in April 2020.

The “Breakthrough Prize 2016 in Fundamental Physics” was awarded to several SKPLUS members for former activities but very much related to SKPLUS.

The Noble Prize in Physics 2015 was awarded to our colleague, Prof. Takaaki Kajita, director and leader of our Partner Institute in SKPLUS, ICRR, U. Tokyo. The awarded research was conducted with the SK detector. The neutrino studies of this Project are continuation and extension of the discoveries honored with the Nobel Price 2015.