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We propose a novel idea of revealing the origin of enhanced circumstellar material (CSM) surrounding a core-collapse supernova by detecting neutrinos at multiple energies. In this method, assuming CSM is formed by the release of neutrinos in a pre-explosion burning phase, the following emission of high-energy neutrinos is considered. As a demonstration of the proposed idea, we estimated the signal event rate at JUNO and IceCube in comparison with background rate expected at each detector. This paper initiates astrophysical studies using neutrinos from diverse energy regimes, i.e., multi energy neutrino astronomy. The results are presented in a paper (link).

I proposed a new comprehensive framework for the diffuse fluxes of core-collapse supernova-origin neutrinos due to two distinctive physical processes. In this scheme, I made use of models of thermal MeV neutrinos emitted from the core and non-thermal high-energy neutrinos produced from accelerated cosmic rays interacting with stellar circumstellar material. These two energy regimes are connected through conventional supernova classification based on the optical survey and progenitor mass on the Salpeter initial mass function. I calculated the diffuse fluxes at Earth of both models by using the common cosmological parameters and core-collapse rate. The results are summarized in a paper (link).

I used IceCube starting track events to measure inelasticity in high energy neutrino interactions. Inelasticity is the energy fraction of the hadronic part in the neutrino interaction. Measuring inelasticity for high energy neutrinos is beneficial to test the Standard Model of particle physics as well as to search for astrophysical tau neutrinos. The Asimov sensitivity estimation I performed shows a 3 times better precision over the previous IceCube study with a different sample.

As a part of the IceCube-Upgrade plan, CMOS cameras will be installed together with new optical modules. These will be used to capture images of surrounding ice and calibrate its optical properties. I worked on developing analysis framework of camera images for extracting ice properties.

We developed a model of the successful supernova that results in a black hole at later times and investigated the impact of such type on the diffuse supernova neutrino background. From the sensitivity study, we found that contributions from such type supernovae can reduce the required operation time of Hyper-Kamiokande until detection. The results are presented in a paper (link).

I invented a novel data-based evaluation method for IceCube cascade angular reconstructoon by utilizing a starting track sample. The strategy is to reconstruct directions of the hadronic cascade part in muon neutrino charged-current events and compare them to track directions on a per-event basis. I found a significant bias in zenith angle reconstruction of cascades. From the detailed study of systematic uncertainty effects, I found such bias is most likely to stem from mis-modeling of the refrozen ice in drill holes. The study results are presented in a conference proceedings (link).

We calculated the diffuse supernova neutrino background flux based on a novel model of galactic chemical evolution. Here we used a variable initial mass function depending on the galaxy type and assumed failed supernovae forming a black hole for progenitors heavier than 18 solar-mass. The resulting electron antineutrino flux shows characteristic enhancements at high and low energies. We also investigated detectability of our new model at future water Cherenkov detectors, SK-Gd and Hyper-Kamiokande. The results are shown in a paper (link).

It is essential to study conditions of triggering in the data acquisition system at IceCube-Gen2, for giving feedback to hardware development. I studied trigger and band width for data transfer by using the Corsika simulation sets. I introduced some variables that can be collected in the data acquisition flow and found benchmark conditions to achieve requirement on bandwidth. I also investigated the impact of these benchmark conditions on the physics analysis by using the neutrino events simulated with NuGen. From this study, a trigger efficiency of >90% is ensured for good neutrino events. I performed multiple case studies about bandwidth and neutrino detection efficiency for different benchmark conditions in order for hardware people to refer to numbers easily. These results are shown in the technical design report of IceCube-Gen2 (link).

We focused on the fact that the emitted neutrino spectrum from core-collapse supernovae changes depending on the fate of stellar core collapse and therefore the diffuse supernova neutrino background measurement can potentially provide constraints on this part. We investigated their flux shape for different cases of the core-collapse remnant being a typical mass neutron star, a higher mass neutron star, or a black hole from the failed supernova. We also performed the sensitivity study to make constraints on the composition of these three, based on assumptions for the future water-based Cherenkov detectors, SK-Gd and Hyper-Kamiokande. The results are summarized in a paper (link). This work is introduced on astrobites (link).

I analyzed the Super-Kamiokande data to perform a diffuse supernova neutrino background search. Here I was especially working on improving spallation background reduction, systematic uncertainty estimation of background predictions, and the final statistical analysis. These efforts lead, in the end, to the world’s most sensitive search for electron antineutrinos from outside the Earth. The results are presented in a paper (link). Related to this work, I contributed to a similar analysis but motivated by another physics, Lorentz violation, as searching for electron antineutrinos from the Sun. The results are shown in a paper (link).

Neutrino-oxygen neutral-current quasielastic interactions are among the major backgrounds in diffuse supernova neutrino background searches at water Cherenkov detectors. I analyzed the Super-Kamipkande data from T2K beams to measure this channel precisely, utilizing nuclear γ-rays as a signal. The results serve the world’s most precise measurement for both neutrino and antineutrino cross sections of this channel. Especially the result for antineutrinos is the first ever experimental measurement. These results are published in a paper (link).

I joined the Super-Kamokande detector refurbishment work, which was for the later Gd loading. I was dedicated to especially the outer detector work, including replacement of old PMTs and shading sheets, rust removal, etc. Photos can be found in the Super-Kamiokande official website (link; a smiling guy in No.27 is me!).

In the T2K long baseline neutrino experiment, I worked as an expert of the muon beam monitor. Here I managed detector calibration and trouble shooting in the operation and performed various studied for the current and future beam monitors. Especially, we found serious issues with the current detectors, silicon sensors and ion chambers, in the high intensity beam operation. As a new candidate for the future detector, replacing with the current ones, we proposed electron-multiplier tubes (EMTs) and tested with the prototype sensors. The results are published in a paper (link). The follow-up studies at an electron beam facility have been made towards actual installation in J-PARC, as summarized in a paper (link).

We conducted test beam experiments at Osaka University’s Research Center for Nuclear Physics (RCNP) to measure production cross sections of γ-rays arising from the neutron-oxygen reaction. This is motivated by a large systematic uncertainty regarding the relevant cross sections in the neutrino neutral-current quasielastic interaction measurement at water-Cherenkov detectors. The measurement results are summarized in papers (link, link). As a necessary technique to estimate the neutron background in the γ-ray spectroscopy, we investigated a CsI(Tl) scintillator capability of the pulse shape discrimination (link).

I worked in the LHC-ATLAS experiment as a research project in the CERN summer student programme. My project was focused on a new inner detector, Insertable B-Layer (IBL), for the beam luminosity measurement based on the van der Meer (vdM) scan. In the vdM scan, a Gaussian shape is usually assumed as a beam profile, which may lead to potential systematic uncertainties. In order to investigate such effect, I investigated the luminosity result by changing the beam profile in a Pythia-based Monte Carlo simulation. I also analyzed the actual data from the IBL detector and measured the beam luminosity. The study results are summarized in a report (link).

I investigated impacts of cosmilogical parameters on the Cosmic Microwave Background B-mode using the CAMB simulator.