2020 Fiscal Year Annual Research Report
Studying supernova explosions via their neutrino emissions
| Project Area | Gravitational wave physics and astronomy: Genesis |
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| Project/Area Number |
17H06365
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| Research Institution | The University of Tokyo |
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Principal Investigator |
ヴァギンズ マーク 東京大学, カブリ数物連携宇宙研究機構, 教授 (90509902)
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| Co-Investigator(Kenkyū-buntansha) |
松古 栄夫 大学共同利用機関法人高エネルギー加速器研究機構, 計算科学センター, 助教 (10373185)
住吉 光介 沼津工業高等専門学校, 教養科, 教授 (30280720)
小汐 由介 岡山大学, 自然科学研究科, 准教授 (80292960)
原田 了 東京大学, 宇宙線研究所, 特任研究員 (80844795)
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| Project Period (FY) |
2017-06-30 – 2022-03-31
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| Keywords | gravitational wave |
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| Outline of Annual Research Achievements |
Core collapse supernova [SN] explosions comprise one of the primary sources of near-field gravitational waves [GW]. SN neutrinos, famously observed from SN1987A, also carry information regarding the end state of the star: for explosions within our galaxy, collapses into neutron stars or black holes, the eventual sources of far-field GW, can be differentiated via observations of neutrino emissions.
EXPERIMENT
On the experimental side, the primary research achievement in FY2020 - and it was a major one - was the loading of the first batch, 13.2 tons, of specially developed, ultra-radiopure gadolinium sulfate octahydrate (Gd2(SO4)3)*8H2O) into Super-Kamiokande [SK]. This transformative enhancement of the famous detector makes it much more sensitive to supernova neutrinos - especially those made by very distant explosions - by tagging the neutrons produced in inverse beta decays (IBD) in the water. Doing so reduces the backgrounds to relic supernova neutrinos by a factor of about 10,000.
THEORY
On the theory side, we have succeeded in performing the three-dimensional simulations of the post-bounce evolution of a supernova core by the Boltzmann-Hydro code for the first time. We revealed the feature of 3D prompt convection and the associated asymmetry in the neutrino angle distributions. We have clarified the role of nuclear composition in the early occurrence of convection due to the difference in the equation of state. We prepared the framework of long-term supernova simulations and the analytic formulae of neutrino emission for the observational signals at Super-Kamiokande.
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| Current Status of Research Progress |
Current Status of Research Progress
2: Research has progressed on the whole more than it was originally planned.
Reason
Our ultimate goal is to make theory and experiment work together so we will be ready to make the best possible observations of the next galactic explosion and maximize our extraction of information on the explosion mechanism, progenitor, and nuclear physics ingredients.
EXPERIMENT
For almost twenty years, the prospect of adding gadolinium to Super-Kamiokande to enhance its ability to detect supernova neutrinos has fueled intensive R&D work in Kamioka and elsewhere around Japan and the world. Now, with the first batch of gadolinium finally in the detector, neutrons - and hence neutrino-initiated IBD events - can be detected with approximately 50% efficiency. The transparency of the Gd-loaded water in SK has remained excellent, and in general everything is going as planned.
THEORY
We have successfully extended the numerical code to the general relativistic Boltzmann solver to describe the neutrino transport in curved space. Further extension of the numerical code toward the full treatment of general relativity is underway. We have performed additional simulations of the 2D core-collapse simulations with the Boltzmann-Hydro code. We are analyzing the numerical data of simulation results to examine the difference in stellar models, the effects of the equation of state, and the approximations of the Eddington tensor, which has been routinely used in other groups.
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| Strategy for Future Research Activity |
EXPERIMENT
In FY2021 we plan to steadily collect data after having finally - the idea was first proposed in 2002 - added the very first gadolinium to the water of Super-Kamiokande during FY2020. With a 0.01% by mass concentration of aqueous Gd3+ ions now in the SK water, we will accumulate subtle, previously hidden supernova neutrino interactions from extremely distant (z=1) explosions at roughly 50% efficiency. In the meantime, SK's realtime supernova detection capabilities will be enhanced via more powerful computing using Gd's neutron tagging to provide earlier, more useful warning of a galactic SN to the wider community. Assuming all continues to go well, increased Gd loading of SK to 0.03% Gd3+ resulting in 75% neutron detection efficiency is expected during FY2022.
THEORY
We plan to perform extended simulations of core-collapse supernovae by the Boltzmann-Hydro code to explore the outcome of the shock propagation. We will reveal the differences due to the stellar rotations and new sets of the equation of state and clarify the validity of conventional approximations. We will construct the templates of the neutrino light curve for supernovae by covering the equation of state and the stellar mass. We want to establish the method to extract information about proto-neutron stars from the burst information through the collaboration of theory and experiment members.
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