Research

Overview


My research field is experimental (astro-)particle physics focusing on neutrinos.  Neutrinos are still very mysterious even though several breakthrough discoveries have been made since their conceptual birth by Pauli in 1930. I have been working on neutrino physics for more than fifteen years, from astroparticle physics in IceCube (2004-2012) to reactor neutrino physics in RENO (2012-2018) and NEOS-II (2018-2023), and accelerator/astro-particle neutrino physics in Hyper-Kamiokande (2016-2020) and a future large neutrino detector (aka LSC, 2018-2023) at Yemilab. In Fermilab, I plan to work on SBN program (ICARUS and SBND) and DUNE. 

LSC  (Liquid Scintillation Counter)


Yemilab, constructed in the Handuk iron mine, Jeongseon-gun, is the 1st deep (1 km) underground laboratory dedicated to science in Korea. After five years of construction in two phases, Yemilab is finally ready for neutrino, dark matter, and other scientific experiments as of October 2022, where a large cylindrical cavern (D: 20 m, H: 20 m) was also constructed for a future large-scale neutrino experiment. In this cavern, we could install a liquid scintillator detector (LSC) of a ~2 kilo-ton target to carry out exciting research topics, from particle physics to astroparticle physics. 


I have been responsible for the LSC's design and the physics potential study. With the detector alone, we can precisely measure solar neutrino (pp, 7Be, pep, 8B, and CNO) fluxes, assuming we achieve the level of radiopurity of the liquid scintillator to Borexino's, thanks to an ~8 times larger detector than Borexino. Detecting Supernova burst neutrinos, DSNB, and geo-neutrinos is possible, too. With a 100 MeV-100 kW electron linac facility, we can obtain the best sensitivity on a direct search of dark photons [JHEP 04 (2021) 135] in the mass range between O(eV) and 30 MeV.  With an IsoDAR facility (60 MeV-600 kW proton cyclotron and 9Be target with 7Li sleeve) [arXiv:2110.10635 and JINST 17 (2022) 09, P09042], we could test several scenarios of sterile neutrinos and set the best exclusion sensitivity in electron antineutrino disappearance channel [Phys.Rev.D 105 (2022) 5, 052009] if no sterile neutrino is found. We could search sterile neutrinos using a radioactive source such as 144Ce (100 kCi).  After all these studies, we could upgrade LSC to carry out a neutrinoless double beta decay search. 


We have been working on the studies with international colleagues, and a white paper on LSC will be available to the public soon. 

LSC Physics Program

LSC Dark Photon Sensitivity

IsoDAR@LSC Sensitivity

NEOS-II

NEOS (Neutrino Experiment for Oscillation at Short baseline) experiment aiming for a sterile neutrino search using reactor neutrinos from Hanbit Nuclear Power Plant in Yeonggwang, Korea. We refurbished the NEOS phase-I detector, including the production of a new liquid scintillator target (~1 ton, doped with ~0.5% Gd), and have successfully taken data from Sept. 2018 to Oct. 2020 at a tendon gallery of the 5th reactor core with a ~24 m baseline. The main goal of the phase-II (NEOS-II) is to perform a spectral decomposition of primary fission isotopes, U-235 and Pu-239, by taking an entire cycle of reactor fuel data (~500 calendar days) to study the origin of the flux/shape anomalies of the reactor neutrinos as well as to search for sterile neutrinos. 

Using 388 (112) live days of reactor-on (-off) data, we are finalizing an analysis of the spectral decomposition of U-235/Pu-239 and IBD yield measurement. A preliminary result on a sterile neutrino search is also expected soon. 

Hyper-K/T2HKK



Hyper-K [arXiv:1805.04163] is a future version of Super-K but with more improved capabilities in its size and technology. It consists of two water Cherenkov detectors (2x260 kiloton). By locating one detector in Japan and the other in Korea, its physics potentials are improved thanks to a more than three times longer baseline in Korea from the J-PARC neutrino beam. In 2016, I proposed to locate the Hyper-K 2nd detector in Korea, and it was officially accepted by the Hyper-K collaboration. Since then, I have served as a working group convener for Hyper-K 2nd detector in Korea, a.k.a. T2HKK or Korean Neutrino Observatory (KNO).


More details on the improved physics potentials are found in the T2HKK white paper [PTEP 2018, no. 6, 063C01 (2018)], which I was responsible for as an editor and corresponding author of this paper. Hyper-K is a multi-purpose experiment covering particle physics, astroparticle physics, and a proton decay search. If the 2nd detector is built in Korea, then Korea will be one of the three countries that led world neutrino physics with Japan and the USA for more than 30 years after its construction. 


Photo sensors are a pivotal but most expensive part of the Hyper-K detector components, and high-quality photo sensors can significantly enhance the experimental outcomes. As a member of the photo-sensor committee, I have made crucial contributions to the current design of the Hyper-K photo-sensors, especially towards improving Hyper-K sensitivities to low-energy physics by suggesting the replacement of photo-sensor glasses with low radioactivities. Following my persistent requests for collaboration, the low radioactive glasses have been applied to the new photo sensors by the Hamamatsu company. As a result, the dark rate of the photo sensors is reduced to ~4 kHz  [Nucl.Instrum.Meth.A 958 (2020) 162993]. 

Map showing the baseline and off-axis angle of the J-PARC beam in Japan and Korea [K. Hagiwara, N. Okamura, K. Senda, Phys. Rev. D 76, 093002 (2007)]

RENO



RENO (Reactor Experiment for Neutrino Oscillation) is located in Hanbit Nuclear Power Plant in Yonggwang to measure the smallest neutrino mixing angle (theta_13) in the Pontecorvo-Maki-Nakaga-Sakata (PMNS).  The theta_13 was independently measured in 2012 as a discovery by RENO Daya Bay, confirming earlier but less significant results by T2K, MINOS, and Double Chooz. 


In RENO, I contributed significantly to the first measurement of the oscillation frequency (Dm2ee) and amplitude (theta_13) using 500 live days of data, as described in Phys.Rev.Lett. 116 (2016) 21, 211801 and Phys.Rev.D98, 012002 (2018). I initially performed the "5 MeV excess" study, which was announced at the most prestigious neutrino conference in Boston in 2014 [AIP Conf. Proc. 1666 (2015) 080002], resulting in the second big news of the conference, and many follow-up studies are still on-going to understand the origin of the excess not explained by current reactor neutrino flux models. 


I've also been leading an effort for an alternative analysis (n-H capture) which is more challenging due to the larger background than that of n-Gd analysis. The first result on the alternative analysis using 1500 live days of data was obtained [JHEP 04 (2020) 029], which is consistent with the primary analysis result. I was also involved in a study on using neutrino detectors for nuclear monitoring applications with 16 world experts [Science 09 Nov 2018, Vol. 362, Issue 6415, pp. 649-650]. 


There was no clue at all at the beginning when strange events were suddenly observed in RENO data (through distortion of prompt signal) since October 2012. After some investigation, I found the cause was the Cf-252 source contamination due to the loose O-ring in the capsule containing the source. The loose O-ring resulted in a tiny fraction of the Cf-252 source leaking into the detector. By remaking the Cf-252 source capsule, we could stop contaminating data further. We observed the Cf-252 background had been reduced as time went on according to the half-life (~2.6 years) of the Cf-252. leaked into the detector.  

 

After some operation of RENO, we observed a light yield decrease. Upon my suggestion, nitrogen purging of the detector was performed. After providing a continuous nitrogen inflow to the detector, we observed that the light yield decrease had stopped. 


RENO Detector

RENO result using 500 days [Phys.Rev.D98, 012002 (2018)]

5 MeV Excess [Neutrino 2014 @Boston, AIP Conf. Proc. 1666 (2015) 080002]

IceCube



IceCube is a neutrino telescope consisting of about 5,600 digital optical modules (DOMs) frozen in deep ice over a cubic kilometer volume at the S. Pole, mainly to search for astrophysical sources of neutrinos and dark matter search and neutrino mass ordering determination and other neutrino oscillation parameters. 

I was a member of IceCube from 2004 to early 2012, the construction phase [Astropart.Phys. 26 (2006) 155-173], and I was responsible for designing, implementing, and maintaining the IceCube global trigger system (part of the Data Acquisition System [Nucl.Instrum.Meth.A 601 (2009) 294-316]). It was successfully tested at the South Pole in 2006. I also participated in a joint trigger system between IceCube and AMANDA for a joint search for neutrino signals. I was also responsible for managing simulation software in IceCube. 

While most people in IceCube worked on muon neutrino search, I took a path not explored before by working on an ultrahigh energy (UHE) tau neutrino search. I invented the search algorithms optimized for the UHE tau neutrino event topologies such as “Double Bang” and “(inverted-)Lollipops.” This algorithm was applied to IceCube 22-string data and obtained the first result on UHE tau neutrino search with no signal observation [Phys. Rev. D 86, 022005 (2012)]. Even though my pioneering work on the astrophysical UHE tau neutrino search in IceCube, no such UHE tau neutrino event with clear topological signatures (Double-Bangs and (inverted-)Lollipops), has been observed so far in IceCube with its whole detector, 86 strings in ~1 km3 volume. The future IceCube Gen-2, with its increased detection volume, could observe the UHE tau neutrino events unless there is new physics.

Three events survived UHE Tau neutrino selection criteria in IC-22 data [Phys. Rev. D 86, 022005 (2012)]

Sunny @S.Pole