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S. K. Lee

Publications and source records attributed to S. K. Lee.

14 recordsLinked to original sources
arXiv2026

The Search for $K_L \rightarrow π^0π^0γγ$ and $K_L\rightarrow π^0π^0X$ where $X\rightarrow 2γ$ at the KOTO Experiment

We performed searches for $K_L\rightarrow π^0π^0X$ where $X$ may be an axion-like particle which promptly decays to two photons, and the first search for $K_L \rightarrow π^0π^0γγ$ at the KOTO experiment using data taken in 2021. The search is performed for $X$ mass in the range of 160$\unicode{x2013}$220 MeV/$c^2$. Three events were observed in the signal region, with two events near an $X$ mass of 177 MeV/$c^2$. This result led to a range of upper limits on the branching ratio, BR($K_L\rightarrow π^0π^0X$) $< (1\unicode{x2013}20) \times 10^{-7}$ at the 95% confidence level (C.L.). No events were observed for the analysis of $K_L \rightarrow π^0π^0γγ$, setting an upper limit on the branching ratio, BR($K_L \rightarrow π^0π^0γγ$) $< 1.69 \times 10^{-6}$ at the 95% C.L.

hep-ex↗
arXiv2025

Search for the $K_{L} \to π^{0} ν\barν$ Decay at the J-PARC KOTO Experiment

We performed a search for the $K_L \to π^{0} ν\barν$ decay using the data taken in 2021 at the J-PARC KOTO experiment. With newly installed counters and new analysis method, the expected background was suppressed to $0.252\pm0.055_{\mathrm{stat}}$$^{+0.052}_{-0.067}$$_{\mathrm{syst}}$. With a single event sensitivity of $(9.33 \pm 0.06_{\rm stat} \pm 0.84_{\rm syst})\times 10^{-10}$, no events were observed in the signal region. An upper limit on the branching fraction for the decay was set to be $2.2\times10^{-9}$ at the 90% confidence level (C.L.), which improved the previous upper limit from KOTO by a factor of 1.4. With the same data, a search for $K_L \to π^{0} X^{0}$ was also performed, where $X^{0}$ is an invisible boson with a mass ranging from 1 MeV/$c^{2}$ to 260 MeV/$c^{2}$. For $X^{0}$ with a mass of 135 MeV/$c^{2}$, an upper limit on the branching fraction of $K_L \to π^{0} X^{0}$ was set to be $1.6\times10^{-9}$ at the 90% C.L.

hep-ex↗
arXiv2023

Suppression of Neutron Background using Deep Neural Network and Fourier Frequency Analysis at the KOTO Experiment

We present two analysis techniques for distinguishing background events induced by neutrons from photon signal events in the search for the rare $K^0_L\rightarrowπ^0ν\barν$ decay at the J-PARC KOTO experiment. These techniques employed a deep convolutional neural network and Fourier frequency analysis to discriminate neutrons from photons, based on their variations in cluster shape and pulse shape, in the electromagnetic calorimeter made of undoped CsI. The results effectively suppressed the neutron background by a factor of $5.6\times10^5$, while maintaining the efficiency of $K^0_L\rightarrowπ^0ν\barν$ at $70\%$.

hep-ex↗
arXiv2023

Search for the Pair Production of Dark Particles $X$ with $K_L^0 \to XX$, $X \to γγ$

We present the first search for the pair production of dark particles $X$ via $K_L^0\to XX$ with $X$ decaying into two photons using the data collected by the KOTO experiment. No signal was observed in the mass range of 40 - 110 MeV/c$^2$ and 210 - 240 MeV/c$^2$. This sets upper limits on the branching fractions as $\mathcal{B}(K_L^0 \to XX)$ $<$ (1-4) $\times$ 10$^{-7}$ and $\mathcal{B}(K_L^0 \to XX)$ $<$ (1-2) $\times$ 10$^{-6}$ at the 90% confidence level for the two mass regions, respectively.

hep-ex↗
arXiv2021

Study of the $K_L \!\to\! π^0 ν\overlineν$ Decay at the J-PARC KOTO Experiment

The rare decay $K_L \!\to\! π^0 ν\overlineν$ was studied with the dataset taken at the J-PARC KOTO experiment in 2016, 2017, and 2018. With a single event sensitivity of $( 7.20 \pm 0.05_{\rm stat} \pm 0.66_{\rm syst} ) \times 10^{-10}$, three candidate events were observed in the signal region. After unveiling them, contaminations from $K^{\pm}$ and scattered $K_L$ decays were studied, and the total number of background events was estimated to be $1.22 \pm 0.26$. We conclude that the number of observed events is statistically consistent with the background expectation. For this dataset, we set an upper limit of $4.9 \times 10^{-9}$ on the branching fraction of $K_L \!\to\! π^0 ν\overlineν$ at the 90% confidence level.

hep-ex↗
arXiv2016

A new search for the $K_{L} \to π^0 ν\overlineν$ and $K_{L} \to π^{0} X^{0}$ decays

We searched for the $CP$-violating rare decay of neutral kaon, $K_{L} \to π^0 ν\overlineν$, in data from the first 100 hours of physics running in 2013 of the J-PARC KOTO experiment. One candidate event was observed while $0.34\pm0.16$ background events were expected. We set an upper limit of $5.1\times10^{-8}$ for the branching fraction at the 90\% confidence level (C.L.). An upper limit of $3.7\times10^{-8}$ at the 90\% C.L. for the $K_{L} \to π^{0} X^{0}$decay was also set for the first time, where $X^{0}$ is an invisible particle with a mass of 135 MeV/$c^{2}$.

hep-ex↗
arXiv2016

A review of the discovery reach of directional Dark Matter detection

Cosmological observations indicate that most of the matter in the Universe is Dark Matter. Dark Matter in the form of Weakly Interacting Massive Particles (WIMPs) can be detected directly, via its elastic scattering off target nuclei. Most current direct detection experiments only measure the energy of the recoiling nuclei. However, directional detection experiments are sensitive to the direction of the nuclear recoil as well. Due to the Sun's motion with respect to the Galactic rest frame, the directional recoil rate has a dipole feature, peaking around the direction of the Solar motion. This provides a powerful tool for demonstrating the Galactic origin of nuclear recoils and hence unambiguously detecting Dark Matter. Furthermore, the directional recoil distribution depends on the WIMP mass, scattering cross section and local velocity distribution. Therefore, with a large number of recoil events it will be possible to study the physics of Dark Matter in terms of particle and astrophysical properties. We review the potential of directional detectors for detecting and characterizing WIMPs.

astro-ph.CO↗
arXiv2016

Long-lived neutral-kaon flux measurement for the KOTO experiment

The KOTO ($K^0$ at Tokai) experiment aims to observe the CP-violating rare decay $K_L \rightarrow π^0 ν\barν$ by using a long-lived neutral-kaon beam produced by the 30 GeV proton beam at the Japan Proton Accelerator Research Complex. The $K_L$ flux is an essential parameter for the measurement of the branching fraction. Three $K_L$ neutral decay modes, $K_L \rightarrow 3π^0$, $K_L \rightarrow 2π^0$, and $K_L \rightarrow 2γ$ were used to measure the $K_L$ flux in the beam line in the 2013 KOTO engineering run. A Monte Carlo simulation was used to estimate the detector acceptance for these decays. Agreement was found between the simulation model and the experimental data, and the remaining systematic uncertainty was estimated at the 1.4\% level. The $K_L$ flux was measured as $(4.183 \pm 0.017_{\mathrm{stat.}} \pm 0.059_{\mathrm{sys.}}) \times 10^7$ $K_L$ per $2\times 10^{14}$ protons on a 66-mm-long Au target.

physics.ins-det↗
arXiv2015

Effect of symmetry mismatch on the domain structure of rhombohedral BiFeO3 thin films

Considerable work has focused on the use of epitaxial strain to engineer domain structures in ferroic materials. Here, we revisit the observed reduction of domain variants in rhombohedral BiFeO3 films on rare-earth scandate substrates. Prior work has attributed the reduction of domain variants to anisotropic in-plane strain, but our findings suggest that the monoclinic distortion of the substrate, resulting from oxygen octahedral rotation, is the driving force for variant selection. We study epitaxial BiFeO3 DyScO3 (110)o heterostructures with and without ultrathin, cubic SrTiO3 buffer layers as a means to isolate the effect of symmetry mismatch on the domain formation. Two variant stripe domains are observed in films grown directly on DyScO3, while four-variant domains are observed in films grown on SrTiO3 buffered DyScO3 when the buffer layer is >2 nm thick. This work provides insights into the role of the substrate beyond just lattice mismatch in manipulating and controlling domain structure evolution in materials.

cond-mat.mtrl-sci↗
arXiv2014

Emergence of stationary many-body entanglement in driven-dissipative Rydberg lattice gases

Non-equilibrium quantum dynamics represents an emerging paradigm for condensed matter physics, quantum information science, and statistical mechanics. Strongly interacting Rydberg atoms offer an attractive platform to study driven-dissipative dynamics of quantum spin models with long-range order. Here, we explore the conditions under which stationary many-body entanglement persists with near-unit fidelity and high scalability. In our approach, coherent many-body dynamics is driven by Rydberg-mediated laser transitions, while atoms at the lattice boundary reduce the entropy of the many-body state. Surprisingly, the many-body entanglement is established by continuously evolving a locally dissipative Rydberg system towards the steady-state, as with optical pumping. We characterize the dynamics of multipartite entanglement in a 1D lattice by way of quantum uncertainty relations, and demonstrate the long-range behavior of the stationary entanglement with finite-size scaling, reaching "hectapartite" entanglement under experimental conditions. Our work opens a route towards dissipative preparation of many-body entanglement with unprecedented scaling behavior.

quant-ph↗
arXiv2012

Phase Space Tomography of Matter-Wave Diffraction in the Talbot Regime

We report on the theoretical investigation of Wigner distribution function (WDF) reconstruction of the motional quantum state of large molecules in de Broglie interference. De Broglie interference of fullerenes and as the like already proves the wavelike behaviour of these heavy particles, while we aim to extract more quantitative information about the superposition quantum state in motion. We simulate the reconstruction of the WDF numerically based on an analytic probability distribution and investigate its properties by variation of parameters, which are relevant for the experiment. Even though the WDF described in the near-field experiment cannot be reconstructed completely, we observe negativity even in the partially reconstructed WDF. We further consider incoherent factors to simulate the experimental situation such as a finite number of slits, collimation, and particle-slit van der Waals interaction. From this we find experimental conditions to reconstruct the WDF from Talbot interference fringes in molecule Talbot-Lau interferometry.

quant-ph↗
arXiv1995

Electroweak Sphaleron For Effective Theory In The Limit Of Large Higgs Boson Mass

Theoretical arguments suggest that the Higgs sector of the standard model is an effective theory. We parametrize the new physics by means of an effective Lagrangian technique and study its effect on the energy of the electroweak sphaleron. We found that in the presence of a certain class of higher dimension operators the sphaleron energy becomes arbitrarily large as the Higgs boson mass increases. The physical meaning of this result and its implications to the electroweak baryogenesis in the dynamical models are discussed.

hep-ph↗
arXiv1994

Phenomenology of a non-standard top quark Yukawa coupling

There are theoretical speculations that the top quark may have different properties from that predicted by the standard model. We use an effective Lagrangian technique to model such a non-standard top quark scenario and study its effects on the electroweak baryogenesis, low and high energy physics.

hep-ph↗