arXiv subjects

D. Y. Kim

Publications and source records attributed to D. Y. Kim.

At least 37 records · Page 2Linked to original sources

arXiv2024

Evidence of $h_{b}(\text{2P}) \to Υ(\text{1S})η$ decay and search for $h_{b}(\text{1P,2P}) \to Υ(\text{1S})π^0$ with the Belle detector

We report the first evidence for the $h_{b}(\text{2P}) \to Υ(\text{1S})η$ transition with a significance of $3.5$ standard deviations. The decay branching fraction is measured to be $\mathcal{B}[h_{b}(\text{2P}) \to Υ(\text{1S})η]=(7.1 ~^{+3.7} _{-3.2}\pm 0.8)\times10^{-3}$, which is noticeably smaller than expected. We also set upper limits on $π^0$ transitions of $\mathcal{B}[h_{b}(\text{2P}) \to Υ(\text{1S})π^0] < 1.8\times10^{-3}$, and $\mathcal{B}[h_{b}(\text{1P})\to Υ(\text{1S})π^0] < 1.8\times10^{-3}$, at the $90\%$ confidence level. These results are obtained with a $131.4$~fb$^{-1}$ data sample collected near the $Υ(\text{5S})$ resonance with the Belle detector at the KEKB asymmetric-energy $e^+e^-$ collider.

hep-ex↗

arXiv2024

Search for a heavy neutral lepton that mixes predominantly with the tau neutrino

We report a search for a heavy neutral lepton (HNL) that mixes predominantly with $ν_τ$. The search utilizes data collected with the Belle detector at the KEKB asymmetric energy $e^+ e^-$ collider. The data sample was collected at and just below the center-of-mass energies of the $Υ(4S)$ and $Υ(5S)$ resonances and has an integrated luminosity of $915~\textrm{fb}^{-1}$, corresponding to $(836\pm 12)\times 10^6$ $e^+e^\toτ^+τ^-$ events. We search for production of the HNL (denoted $N$) in the decay $τ^-\to π^- N$ followed by its decay via $N \to μ^+μ^- ν_τ$. The search focuses on the parameter-space region in which the HNL is long lived, so that the $μ^+μ^-$ originate from a common vertex that is significantly displaced from the collision point of the KEKB beams. Consistent with the expected background yield, one event is observed in the data sample after application of all the event-selection criteria. We report limits on the mixing parameter of the HNL with the $τ$ neutrino as a function of the HNL mass.

hep-ex↗

arXiv2024

Search for Two-Body $B$ Meson Decays to $Λ^{0}$ and $Ω^{(*)0}_{c}$

We report the results of the first search for Standard Model and baryon-number-violating two-body decays of the neutral $B$ mesons to $Λ^{0}$ and $Ω^{(*)0}_c$ using 711~${\rm fb^{-1}}$ of data collected at the $Υ(4S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^+ e^-$ collider. We observe no evidence of signal from any such decays and set 95\% confidence-level upper limits on the products of $B^0$ and $\bar{B}^0$ branching fractions for these two-body decays with $\mathcal{B}(Ω_{c}^{0} \to π^+ Ω^-)$ in the range between 9.5~$\times 10^{-8}$ and 31.2~$\times 10^{-8}$.

hep-ex↗

arXiv2024

Background study of the AMoRE-pilot experiment

We report a study on the background of the Advanced Molybdenum-Based Rare process Experiment (AMoRE), a search for neutrinoless double beta decay (\znbb) of $^{100}$Mo. The pilot stage of the experiment was conducted using $\sim$1.9 kg of \CAMOO~ crystals at the Yangyang Underground Laboratory, South Korea, from 2015 to 2018. We compared the measured $β/γ$ energy spectra in three experimental configurations with the results of Monte Carlo simulations and identified the background sources in each configuration. We replaced several detector components and enhanced the neutron shielding to lower the background level between configurations. A limit on the half-life of $0νββ$ decay of $^{100}$Mo was found at $T_{1/2}^{0ν} \ge 3.0\times 10^{23}$ years at 90\% confidence level, based on the measured background and its modeling. Further reduction of the background rate in the AMoRE-I and AMoRE-II are discussed.

nucl-ex↗

arXiv2024

Search for charged-lepton flavor violation in $Υ(2S) \to \ell^\mpτ^\pm$ ($\ell=e,μ$) decays at Belle

We report a search for the charged-lepton flavor violation in $Υ(2S) \to \ell^\mpτ^\pm$ ($\ell=e,μ$) decays using a $25~\fbi$ $Υ(2S)$ sample collected by the Belle detector at the KEKB $e^{+}$$e^-$ asymmetric-energy collider. We find no evidence for a signal and set upper limits on the branching fractions ($\mathcal{B}$) at 90\% confidence level. We obtain the most stringent upper limits: $\mathcal{B}(\Ytomutau) < 0.23 \times 10^{-6}$ and $\mathcal{B}(\Ytoetau) < 1.12 \times 10^{-6}$.

hep-ex↗

arXiv2024

Search for a dark leptophilic scalar produced in association with $τ^+τ^-$ pair in $e^+e^-$ annihilation at center-of-mass energies near 10.58 GeV

A dark leptophilic scalar $(ϕ_L)$ is a hypothetical particle that couples only to leptons rather than quarks. We report on a search for $ϕ_L$ in the $e^+e^- \to τ^+ τ^- ϕ_L, ~ϕ_L \to \ell^+ \ell^- ~(\ell = e, μ)$ process using 626~\invfb of data collected by the Belle experiment near the \Y4S resonance. We validate the backgrounds with multiple control regions in data, using a novel multiclass multivariate event classifier. In absence of a signal, we quote upper limits at the 90\% confidence level on the coupling between $ϕ_L$ and leptons. Our bounds, obtained in a blinded approach, are 19\% more constraining than the previous limits, averaged over the mass range $0.04 \leq m_{ϕ_L} \leq 6.5~\gev$. We exclude the parameter space below 4~\gev favored by measurement of the anomalous magnetic moment of the muon.

hep-ex↗

arXiv2024

Search for Baryon-Number-Violating Processes in $B^-$ Decays to the $\barΞ_{c}^{0} \barΛ_{c}^{-}$ Final State

We report the results of the first search for $B^-$ decays to the $\barΞ_{c}^{0} \barΛ_{c}^{-}$ final state using 711~${\rm fb^{-1}}$ of data collected at the $Υ(4S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^+ e^-$ collider. The results are interpreted in terms of both direct baryon-number-violating $B^-$ decay and $Ξ_{c}^{0}-\barΞ_{c}^{0}$ oscillations which follow the Standard Model decay $B^- \to Ξ_{c}^{0} \barΛ_{c}^{-}$. We observe no evidence for baryon number violation and set the 95\% confidence-level upper limits on the ratio of baryon-number-violating and Standard Model branching fractions ${\mathcal{B}(B^- \rightarrow \barΞ_{c}^{0} \barΛ_{c}^{-})}/{\mathcal{B}(B^- \rightarrow Ξ_{c}^{0} \barΛ_{c}^{-})}$ to be $< 2.7\%$ and on the $Ξ_{c}^{0} - \barΞ_{c}^{0}$ oscillation angular frequency $ω$ to be $< 0.76\ \mathrm{ps}^{-1}$ (equivalent to $τ_{\rm mix} > 1.3$~ps).

hep-ex↗

arXiv2024

Measurements of the branching fraction, polarization, and $CP$ asymmetry for the decay $B^0\rightarrow ωω$

We present a measurement of $B^{0} \rightarrow ωω$, a charmless decay into two vector mesons, using 772 $\times 10^6$ $B\overline{B}$ pairs collected with the Belle detector at the KEKB $e^+e^-$ collider. The decay is observed with a significance of 7.9 standard deviations. We measure a branching fraction $\mathcal{B} = (1.53 \pm 0.29 \pm 0.17) \times 10^{-6}$, a fraction of longitudinal polarization $f_L = 0.87 \pm 0.13 \pm 0.13$, and a time-integrated $CP$ asymmetry $A_{CP}$ = $-0.44 \pm 0.43 \pm 0.11$, where the first uncertainties listed are statistical and the second are systematic. This is the first observation of $B^{0} \rightarrow ωω$, and the first measurements of $f_L$ and $A_{CP}$ for this decay.

hep-ex↗

arXiv2023

Search for the semileptonic decays $Ξ_c^0 \to Ξ^0\ell^+\ell^-$ at Belle

Using the full data sample of 980 $\mathrm{fb}^{-1}$ collected with the Belle detector at the KEKB asymmetric energy electron-positron collider, we report the results of the first search for the rare semileptonic decays $Ξ_c^0 \to Ξ^0\ell^+\ell^-$ ($\ell=e$ or $μ)$. No significant signals are observed in the $Ξ^0\ell^+\ell^-$ invariant-mass distributions. Taking the decay $Ξ_c^0 \to Ξ^- π^+$ as the normalization mode, we report 90\% credibility upper limits on the branching fraction ratios ${\cal{B}} (Ξ_c^0 \to Ξ^0 e^+ e^-) / {\cal{B}}(Ξ_c^0\to Ξ^-π^+) < 6.7 \times 10^{-3}$ and ${\cal{B}} (Ξ_c^0 \to Ξ^0 μ^+ μ^-) / {\cal{B}}(Ξ_c^0\to Ξ^-π^+) < 4.3 \times 10^{-3}$ based on the phase-space assumption for signal decays. The 90\% credibility upper limits on the absolute branching fractions of ${\cal{B}} (Ξ_c^0 \to Ξ^0 e^+ e^-)$ and ${\cal{B}} (Ξ_c^0 \to Ξ^0 μ^+ μ^-)$ are found to be $9.9 \times 10^{-5}$ and $6.5 \times 10^{-5}$, respectively.

hep-ex↗

arXiv2023

Search for a heavy neutrino in $τ$ decays at Belle

We report on a search for a heavy neutrino in the decays $τ^- \to π^- ν_h$, $ν_h \to π^\pm \ell-+$, $\ell = e, μ$. The results are obtained using the full data sample collected with the Belle detector at the KEKB asymmetric energy $e^+e^-$ collider. We observe no significant signal and set 90% CL upper limits on the couplings of the heavy right-handed neutrinos to the conventional SM left-handed neutrinos in the mass range 0.2-1.6 GeV/c$^2$.

hep-ex↗

arXiv2023

Superconductivity of metastable dihydrides at ambient pressure

Hydrogen in metals is a significant research area with far-reaching implications, encompassing diverse fields such as hydrogen storage, metal-insulator transitions, and the recently emerging phenomenon of room-temperature ($\textit{$T_C$}$) superconductivity under high pressure. Hydrogen atoms pose challenges in experiments as they are nearly invisible, and they are considered within ideal crystalline structures in theoretical predictions, which hampers research on the formation of meta-stable hydrides. Here, we propose pressure-induced hydrogen migration from tetrahedral site ($\textit{T}$) to octahedral site ($\textit{O}$),forming $LaH_x^OH_{2-x}^{T}$ in cubic $LaH^2$.Under decompression, it retains $H_x^O$ occupancy, and is dynamically stable even at ambient pressure, enabling a synthesis route of metastable dihydrides via compression-decompression process. We predict that the electron phonon coupling strength of $LaH_x^OH_{2-x}^{T}$ is enhanced with increasing $\textit{x}$, and the associated $\textit{$T_C$}$ reaches up to 10.8 $\textit{K}$ at ambient pressure. Furthermore, we calculated stoichiometric hydrogen migration threshold pressure ($\textit{$P_C$}$) for various lanthanides dihydrides ($\textit{R}$$H_2$, where $\textit{R}$=Y, Sc, Nd, and Lu), and found an inversely linear relation between $\textit{$P_C$}$ and ionic radii of $\textit{R}$. We propose that the highest $\textit{$T_C$}$ in the face-centered-cubic dihydride system can be realized by optimizing the $\textit{O}$/$\textit{T}$-site occupancies.

cond-mat.supr-con↗

arXiv2023

Search for $CP$ violation in $D^{+}_{(s)}\rightarrow K^{+}K^{0}_{S}h^{+}h^{-}$ $(h=K,π)$ decays and observation of the Cabibbo-suppressed decay $D^{+}_{s}\rightarrow K^{+}K^{-}K^{0}_{S}π^{+}$

We search for $CP$ violation by measuring a $T$-odd asymmetry in the Cabibbo-suppressed $D^{+}\rightarrow K^{+}K^{0}_{S}π^{+}π^{-} $ decay, and in the Cabibbo-favored $D^{+}_{s}\rightarrow K^{+}K^{0}_{S}π^{+}π^{-}$ and $D^{+}\rightarrow K^{+}K^{-}K^{0}_{S}π^{+}$ decays. We use 980 ${\rm fb}^{-1}$ of data collected by the Belle detector running at the KEKB asymmetric-energy $e^{+}e^{-}$ collider. The $C\!P$-violating $T$-odd parameter ${a}^{T\text{-}\rm{odd}}_{CP}$ is measured to be ${a}^{T\text{-}\rm{odd}}_{CP}(D^{+}\rightarrow K^{+}K^{0}_{S}π^{+}π^{-})=(0.34\pm0.87\pm0.32)\%,$ ${a}^{T\text{-}\rm{odd}}_{CP}(D^{+}_{s}\rightarrow K^{+}K^{0}_{S}π^{+}π^{-})=(-0.46\pm0.63\pm0.38)\%,$ and ${a}^{T\text{-}\rm{odd}}_{CP}(D^{+}\rightarrow K^{+}K^{-}K^{0}_{S}π^{+})=(-3.34\pm2.66\pm0.35)\%,$ where the first uncertainty is statistical and the second is systematic. We also report the first observation of the Cabibbo-suppressed decay $D^{+}_{s}\rightarrow K^{+}K^{-}K^{0}_{S}π^{+}$. The branching fraction is measured relative to that of the analogous Cabibbo-favored decay : $B(D^{+}_{s}\rightarrow K^{+}K^{-}K^{0}_{S}π^{+}) / B(D^{+}_{s}\rightarrow K^{+}K^{0}_{S}π^{+}π^{-}) = (1.36\pm 0.15\pm 0.04)\%$.

hep-ex↗

arXiv2023

Measurement of Angular Coefficients of $\bar{B} \to D^* \ell \barν_\ell$: Implications for $|V_{cb}|$ and Tests of Lepton Flavor Universality

We measure the complete set of angular coefficients $J_i$ for exclusive $\bar{B} \to D^* \ell \barν_\ell$ decays ($\ell = e, μ$). Our analysis uses the full $711\,\mathrm{fb}^{-1}$ Belle data set with hadronic tag-side reconstruction. The results allow us to extract the form factors describing the $B \to D^*$ transition and the Cabibbo-Kobayashi-Maskawa matrix element $|V_{\rm cb}|$. Using recent lattice QCD calculations for the hadronic form factors, we find $|V_{\rm cb}| = (41.0 \pm 0.7) \times 10^3 $ using the BGL parameterization, compatible with determinations from inclusive semileptonic decays. We search for lepton flavor universality violation as a function of the hadronic recoil parameter $w$, and investigate the differences of the electron and muon angular distributions. We find no deviation from Standard Model expectations.

hep-ex↗

arXiv2023

Search for $B{}^0_s \rightarrow \ell^{\mp} τ^{\pm}$ with the Semi-leptonic Tagging Method at Belle

We present a search for the lepton-flavor-violating decays $B{}^0_s \rightarrow \ell^{\mp}τ^{\pm}$, where $\ell = e, μ$, using the full data sample of $121~\mathrm{fb}^{-1}$ collected at the $Υ(5S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^+e^-$ collider. We use $B{}^0_s \overline{B}{}^0_s$ events in which one $B{}^0_s$ meson is reconstructed in a semileptonic decay mode and the other in the signal mode. We find no evidence for $B{}^0_s \rightarrow \ell^{\mp}τ^{\pm}$ decays and set upper limits on their branching fractions at $90\%$ confidence level as $\mathcal{B}(B{}^0_s \rightarrow e^{\mp}τ^{\pm}) < 14 \times 10^{-4}$ and $\mathcal{B}(B{}^0_s \rightarrow μ^{\mp}τ^{\pm}) < 7.3 \times 10^{-4}$. Our result represents the first upper limit on the $B{}^0_s \rightarrow e^{\mp}τ^{\pm}$ decay rate.

hep-ex↗

arXiv2023

Observation of charmed strange meson pair production in $Υ(2S)$ decays and in $e^{+}e^{-}$ annihilation at $\sqrt{s} = 10.52~ \rm{GeV}$

We observe the process $Υ(2S)\to D_s^{(*)+} D_{sJ}^{-}$ and continuum production $e^+e^- \to D_s^{(*)+} D_{sJ}^- $ at $\sqrt{s} = 10.52$ GeV (and their charge conjugates) using the data samples collected by the Belle detector at KEKB, where $D_{sJ}^-$ is $D_{s1}(2536)^-$ or $D^{*}_{s2}(2573)^-$. Both $D_{sJ}^-$ states are identified through their decay into $\bar{K}\bar{D}^{(*)}$. We measure the products of branching fractions ${\cal B}(Υ(2S) \to D_{s}^{(*)+} D_{sJ}^-) {\cal B}(D_{sJ}^-\to \bar{K} \bar{D}^{(*)})$ and the Born cross sections $σ^{\rm Born}(e^+e^- \to D_{s}^{(*)+} D_{sJ}^-) {\cal B}(D_{sJ}^-\to \bar{K} \bar{D}^{(*)})$, and then compare the ratios $R_1 \equiv {\cal B}(Υ(2S)\to D_{s}^{(*)+} D_{sJ}^-)/{\cal B}(Υ(2S)\toμ^{+}μ^-)$ for $Υ(2S)$ decays and $R_2 \equiv σ^{\rm Born}(e^+e^-\to D_{s}^{(*)+}D_{sJ}^-)/σ^{\rm Born}(e^+e^-\to μ^{+}μ^-)$ for continuum production. We obtain $R_1/R_2 = 9.7\pm 2.3 \pm 1.1$, $6.8 \pm 2.1 \pm 0.8$, $10.2 \pm 3.3 \pm 2.5$, and $3.4 \pm 2.1 \pm 0.5$ for the $D_s^+ D_{s1}(2536)^-$, $D_s^{*+} D_{s1}(2536)^-$, $D_s^+ D_{s2}^{*}(2573)^{-}$, and $D_s^{*+} D_{s2}^{*}(2573)^{-}$ final states in the $D_{sJ}^-\to K^{-} \bar{D}^{(*)0}$ modes, respectively. Therefore, the strong decay is expected to dominate in the $Υ(2S)\to D_{s}^{(*)+}D_{sJ}^-$ processes. We also measure the ratios of branching fractions ${\cal B}(D_{s1}(2536)^-\to K_S^0 D^{*}(2010)^{-})/{\cal B}(D_{s1}(2536)^-\to K^{-} D^{*}(2007)^0) = 0.48 \pm 0.07 \pm 0.02$ and ${\cal B}(D_{s2}^{*}(2573)^- \to K_S^0 D^-)/{\cal B}(D_{s2}^{*}(2573)^- \to K^{-}D^0) = 0.49 \pm 0.10 \pm 0.02$, which are consistent with isospin symmetry. The second ratio is the first measurement of this quantity. Here, the first uncertainties are statistical and the second are systematic.

hep-ex↗

arXiv2023

Measurement of the $e^+e^- \to B_s^0 \bar{B}_s^0 X$ cross section in the energy range from $10.63$ to $11.02$ GeV using inclusive $D_s^{\pm}$ and $D^0$ production

We report the first measurement of the inclusive $e^+e^- \to b\bar{b} \to D_s^{\pm}X$ and $e^+e^- \to b\bar{b} \to D^0/\bar{D}^0X$ cross sections in the energy range from $10.63$ to $11.02$. Based on these results, we determine $σ(e^+ e^- \to B_s^0 \bar{B}_s^0 X)$ and $σ(e^+ e^- \to B\bar{B}X)$ in the same energy range. We measure the fraction of $B_s^0$ events at $Υ(10860)$ to be $f_{\rm s}=(22.0^{+2.0}_{-2.1})\%$. We determine also the ratio of the $B_s^0$ inclusive branching fractions $\mathcal{B}(B_s^0 \to D^0/\bar{D}^0X)/\mathcal{B}(B_s^0 \to D_s^{\pm} X)=0.416 \pm 0.018 \pm 0.092$. The results are obtained using the data collected with the Belle detector at the KEKB asymmetric-energy $e^+e^-$ collider.

hep-ex↗

arXiv2023

Evidence for $B^0 \to p\barΣ^0π^-$ at Belle

We search for the $B^0\to p\barΣ^0π^-$ decay with $\barΣ^0 \to \barΛγ$, where the $γ$ is not measured, using a data sample corresponding to an integrated luminosity of 711 $\rm{fb^{-1}}$ which contains 772 $\times$ $10^{6}$ $B\bar{B}$ pairs, collected around the $Υ$(4S) resonance with the Belle detector at the KEKB asymmetric-energy $e^{+}e^{-}$ collider. We measure for the first time the $B^0\to p\barΣ^0π^-$ branching fraction to be $\mathcal{B}(B^0 \to p \barΣ^0 π^-) = (1.17^{+0.43}_{-0.40}(\text{stat})\pm 0.07(\text{syst})) \times 10^{-6}$ with a significance of $3.0σ$. We simultaneously measure the branching fraction for the related channel $B^{0}\to p\barΛπ^{-}$ with much improved precision.

hep-ex↗

arXiv2023

First Simultaneous Determination of Inclusive and Exclusive $\left|V_{ub}\right|$

The first simultaneous determination of the absolute value of the Cabibbo-Kobayashi-Maskawa matrix element $V_{ub}$ using inclusive and exclusive decays is performed with the full Belle data set at the $Υ(4S)$ resonance, corresponding to an integrated luminosity of 711 fb${}^{-1}$. We analyze collision events in which one $B$ meson is fully reconstructed in hadronic modes. This allows for the reconstruction of the hadronic $X_u$ system of the semileptonic $b \to u \ell \bar ν_\ell$ decay. We separate exclusive $B \to π\, \ell\, \bar ν_{\ell}$ decays from other inclusive $B \to X_u \, \ell\, \bar ν_{\ell}$ and backgrounds with a two-dimensional fit, that utilizes the number of charged pions in the $X_u$ system and the four-momentum transfer $q^2$ between the $B$ and $X_u$ system. Combining our measurement with information from lattice QCD and QCD calculations of the inclusive partial rate as well as external experimental information on the shape of the $B \to π\, \ell\, \bar ν_{\ell}$ form factor, we determine $\left|V_{ub}^{\mathrm{excl.}} \right| = (3.78 \pm 0.23 \pm 0.16 \pm 0.14)\times 10^{-3}$ and $\left|V_{ub}^{\mathrm{incl.}} \right| = (3.88 \pm 0.20 \pm 0.31 \pm 0.09)\times 10^{-3}$, respectively, with the uncertainties being the statistical error, systematic errors, and theory errors. The ratio of $\left|V_{ub}^{\mathrm{excl.}} \right| / \left|V_{ub}^{\mathrm{incl.}} \right| = 0.97 \pm 0.12$ is compatible with unity.

hep-ex↗