arXiv subjects

R. Kulasiri

Publications and source records attributed to R. Kulasiri.

At least 19 recordsLinked to original sources

arXiv2023

Search for the decay $B^{0}\rightarrow K^{\ast 0}τ^{+}τ^{-}$ at the Belle experiment

This letter presents a search for the rare flavor-changing neutral current process $B^{0}\rightarrow K^{\ast 0}τ^{+}τ^{-}$ using data taken with the Belle detector at the KEKB asymmetric energy $e^{+}e^{-}$ collider. The analysis is based on the entire $Υ(4S)$ resonance data sample of 711 $\rm fb^{-1}$, corresponding to $772\times 10^{6} B \bar{B}$ pairs. In our search we fully reconstruct the companion $B$ meson produced in the process $e^{+}e^{-}\rightarrowΥ(4S)\rightarrow B\bar{B}$ from its hadronic decay modes, and look for the decay $B^{0}\rightarrow K^{\ast 0}τ^{+}τ^{-}$ in the rest of the event. No evidence for a signal is found. We report an upper limit on the branching fraction $\mathcal{B}({B^{0}\rightarrow K^{\ast 0}τ^{+}τ^{-}})<3.1\times 10^{-3}$ at 90\% confidence level. This is the first direct limit on $\mathcal{B}({B^{0}\rightarrow K^{\ast 0}τ^{+}τ^{-}})$.

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arXiv2023

Angular analysis of the low $K^{+}K^{-}$ invariant mass enhancement in $B^{+}\to K^{+}K^{-}π^{+}$ decays

We study the decay $B^{+} \to K^+ K^- π^+$ and investigate the angular distribution of $K^{+}K^{-}$ pairs with invariant mass below $1.1$ GeV/$c^2$. This region exhibits both a strong enhancement in signal and very large direct $CP$ violation. We construct a coherent sum model for the angular distribution of $S$- and $P$-wave, and report the ratio of their amplitudes, the relative phase and the forward-backward asymmetry. We also report absolute differential branching fractions and direct $CP$ asymmetry for the decay in bins of $M_{K^+K^-}$ and the differential branching fractions in bins of $M_{K^+π^-}$. The results are based on a data sample that contains $772\times10^6$ $B \bar{B}$ pairs collected at the $Υ(4S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^+ e^-$ collider. The measured overall branching fraction and the direct $CP$ asymmetry are $(5.38\pm0.40\pm0.35)\times 10^{-6}$ and $-0.170\pm0.073\pm0.017$, respectively, where the first uncertainties are statistical and the second are systematic.

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arXiv2022

An improved search for the electric dipole moment of the $τ$ lepton

We report a measurement of the electric dipole moment of the $τ$ lepton ($d_τ$) using an 833~fb$^{-1}$ data sample collected near the $Υ(4S)$ resonance, with the Belle detector at the KEKB asymmetric-energy $e^+ e^-$ collider. Using an optimal observable method, we obtain the real and imaginary parts of $d_τ$ as ${\rm Re}(d_τ) = ( -0.62 \pm 0.63 ) \times 10^{-17} ~e{\rm cm}$ and ${\rm Im}(d_τ) = ( -0.40 \pm 0.32 ) \times 10^{-17} ~e{\rm cm}$, respectively. These results are consistent with null electric dipole moment at the present level of experimental sensitivity and improve the sensitivity by about a factor of three.

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arXiv2022

The study of $γγ\toγψ(2S)$ at Belle

Using $980~\rm fb^{-1}$ of data on and around the $Υ(nS)(n=1,2,3,4,5)$ resonances collected with the Belle detector at the KEKB asymmetric-energy $e^+e^-$ collider, the two-photon process $γγ\to γψ(2S)$ is studied from the threshold to $4.2~{\rm GeV}$ for the first time. Two structures are seen in the invariant mass distribution of $γψ(2S)$: one at $M_{R_1} = 3922.4\pm 6.5 \pm 2.0~{\rm MeV}/c^2$ with a width of $Γ_{R_1} = 22\pm 17\pm 4~{\rm MeV}$, and another at $M_{R_2} = 4014.3\pm 4.0 \pm 1.5~{\rm MeV}/c^2$ with a width of $Γ_{R_2} = 4\pm 11 \pm 6~{\rm MeV}$; the signals are parametrized with the incoherent sum of two Breit-Wigner functions. The first structure is consistent with the $X(3915)$ or the $χ_{c2}(3930)$, and the local statistical significance is determined to be $3.1σ$ with the systematic uncertainties included. The second matches none of the known charmonium or charmoniumlike states, and its global significance is determined to be $2.8σ$ including the look-elsewhere effect. The production rates are $Γ_{γγ}{\cal B}(R_1\toγψ(2S)) = 9.8\pm 3.6\pm 1.2~{\rm eV}$ assuming $(J^{PC}, |λ|) =(0^{++}, 0)$ or $2.0\pm 0.7\pm 0.2~{\rm eV}$ with $(2^{++}, 2)$ for the first structure and $Γ_{γγ}{\cal B}(R_2\toγψ(2S)) = 6.2\pm 2.2\pm 0.8~{\rm eV}$ with $(0^{++}, 0)$ or $1.2\pm 0.4\pm 0.2~{\rm eV}$ with $(2^{++}, 2)$ for the second one. Here, the first errors are statistical and the second systematic, and $λ$ is the helicity.

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arXiv2021

Measurement of Differential Branching Fractions of Inclusive ${B \to X_u \, \ell^+\, ν_{\ell}}$ Decays

The first measurements of differential branching fractions of inclusive semileptonic ${B \to X_u \, \ell^+\, ν_{\ell}}$ decays are performed using the full Belle data set of 711 fb$^{-1}$ of integrated luminosity at the $Υ(4S)$ resonance and for $\ell = e, μ$. Differential branching fractions are reported as a function of the lepton momentum, the four-momentum-transfer squared, light-cone momenta, the hadronic mass, and the hadronic mass squared. They are obtained by subtracting the backgrounds from semileptonic ${B \to X_c \, \ell^+\, ν_{\ell}}$ decays and other processes, and corrected for resolution and acceptance effects. The measured distributions are compared to predictions from inclusive and hybrid ${B \to X_u \, \ell^+\, ν_{\ell}}$ calculations.

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arXiv2021

Measurement of the Masses and Widths of the $Σ_c(2455)^+$ and $Σ_c(2520)^+$ Baryons

Using 980 ${\rm fb}^{-1}$ of data {collected} with the Belle detector operating at the KEKB asymmetric-energy $e^+e^-$ collider, we report the measurements of the masses, and the first measurements of the instrinsic widths, of the $Σ_c(2455)^+$ and $Σ_c(2520)^+$ charmed baryons. We find $M(Σ_c(2455)^+)-M(Λ_c^+) = 166.17\pm 0.05^{+0.16}_{-0.07}\ {\rm MeV}/c^2$, $Γ(Σ_c(2455)^+) = 2.3 \pm 0.3 \pm 0.3\ {\rm MeV/c^2}$, $M(Σ_c(2520)^+)-M(Λ_c^+) = 230.9 \pm 0.5 ^{+0.5}_{-0.1}\ {\rm MeV}/c^2$, and $Γ(Σ_c(2520)^+) = 17.2^{+2.3\ +3.1}_{-2.1\ -0.7}\ {\rm MeV}/c^2$, where the uncertainties are statistical and systematic, respectively. These measurements can be used to test models of the underlying quark structure of the $Σ_c$ states.

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arXiv2021

Search for $B_{s}^{0} \rightarrow η^{\prime} X_{s\bar{s}}$ at Belle using a semi-inclusive method

We report the first search for the penguin-dominated process $B_{s}^{0} \rightarrow η^{\prime} X_{s\bar{s}}$ using a semi-inclusive method. A 121.4 $\mathrm{fb}^{-1}$ integrated luminosity $Υ(5S)$ data set collected by the Belle experiment, at the KEKB asymmetric-energy $e^+e^-$ collider, is used. We observe no statistically significant signal and including all uncertainties, we set a 90\% confidence level upper limit on the partial branching fraction at 1.4 $\times$ 10$^{-3}$ for $M(X_{s\bar{s}})$ $\leq$ 2.4 GeV/$c^{2}$.

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arXiv2021

Measurement of the Resonant and Non-Resonant Branching Ratios in $Ξ_{c}^{0} \rightarrow Ξ^{0} K^+ K^-$

Using the entire data sample of $980$ $fb^{-1}$ integrated luminosity collected with the Belle detector at the KEKB asymmetric-energy $e^{+}e^{-}$ collider, we present an amplitude analysis measuring the branching fractions of the Cabibbo-allowed, $W$-exchange resonant decay $Ξ_{c}^{0} \rightarrow Ξ^{0} ϕ(\to K^+ K^-)$ with a polarized $ϕ$ and the non-resonant decay via a direct process $Ξ_{c}^{0} \rightarrow Ξ^{0} K^+ K^-$. We present these measurements, relative to the normalization mode $Ξ^{-}π^{+}$, and find branching ratios $\frac{\mathcal{B}(Ξ_{c}^{0} \rightarrow Ξ^{0} ϕ(\rightarrow K^{+}K^{-}))}{\mathcal{B}(Ξ_{c}^{0} \rightarrow Ξ^{-} π^{+})} = 0.036 \pm 0.004 (stat.) \pm 0.002 (syst.)$ and $\frac{\mathcal{B}(Ξ_{c}^{0} \rightarrow Ξ^{0} K^{+} K^{-})}{\mathcal{B}(Ξ_{c}^{0} \rightarrow Ξ^{-} π^{+})} = 0.039 \pm 0.004 (stat.) \pm 0.002 (syst.)$ which suggest that only minor cusping peaks occur in the combinatorial background of $Ω^{*-} \to Ξ^{0}K^{-}$ due to these $Ξ_{c}^{0}$ decays.

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arXiv2021

Measurement of the Branching Fraction of the Decay $\boldsymbol{B^{+}\toπ^{+}π^{-}\ell^{+}ν_\ell}$ in Fully Reconstructed Events at Belle

We present an analysis of the exclusive $B^{+}\toπ^{+}π^{-}\ell^{+}ν_{\ell}$ decay, where $\ell$ represents an electron or a muon, with the assumption of charge-conjugation symmetry and lepton universality. The analysis uses the full $Υ(4S)$ data sample collected by the Belle detector, corresponding to 711 fb$^{-1}$ of integrated luminosity. We select the events by fully reconstructing one $B$ meson in hadronic decay modes, subsequently determining the properties of the other $B$ meson. We extract the signal yields using a binned maximum-likelihood fit to the missing-mass squared distribution in bins of the invariant mass of the two pions or the momentum transfer squared. We measure a total branching fraction of ${{\cal B}(B^{+}\to π^{+}π^{-}\ell^{+}ν_{\ell})= [22.7 ^{+1.9}_{-1.6} (\mathrm{stat}) \pm 3.5(\mathrm{syst}) ]\times 10^{-5}}$, where the uncertainties are statistical and systematic, respectively. This result is the first reported measurement of this decay.

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arXiv2021

Evidence for $X(3872)\rightarrow J/ψπ^+π^-$ produced in single-tag two-photon interactions

We report the first evidence for $X(3872)$ production in two-photon interactions by tagging either the electron or the position in the final state, exploring the highly virtual photon region. The search is performed in $e^+e^- \rightarrow e^+e^-J/ψπ^+π^-$, using 825 fb$^{-1}$ of data collected by the Belle detector operated at the KEKB $e^+e^-$ collider. We observe three $X(3872)$ candidates with an expected background of $0.11\pm 0.10$ events, with a significance of 3.2$σ$. We obtain an estimated value for $\tildeΓ_{γγ}{\cal B}(X(3872)\rightarrow J/ψπ^+π^-$) assuming the $Q^2$ dependence predicted by a $c\bar{c}$ meson model, where $-Q^2$ is the invariant mass-squared of the virtual photon. No $X(3915)\rightarrow J/ψπ^+π^-$ candidates are found.

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arXiv2021

Measurement of Branching Fraction and Search for $CP$ Violation in $B\to ϕϕK$

We report the measurement of branching fractions and $CP$-violation asymmetries in $B\to ϕϕK$ decays based on a $711\,{\rm fb}^{-1}$ data sample containing $772\times 10^6$ $B\bar{B}$ events. The data were recorded at the $Υ(4S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^+ e^-$ collider. For $B^+ \to ϕϕK^+$, the branching fraction and $CP$-violation asymmetry measured below the $η_{c}$ threshold ($m_{ϕϕ}<2.85\,{\rm GeV}/c^2$) are $[3.43^{\,+\,0.48}_{\,-\,0.46}({\rm stat})\pm 0.22({\rm syst})] \times10^{-6}$ and $-0.02\pm0.11({\rm stat})\pm0.01({\rm syst})$, respectively. Similarly, the branching fraction obtained for $B^0 \toϕϕK^0$ below the $η_{c}$ threshold is $[3.02^{\,+\,0.75}_{\,-\,0.66} ({\rm stat})\pm \,0.20({\rm syst})]\times10^{-6}$. We also measure the $CP$-violation asymmetry for $B^+ \toϕϕK^+$ within the $η_{c}$ region ($m_{ϕϕ}\in [2.94,3.02]\,{\rm GeV}/c^2$) to be $+0.12\pm0.12({\rm stat})\pm0.01({\rm syst})$.

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arXiv2020

Search for a doubly-charged $DDK$ bound state in $Υ(1S,2S)$ inclusive decays and via direct production in $e^+e^-$ collisions at $\sqrt{s}$ = 10.520, 10.580, and 10.867 GeV

We report the results of a first search for a doubly-charged $DDK$ bound state, denoted the $R^{++}$, in $Υ(1S)$ and $Υ(2S)$ inclusive decays and via direct production in $e^+e^-$ collisions at $\sqrt{s}$ = 10.520, 10.580, and 10.867 GeV. The search uses data accumulated with the Belle detector at the KEKB asymmetric-energy $e^+e^-$ collider. No significant signals are observed in the $D^{+}D_{s}^{*+}$ invariant-mass spectra of all studied modes. The 90\% credibility level upper limits on their product branching fractions in $Υ(1S)$ and $Υ(2S)$ inclusive decays (${\cal B}(Υ(1S,2S) \to R^{++} + anything) \times {\cal B}(R^{++} \to D^{+} D_{s}^{*+})$), and the product values of Born cross section and branching fraction in $e^+e^-$ collisions ($σ(e^+e^- \to R^{++} + anything) \times {\cal B}(R^{++} \to D^{+} D_{s}^{*+})$) at $\sqrt{s}$ = 10.520, 10.580, and 10.867 GeV under different assumptions of $R^{++}$ masses varying from 4.13 to 4.17 GeV/$c^2$, and widths varying from 0 to 5 MeV are obtained.

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arXiv2020

Measurement of time-dependent CP violation parameters in $B^0 \to K_0^SK_0^SK_0^S$ decays at Belle

We measure the time-dependent $C\!P$ violation parameters in $B^{0} \to K^{0}_{S}K^{0}_{S}K^{0}_{S}$ decays using $772 \times 10^6$ $B {\mkern 1.5mu\overline{\mkern-1.5mu B \mkern-0.1mu}\mkern 0.1mu}$ pairs collected at the $Υ(4S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^+ e^-$ collider. The obtained mixing-induced and direct $C\!P$ asymmetries are $-0.71 \pm 0.23~{\rm(stat)} \pm 0.05~{\rm(syst)}$ and $0.12 \pm 0.16~{\rm(stat)} \pm 0.05~{\rm(syst)}$, respectively. These values are consistent with the Standard Model predictions. The significance of $C\!P$ violation differs from zero by 2.5 standard deviations.

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arXiv2020

Study of Electromagnetic Decays of Orbitally Excited $Ξ_c$ Baryons

Using 980 $fb^-1$ of data collected with the Belle detector operating at the KEKB asymmetric-energy e^+e^- collider, we report a study of the electromagnetic decays of excited {charmed baryons} $Ξ_c(2790)$ and $Ξ_c(2815)$. A clear signal (8.6 standard deviations) is observed for $Ξ_c(2815)^0 \to Ξ_c^0γ$, and we measure: $B[Ξ_c(2815)^0 \to Ξ_c^0γ]/B[Ξ_c(2815)^0 \to Ξ_c(2645)^+π^- \to Ξ_c^0π^+π^-] = 0.41 \pm 0.05 \pm 0.03$. We also present evidence (3.8 standard deviations) for the similar decay of the $Ξ_c(2790)^0$ and measure: $B[Ξ_c(2790)^{0}\toΞ_c^{0}γ]/B[Ξ_c(2790)^0\toΞ_c^{\prime +}π^{-}\toΞ_c^{+}γπ^-] = 0.13 \pm 0.03 \pm 0.02$. The first quoted uncertainties are statistical and the second systematic. We find no hint of the analogous decays of the $Ξ_c(2815)^+$ and $Ξ_c(2790)^+$ baryons and set upper limits at the 90% confidence level of: $B[Ξ_c(2815)^{+}\toΞ_c^{+}γ]/B[Ξ_c(2815)^+\toΞ_c(2645)^0π^+\toΞ_c^+π^-π^+] < 0.09,$ and $B[Ξ_c(2790)^{+}\toΞ_c^{+}γ]/B[Ξ_c(2790)^+\toΞ_c^{\prime 0}π^{+}\toΞ_c^{0}γπ^+] < 0.06.$ Approximate values of the partial widths of the decays are extracted, which can be used to discriminate between models of the underlying quark structure of these excited states.

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arXiv2020

Test of lepton-flavor universality in ${B\to K^\ast\ell^+\ell^-}$ decays at Belle

We present a measurement of $R_{K^{\ast}}$, the branching fraction ratio ${{\cal B}(B\to K^\ast μ^+ μ^-)}$/ ${{\cal B}(B\to K^\ast e^+ e^-)}$, for both charged and neutral $B$ mesons. The ratio for the charged case, $R_{K{^{\ast +}}}$, is the first measurement ever performed. In addition, we report absolute branching fractions for the individual modes in bins of the squared dilepton invariant mass, $q^2$. The analysis is based on a data sample of $711~\mathrm{fb}^{-1}$, containing $772\times 10^{6}$ $B\bar B$ events, recorded at the $Υ(4S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^+e^-$ collider. The obtained results are consistent with Standard Model expectations.

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arXiv2020

Search for transitions from $Υ(4S)$ and $Υ(5S)$ to $η_b(1S)$ and $η_b(2S)$ with emission of an $ω$ meson

Using data collected in the Belle experiment at the KEKB asymmetric-energy $e^+e^-$ collider we search for transitions $Υ(4S) \rightarrow η_b(1S)ω$, $Υ(5S) \rightarrow η_b(1S)ω$ and $Υ(5S) \rightarrow η_b(2S)ω$. No significant signals are observed and we set 90\% confidence level upper limits on the corresponding visible cross sections: $0.2 ~\textrm{pb}, 0.4 ~\textrm{pb}$ and $1.9 ~\textrm{pb}$, respectively.

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arXiv2020

Measurement of Branching Fractions of $Λ_{c}^{+} \rightarrow ηΛπ^{+}$, $ηΣ^{0} π^{+}$, $Λ(1670) π^{+}$, and $ηΣ(1385)^{+}$

We report branching fraction measurements of four decay modes of the $Λ_{c}^{+}$ baryon, each of which includes an $η$ meson and a $Λ$ baryon in the final state, and all of which are measured relative to the $Λ_{c}^{+} \rightarrow p K^{-} pi^{+}$ decay mode. The results are based on a $980~\mathrm{fb^{-1}}$ data sample collected by the Belle detector at the KEKB asymmetric-energy $e^{+}e^{-}$ collider. Two decays, $ηΣ^{0} π^{+}$ and $Λ(1670) π^{+}$, are observed for the first time, while the measurements of the other decay modes, $Λ_{c}^{+} \rightarrow ηΛπ^{+}$ and $ηΣ(1385)^{+}$, are more precise than those made previously. We obtain $\mathcal{B}(Λ_{c}^{+} \rightarrow ηΛπ^{+})/\mathcal{B}(Λ_{c}^{+} \rightarrow p K^{-} π^{+})$ = $0.293 \pm 0.003 \pm 0.014$, $\mathcal{B}(Λ_{c}^{+} \rightarrow ηΣ^{0} π^{+})/\mathcal{B}(Λ_{c}^{+} \rightarrow p K^{-} π^{+})$ = $0.120 \pm 0.006 \pm 0.006$, $\mathcal{B}(Λ_{c}^{+} \rightarrow Λ(1670) π^{+}) \times \mathcal{B}(Λ(1670) \rightarrow ηΛ)/\mathcal{B}(Λ_{c}^{+} \rightarrow p K^{-} π^{+})$ = $(5.54 \pm 0.29 \pm 0.73 ) \times 10^{-2}$, and $\mathcal{B}(Λ_{c}^{+} \rightarrow ηΣ(1385)^{+})/\mathcal{B}(Λ_{c}^{+} \rightarrow p K^{-} π^{+})$ = $0.192 \pm 0.006 \pm 0.016$. The mass and width of the $Λ(1670)$ are also precisely determined to be $1674.3 \pm 0.8 \pm 4.9~{\rm MeV}/c^{2}$ and $36.1 \pm 2.4 \pm 4.8~{\rm MeV}$, respectively, where the uncertainties are statistical and systematic, respectively.

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arXiv2020

Dalitz analysis of $D^{0}\to K^{-}π^{+}η$ decays at Belle

We present the results of the first Dalitz plot analysis of the decay $D^{0}\to K^{-}π^{+}η$. The analysis is performed on a data set corresponding to an integrated luminosity of 953 $\rm{fb}^{-1}$ collected by the Belle detector at the asymmetric-energy $e^{+}e^{-}$ KEKB collider. The Dalitz plot is well described by a combination of the six resonant decay channels $\bar{K}^{*}(892)^0η$, $K^{-}a_0(980)^+$, $K^{-}a_2(1320)^+$, $\bar{K}^{*}(1410)^0η$, $K^{*}(1680)^-π^{+}$ and $K_2^{*}(1980)^-π^{+}$, together with $Kπ$ and $Kη$ S-wave components. The decays $K^{*}(1680)^{-}\to K^{-}η$ and $K_{2}^{*}(1980)^{-}\to K^{-}η$ are observed for the first time. We measure ratio of the branching fractions, $\frac{\mathcal{B}(D^{0}\to K^{-}π^{+}η)}{\mathcal{B}(D^{0}\to K^{-}π^{+})}=0.500\pm0.002{\rm(stat)}\pm0.020{\rm(syst)}\pm0.003{\rm (\mathcal{B}_{PDG})}$. Using the Dalitz fit result, the ratio $\frac{\mathcal{B}(K^{*}(1680)\to Kη)}{\mathcal{B}(K^{*}(1680)\to Kπ)}$ is measured to be $0.11\pm0.02{\rm(stat)}^{+0.06}_{-0.04}{\rm(syst)}\pm0.04{\rm(\mathcal{B}_{\text{PDG}})}$; this is much lower than the theoretical expectations ($\approx1$) made under the assumption that $K^{*}(1680)$ is a pure $1^{3}D_1$ state. The product branching fraction $\mathcal{B}(D^0\to [K_2^{*}(1980)^-\to K^{-}η]π^{+})=(2.2^{+1.7}_{-1.9})\times10^{-4}$ is determined. In addition, the $πη^{\prime}$ contribution to the $a_0(980)^{\pm}$ resonance shape is confirmed with 10.1$σ$ statistical significance using the three-channel Flatté model. We also measure $\mathcal{B}(D^0\to\bar{K}^{*}(892)^0η)=(1.41^{+0.13}_{-0.12})\%$. This is consistent with, and more precise than, the current world average $(1.02\pm0.30)\%$, deviates with a significance of more than $3σ$ from the theoretical predictions of (0.51-0.92)%.

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