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D. Epifanov

Publications and source records attributed to D. Epifanov.

At least 91 records · Page 5Linked to original sources
arXiv2023

Search for a dark photon and an invisible dark Higgs boson in $μ^+μ^-$ and missing energy final states with the Belle II experiment

The dark photon $A^\prime$ and the dark Higgs boson $h^\prime$ are hypothetical particles predicted in many dark sector models. We search for the simultaneous production of $A^\prime$ and $h^\prime$ in the dark Higgsstrahlung process $e^{+}e^{-}\rightarrow A^\prime \, h^\prime$ with $A^\prime \rightarrow μ^+μ^-$ and $ h^\prime$ invisible in electron-positron collisions at a center-of-mass energy of 10.58 GeV collected by the Belle II experiment in 2019. With an integrated luminosity of 8.34 fb$^{-1}$, we observe no evidence for signal. We obtain exclusion limits at 90% Bayesian credibility in the range of 1.7--5.0 fb on the cross section and in the range of $1.7 \times10^{-8}$--$200 \times10^{-8}$ on the effective coupling $\varepsilon^2 \times α_D$ for the $A^\prime$ mass in the range of 4.0 GeV/$c^2$ $< M_{A^\prime}< 9.7$ GeV/$c^2$ and for the $h^\prime$ mass $M_{h^\prime} < M_{A^\prime}$, where $\varepsilon$ is the mixing strength between the standard model and the dark photon and $α_D$ is the coupling of the dark photon to the dark Higgs boson. Our limits are the first in this mass range.

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arXiv2023

Study of the lineshape of $X(3872)$ using $B$ decays to $D^0\overline{D}{}^{*0}K$

We present a study of the $X(3872)$ lineshape in the decay $B \to X(3872)K\to D^0\overline{D}{}^{*0}K$ using a data sample of $772\times 10^6$ $B\overline{B}$ pairs collected at the $Υ(4S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^+e^-$ collider. The peak near the threshold in the $D^0\overline{D}{}^{*0}$ invariant mass spectrum is fitted using a relativistic Breit-Wigner lineshape. We determine the mass and width parameters to be $m = 3873.71 ^{+0.56}_{-0.50} ({\rm stat}) \pm0.13 ({\rm syst}) ~{\rm MeV}/c^2$ and $Γ_0 = 5.2 ^{+2.2}_{-1.5} ({\rm stat}) \pm 0.4 ({\rm syst})~{\rm MeV}$, respectively. The branching fraction is found to be ${\cal B} (B^+\to X(3872)K^+) \times {\cal{B}}(X(3872) \to D^0\overline{D}{}^{*0}) = (0.97 ^{+0.21}_{-0.18} ({\rm stat}) \pm 0.10 ({\rm syst})) \times 10^{-4}$. The signal from $B^0$ decays is observed for the first time with $5.2σ$ significance, and the ratio of branching fractions between charged and neutral $B$ decays is measured to be ${\cal B}(B^0\to X(3872)K^0)/{\cal B}(B^+ \to X(3872)K^+) = 1.34^{+0.47}_{-0.40} ({\rm stat}) ^{+0.10}_{-0.12} ({\rm syst})$. The peak is also studied using a Flatté lineshape. We determine the lower limit on the $D\overline{D}{}^{*}$ coupling constant $g$ to be $0.075$ at 95% credibility in the parameter region where the ratio of $g$ to the mass difference from the $D^0\overline{D}{}^{*0}$ threshold is equal to $-15.11~{\rm GeV}^{-1}$, as measured by LHCb.

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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

Search for $C\!P$ violation using $T$-odd correlations in $D_{(s)}^{+}\to K^{+} K^{-}π^{+}π^{0}$, $D_{(s)}^{+}\to K^{+} π^{-}π^{+}π^{0}$, and $D^{+}\to K^{-}π^{+}π^{+}π^{0}$ decays

We search for $C\!P$ violation using $T$-odd correlations in five $D_{(s)}^{+}$ and $D_{(s)}^{-}$ four-body decays. Our analysis is based on 980 $\rm fb^{-1}$ of data collected by the Belle detector at the KEKB energy-asymmetric $e^+e^-$ collider. Our results for the $T$-odd $C\!P$-violating parameter $a^{T\text{-odd}}_{C\!P}$ are: $a^{T\text{-odd}}_{C\!P}({D^{+}\to K^{-}K^{+}π^{+}π^{0}}) = (+2.6\pm 6.6\pm 1.3 )\times10^{-3}$, $a^{T\text{-odd}}_{C\!P}({D^{+}\to K^{+}π^{-}π^{+}π^{0}}) = (-1.3\pm 4.2\pm 0.1 )\times10^{-2}$, $a^{T\text{-odd}}_{C\!P}({D^{+}\to K^{-}π^{+}π^{+}π^{0}}) = (+0.2\pm 1.5\pm 0.8 )\times10^{-3}$, $a^{T\text{-odd}}_{C\!P}({D_s^{+}\to K^{+}π^{-}π^{+}π^{0}}) = (-1.1\pm 2.2\pm 0.1 )\times10^{-2}$, and $a^{T\text{-odd}}_{C\!P}({D_s^{+}\to K^{-}K^{+}π^{+}π^{0}}) = (+2.2\pm 3.3\pm 4.3 )\times10^{-3}$, where the uncertainties are statistical and systematic, respectively. These results are the first such measurements and are all consistent with zero. They include the first measurement for a $D^+_s$ singly Cabibbo-suppressed decay, and the first measurement for a $D$ meson doubly Cabibbo-suppressed decay. We also measure $a^{T\text{-odd}}_{C\!P}$ in different subregions of phase space, where the decays are dominated by different intermediate resonance states such as $D^+\toϕρ^+$, $\bar{K}^{*0}K^{*+}$, and $\bar{K}^{*0}ρ^+$; and $D_s^+\to K^{*+}ρ^{0}$, $K^{*0}ρ^{+}$, $ϕρ^+$, and $\bar{K}^{*0}K^{*+}$. No evidence for $C\!P$ violation is found.

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arXiv2023

Search for Lepton-Flavor-Violating $τ$ Decays to a Lepton and an Invisible Boson at Belle II

We search for lepton-flavor-violating $τ^-\to e^-α$ and $τ^-\toμ^-α$ decays, where $α$ is an invisible spin-0 boson. The search uses electron-positron collisions at $10.58$ GeV center-of-mass energy with an integrated luminosity of $62.8$ fb$^{-1}$, produced by the SuperKEKB collider and collected with the Belle II detector. We search for an excess in the lepton-energy spectrum of the known $τ^-\to e^-\barν_e ν_τ$ and $τ^-\to μ^-\barν_μν_τ$ decays. We report 95\% confidence-level upper limits on the branching-fraction ratio ${\mathcal B}(τ^-\to e^- α) / {\mathcal B}(τ^- \to e^- \barν_e ν_τ)$ in the range $(1.1-9.7) \times 10^{-3}$ and on ${\mathcal B}(τ^-\to μ^- α) / {\mathcal B}(τ^- \to μ^- \barν_μν_τ)$ in the range $(0.7-12.2) \times 10^{-3}$ for $α$ masses between 0 and 1.6 GeV/c$^2$. These results provide the most stringent bounds on invisible boson production from $τ$ decays.

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arXiv2023

Search for lepton-flavor-violating $τ$ decays into a lepton and a vector meson using the full Belle data sample

Charged-lepton-flavor-violation is predicted in several new physics scenarios. We update the analysis of $τ$ lepton decays into a light charged lepton ($\ell$ = $e^{\pm}$ or $μ^{\pm}$) and a vector meson ($V^0$ = $ρ^0$, $ϕ$, $ω$, $K^{\ast0}$, or $\overline{K}{}^{\ast0}$) using 980 fb$^{-1}$ of data collected with the Belle detector at the KEKB collider. No significant excess of such signal events is observed, and thus 90% credibility level upper limits are set on the $τ\rightarrow \ell V^0$ branching fractions in the range of (1.7--$4.3) \times 10^{-8}$. These limits are improved by 30% on average from the previous results.

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arXiv2023

First Observation of $Λπ^+$ and $Λπ^-$ Signals near the $\bar{K}N (I=1)$ Mass Threshold in $Λ_c^+\rightarrowΛπ^+π^+π^-$ Decay

Using the data sample of 980 fb$^{-1}$ collected with the Belle detector operating at the KEKB asymmetric-energy $e^+e^-$ collider, we present the results of an investigation of the $Λπ^+$ and $Λπ^-$ invariant mass distributions looking for substructure in the decay $Λ_c^+\rightarrowΛπ^+π^+π^-$. We find a significant signal in each mass dis\ tribution. When interpreted as resonances, we find for the $Λπ^+$ ($Λπ^-$) combination a mass of $1434.3 \pm 0.6 (\mathrm{stat}) \pm 0.9(\mathrm{syst})$ MeV/$c^2$ [$1438.5 \pm 0.9 (\mathrm{stat}) \pm 2.5(\mathrm{syst})$ MeV/$c^2$], an intrinsic width of $11.5 \pm 2.8 (\mathrm{stat}) \pm 5.3(\mathrm{syst})$ MeV/$c^2$ [$33.0 \pm 7.5 (\mathrm{stat}) \pm 23.6(\mathrm{syst})$ MeV/$c^2$] with a significance of 7.5$σ$ (6.2$σ$). As these two signals are very close to the $\bar{K}N$ threshold, we also investigate the possibility of a $\bar{K}N$ cusp, and find that \ we cannot discriminate between these two interpretations due to the limited size of the data sample.

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arXiv2023

Study of $e^+e^- \rightarrow Σ^0 \overlineΣ{}^0$ and $Σ^+\overlineΣ{}^- $ by Initial State Radiation Method at Belle

The processes $ e^+e^-\rightarrow Σ^0\overlineΣ{}^0 $ and $ e^+e^-\rightarrowΣ^+\overlineΣ{}^-$ are studied using initial-state-radiation events in a sample of 980 $\,\mbox{fb}^{-1}$ collected with the Belle detector at the KEKB asymmetric-energy $ e^+e^- $ collider. The cross sections from the mass threshold to $ 3{\mathrm{\,Ge\kern -0.1em V\!/}c^2} $ and the effective form factors of $ Σ^0 $ and $ Σ^+ $ are measured. In the charmonium region, we observe the decays $J/ψ\rightarrowΣ^0\overlineΣ{}^0$ and $J/ψ\rightarrowΣ^+\overlineΣ{}^-$ and determine the respective branching fractions.

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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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arXiv2023

Measurement of the $Ω_c^0$ lifetime at Belle II

We report on a measurement of the $Ω_c^0$ lifetime using $Ω_c^0 \to Ω^-π^+$ decays reconstructed in $e^+e^-\to c\bar{c}$ data collected by the Belle II experiment and corresponding to $207~{\rm fb^{-1}}$ of integrated luminosity. The result, $\rmτ(Ω_c^0)=243\pm48( stat)\pm11(syst)~fs$, agrees with recent measurements indicating that the $Ω_c^0$ is not the shortest-lived weakly decaying charmed baryon.

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arXiv2023

Measurement of the $B^+/B^0$ production ratio in $e^+e^-$ collisions at the $Υ(4S)$ resonance using $B \rightarrow J/ψ(\ell\ell) K$ decays at Belle

We measure the ratio of branching fractions for the $Υ(4S)$ decays to $B^+B^-$ and $B^0\bar{B}{}^0$ using $B^+ \rightarrow J/ψ(\ell\ell) K^+$ and $B^0 \rightarrow J/ψ(\ell\ell) K^0$ samples, where $J/ψ(\ell\ell)$ stands for $J/ψ\to \ell^+\ell^-$ ($\ell = e$ or $μ$), with $711$ fb$^{-1}$ of data collected at the $Υ(4S)$ resonance with the Belle detector. We find the decay rate ratio of $Υ(4S) \rightarrow B^+B^-$ over $Υ(4S) \rightarrow B^0\bar{B}{}^0$ to be $1.065\pm0.012\pm 0.019 \pm 0.047$, which is the most precise measurement to date. The first and second uncertainties are statistical and systematic, respectively, and the third uncertainty is systematic due to the assumption of isospin symmetry in $B \to J/ψ(\ell\ell) K$.

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arXiv2023

Measurement of the $B^0$ lifetime and flavor-oscillation frequency using hadronic decays reconstructed in 2019-2021 Belle II data

We measure the $B^0$ lifetime and flavor-oscillation frequency using $B^0\to D^{(*)-}π^+$ decays collected by the Belle II experiment in asymmetric-energy $e^+e^-$ collisions produced by the SuperKEKB collider operating at the $Υ(4S)$ resonance. We fit the decay-time distribution of signal decays, where the initial flavor is determined by identifying the flavor of the other $B$ meson in the event. The results, based on $33000$ signal decays reconstructed in a data sample corresponding to ${190}\text{fb}^{-1}$, are $τ_{B^0} = {1.499}\pm{0.013}\pm{0.008}\;\text{ps}$, $Δm_d = {0.516}\pm{0.008}\pm{0.005}\;\text{ps}^{-1}$, where the first uncertainties are statistical and the second are systematic. These results are consistent with the world-average values.

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arXiv2023

Two-particle angular correlations in $e^+ e^-$ collisions to hadronic final states in two reference coordinates at Belle

We present the analysis of two-particle angular correlations using coordinate systems defined with the conventional beam axis and the event thrust axis, and propose the latter to be a useful representation for the correlation structure interpretation in the $e^+ e^-$ collision system. The $e^+ e^-$ collisions to hadronic final states at center-of-mass energies of $\sqrt{s} = 10.52$ GeV and $10.58$ GeV are recorded by the Belle detector at KEKB. In this paper, results on the first dataset are supplementary to the previous Belle publication arXiv:2201.01694 while the latter one is the first two-particle correlation measurement at a collision energy on the $Υ(4S)$ resonance and sensitive to its decay products. Measurements are reported as a function of the charged-particle multiplicity. Finally, a qualitative understanding of the correlation structure is discussed using a combination of Monte Carlo simulations and experimental data.

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arXiv2023

Search for $C\!P$ violation and measurement of branching fractions and decay asymmetry parameters for $Λ_c^+\toΛh^+$ and $Λ_c^+\toΣ^{0} h^+$ ($h\!=\!K,\,π$)

We report a study of $Λ_c^+\toΛh^+$ and $Λ_c^+\toΣ^{0} h^+$ ($h\!=\!K,\,π$) decays based on a data sample of 980~${\rm fb}^{-1}$ collected with the Belle detector at the KEKB energy-asymmetric $e^+e^-$ collider. The first results of direct $C\!P$ asymmetry in two-body singly Cabibbo-suppressed (SCS) decays of charmed baryons are measured, $A_{C\!P}^{\rm{dir}}(Λ_c^+\toΛK^+)\!=\!+0.021\pm0.026\pm0.001$ and $A_{C\!P}^{\rm{dir}}(Λ_c^+\toΣ^0K^+)\!=\!+0.025\pm0.054\pm0.004$. We also make the most precise measurement of the decay asymmetry parameters ($α$) for the four modes of interest and search for $C\!P$ violation via the $α$-induced $C\!P$ asymmetry ($A_{C\!P}^α$). We measure $A_{C\!P}^α(Λ_c^+\toΛK^+)\!=\!{-0.023\pm0.086\pm0.071}$ and $A_{C\!P}^α(Λ_c^+\toΣ^0K^+)\!=\!{+0.08\pm 0.35\pm 0.14}$, which are the first $A_{C\!P}^α$ results for SCS decays of charmed baryons. We search for $Λ$-hyperon $C\!P$ violation in $Λ_c^+\to(Λ,\,Σ^0)π^+$ and find $A_{C\!P}^α(Λ\to pπ^{-})\!=\!{+0.013\pm0.007\pm0.011}$. This is the first time that hyperon $C\!P$ violation has been measured via Cabibbo-favored charm decays. No evidence of baryon $C\!P$ violation is found. We also obtain the most precise branching fractions for two SCS $Λ_c^+$ decays, $\mathcal{B}(Λ_c^+\toΛK^+)\!=\!(6.57\pm0.17\pm0.11\pm0.35)\times10^{-4}$ and $\mathcal{B}(Λ_c^+\toΣ^0K^+)\!=\!(3.58\pm0.19\pm0.06\pm0.19)\times10^{-4}$. The first uncertainties are statistical and the second systematic, while the third uncertainties come from the uncertainties on the world average branching fractions of $Λ_c^+\to(Λ,\,Σ^0)π^+$.

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arXiv2023

Observation of $e^+e^-\toωχ_{bJ}(1P)$ and search for $X_b \to ωΥ(1S)$ at $\sqrt{s}$ near 10.75 GeV

We study the processes $e^+e^-\toωχ_{bJ}(1P)$ ($J$ = 0, 1, or 2) using samples at center-of-mass energies $\sqrt{s}$ = 10.701, 10.745, and 10.805 GeV, corresponding to 1.6, 9.8, and 4.7 fb$^{-1}$ of integrated luminosity, respectively. These data were collected with the Belle II detector during special operations of the SuperKEKB collider above the $Υ(4S)$ resonance. We report the first observation of $ωχ_{bJ}(1P)$ signals at $\sqrt{s}$ = 10.745 GeV. By combining Belle II data with Belle results at $\sqrt{s}$ = 10.867 GeV, we find energy dependencies of the Born cross sections for $e^+e^-\to ωχ_{b1,b2}(1P)$ to be consistent with the shape of the $Υ(10753)$ state. These data indicate that the internal structures of the $Υ(10753)$ and $Υ(10860)$ states may differ. Including data at $\sqrt{s}$ = 10.653 GeV, we also search for the bottomonium equivalent of the $X(3872)$ state decaying into $ωΥ(1S)$. No significant signal is observed for masses between 10.45 and 10.65 GeV/$c^2$.

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arXiv2023

Measurement of two-photon decay width of $χ_{c2}(1P)$ in $ γγ\rightarrow χ_{c2}(1P) \rightarrow J/ψγ$ at Belle

We report the measurement of the two-photon decay width of $χ_{c2}(1P)$ in two-photon processes at the Belle experiment. We analyze the process $ γγ\rightarrow χ_{c2}(1P) \rightarrow J/ψγ$, $J/ψ\rightarrow \ell^{+} \ell^{-}$ $(\ell = e \ {\rm or} \ μ)$ using a data sample of 971 fb$^{-1}$ collected with the Belle detector at the KEKB $e^{+} e^{-}$ collider. In this analysis, the product of the two-photon decay width of $χ_{c2}(1P)$ and the branching fraction is determined to be $Γ_{γγ}(χ_{c2}(1P)) \mathcal{B}( χ_{c2}(1P) \rightarrow J/ψ\, γ)\mathcal{B}( J/ψ\rightarrow \ell^+\ell^- ) = {\rm 14.8}$ $\pm$ ${\rm 0.3}({\rm stat.})$ $\pm$ ${\rm 0.7}({\rm syst.})$ eV, which corresponds to $ Γ_{γγ}(χ_{c2}(1P)) $ = 653 $\pm$ 13(stat.) $\pm$ 31(syst.) $\pm$ 17(B.R.) eV, where the third uncertainty is from $\mathcal{B}( χ_{c2}(1P) \rightarrow J/ψ\, γ)$ and $\mathcal{B}( J/ψ\rightarrow \ell^+\ell^- )$.

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arXiv2023

Measurement of the mass and width of the $Λ_c(2625)^+$ and the branching ratios of $Λ_c(2625)^+ \to Σ_c^{0}π^{+}$ and $Λ_c(2625)^+ \to Σ_c^{++}π^{-}$

Using the entire data sample of $980\,\textrm{fb}^{-1}$ collected with the Belle detector operating at the KEKB asymmetric-energy collider, we report the measurement of the mass, width, and the relative branching ratios of the $Λ_c(2625)^+$ charmed baryon. The mass difference between $Λ_c(2625)^+$ and $Λ_c^+$ is measured to be $M(Λ_c(2625)^{+}) - M(Λ_c^{+}) = 341.518 \pm 0.006 \pm 0.049\ \mathrm{MeV}/\mathit{c}^2$. The upper limit on the width is measured to be $Γ(Λ_c(2625)^+) < 0.52\,\mathrm{MeV}/\textit{c}^2$ at 90\% credibility level. Based on a full Dalitz plot fit, branching ratios with respect to the mode $Λ_c(2625)^+ \to Λ_c^+ π^+ π^-$ are measured to be $\frac{\mathcal{B}(Λ_c(2625)^+ \to Σ_c^{0} π^{+})} {\mathcal{B}(Λ_c(2625)^+ \to Λ_c^+ π^{+} π^{-})} = (5.19 \pm 0.23 \pm 0.40) \%$ and $\frac{\mathcal{B}(Λ_c(2625)^+ \to Σ_c^{++} π^{-})} {\mathcal{B}(Λ_c(2625)^+ \to Λ_c^+ π^{+} π^{-})} = (5.13 \pm 0.26 \pm 0.32) \%$. These measurements can be used to further constrain the parameters of the underlying theoretical models.

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arXiv2023

Search for the decay $B^0_s \rightarrow π^0 π^0$ at Belle

We report the results of the first search for the decay $B_s^0\rightarrowπ^0π^0$ using $121.4\ \rm fb^{-1}$ of data collected at the $Υ(5\rm S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^+e^-$ collider. We observe no signal and set a 90\% confidence level upper limit of $7.7\times 10^{-6}$ on the $B_s^0\rightarrowπ^0π^0$ decay branching fraction.

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