arXiv subjects

L. Cao

Publications and source records attributed to L. Cao.

At least 145 records · Page 8Linked to original sources

arXiv2021

Search for $B^{+}\to K^{+}ν\barν$ decays using an inclusive tagging method at Belle II

A search for the flavor-changing neutral-current decay $B^{+}\to K^{+}ν\barν$ is performed at the Belle II experiment at the SuperKEKB asymmetric energy electron-positron collider. The results are based on a data sample corresponding to an integrated luminosity of $63\,\mbox{fb}^{-1}$ collected at the $Υ{(4S)}$ resonance and a sample of $9\,\mbox{fb}^{-1}$ collected at an energy $60\mathrm{\,Me\kern -0.1em V}$ below the resonance. A novel measurement method is employed, which exploits topological properties of the $B^{+}\to K^{+}ν\barν$ decay that differ from both generic bottom-meson decays and light-quark pair production. This inclusive tagging approach offers a higher signal efficiency compared to previous searches. No significant signal is observed. An upper limit on the branching fraction of $B^{+}\to K^{+}ν\barν$ of $4.1 \times 10^{-5}$ is set at the 90% confidence level.

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arXiv2021

Measurements of the branching fractions for $B \to K^{*}γ$ decays at Belle II

This paper reports a study of $B \to K^{*}γ$ decays using $62.8\pm 0.6$ fb$^{-1}$ of data collected during 2019--2020 by the Belle II experiment at the SuperKEKB $e^{+}e^{-}$ asymmetric-energy collider, corresponding to $(68.2 \pm 0.8) \times 10^6$ $B\overline{B}$ events. We find $454 \pm 28$, $50 \pm 10$, $169 \pm 18$, and $160 \pm 17$ signal events in the decay modes $B^{0} \to K^{*0}[K^{+}π^{-}]γ$, $B^{0} \to K^{*0}[K^0_{\rm S}π^{0}]γ$, $B^{+} \to K^{*+}[K^{+}π^{0}]γ$, and $B^{+} \to K^{*+}[K^{+}π^{0}]γ$, respectively. The uncertainties quoted for the signal yield are statistical only. We report the branching fractions of these decays: $$\mathcal{B} [B^{0} \to K^{*0}[K^{+}π^{-}]γ] = (4.5 \pm 0.3 \pm 0.2) \times 10^{-5}, $$ $$\mathcal{B} [B^{0} \to K^{*0}[K^0_{\rm S}π^{0}]γ] = (4.4 \pm 0.9 \pm 0.6) \times 10^{-5},$$ $$\mathcal{B} [B^{+} \to K^{*+}[K^{+}π^{0}]γ] = (5.0 \pm 0.5 \pm 0.4)\times 10^{-5},\text{ and}$$ $$\mathcal{B} [B^{+} \to K^{*+}[K^0_{\rm S}π^{+}]γ] = (5.4 \pm 0.6 \pm 0.4) \times 10^{-5},$$ where the first uncertainty is statistical, and the second is systematic. The results are consistent with world-average values.

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arXiv2021

Measurement of the branching fraction for $B^{0} \rightarrow π^{0} π^{0}$ decays reconstructed in 2019-2020 Belle II data

We report the first reconstruction of the $B^{0} \to π^{0} π^{0}$ decay mode at Belle II using samples of 2019 and 2020 data that correspond to 62.8 fb$^{-1}$ of integrated luminosity. We find $14.0^{+6.8}_{-5.6}$ signal decays, corresponding to a significance of 3.4 standard deviations and determine a branching ratio of $\mathcal{B}(B^{0} \rightarrow π^{0} π^{0}) = [0.98^{+0.48}_{-0.39} \pm 0.27] \times 10^{-6}$. The results agree with previous determinations and contribute important information to an early assessment of detector performance and Belle II's potential for future determinations of $α/ϕ_2$ using $B \rightarrow ππ$ modes.

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arXiv2021

Measurements of branching fractions and CP-violating charge asymmetries in multibody charmless $B$ decays reconstructed in 2019-2020 Belle II data

We report on measurements of branching fractions ($\mathcal{B}$) and CP-violating charge asymmetries ($\mathcal{A}_{\rm CP}$) of multibody charmless $B$ decays reconstructed by the Belle II experiment at the SuperKEKB electron-positron collider. We use a sample of collisions collected in 2019 and 2020 at the $Υ(4S)$ resonance and corresponding to $62.8$ fb$^{-1}$ of integrated luminosity. We use simulation to determine optimized event selections. The $ΔE$ and $M_{\rm bc}$ distributions of the resulting samples are fit to determine signal yields of approximately 690, 840, and 380 decays for the channels $B^+ \to K^+K^-K^+$, $B^+ \to K^+π^-π^+$, and $B^0 \to K^+π^-π^0$, respectively. These yields are corrected for efficiencies determined from simulation and control data samples to obtain $\mathcal{B}(B^+ \to K^+K^-K^+) = [35.8 \pm 1.6(\rm stat) \pm 1.4 (\rm syst)]\times 10^{-6}$, $\mathcal{B}(B^+ \to K^+π^-π^+) = [67.0 \pm 3.3 (\rm stat)\pm 2.3 (\rm syst)]\times 10^{-6}$, $\mathcal{B}(B^0 \to K^+π^-π^0) = [38.1 \pm 3.5 (\rm stat)\pm 3.9 (\rm syst)]\times 10^{-6}$, $\mathcal{A}_{\rm CP}(B^+ \to K^+K^-K^+) = -0.103 \pm 0.042(\rm stat) \pm 0.020 (\rm syst)$, $\mathcal{A}_{\rm CP}(B^+ \to K^+π^-π^+) = -0.010 \pm 0.050 (\rm stat)\pm 0.021(\rm syst)$, and $\mathcal{A}_{\rm CP}(B^0 \to K^+π^-π^0) = 0.207 \pm 0.088 (\rm stat)\pm 0.011(\rm syst)$. Results are consistent with previous measurements and demonstrate detector performance comparable with the best Belle results.

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arXiv2021

Angular analysis of $B^+ \to ρ^+ρ^0$ decays reconstructed in 2019-2020 Belle II data

We report on the first Belle II measurement of the branching fraction ($\mathcal{B}$) and longitudinal polarization fraction ($f_L$) of $B^+\to ρ^+ρ^0$ decays. We reconstruct $B^+\to ρ^+(\to π^+π^0(\to γγ))ρ^0(\to π^+π^-)$ decays in a sample of SuperKEKB electron-positron collisions collected by the Belle II experiment in 2019 and 2020 at the $Υ$(4S) resonance and corresponding to $62.8$ fb$^{-1}$ of integrated luminosity. We fit the distributions of the difference between expected and observed $B$ candidate energy, continuum-suppression variable, dipion masses, and angular distributions of the resulting samples, to determine a signal yield of $104\pm16$ events. The signal yields are corrected for efficiencies determined from simulation and control data samples to obtain $\mathcal{B}(B^+ \to ρ^+ρ^0) = [20.6 \pm 3.2(\rm stat) \pm 4.0(\rm syst)]\times 10^{-6}$, and $f_L(B^+ \to ρ^+ρ^0) = 0.936 ^{+0.049}_{-0.041}(\rm stat)\pm 0.021(\rm syst)$. This first Belle II $B^+ \to ρ^+ρ^0$ angular analysis yields results compatible with previous determinations, and indicates Belle II performance superior to early Belle results.

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arXiv2021

Study of $χ_{bJ}(nP) \rightarrow ωΥ(1S)$ at Belle

We report results from a study of hadronic transitions of the $χ_{bJ}(nP)$ states of bottomonium at Belle. The $P$-wave states are reconstructed in transitions to the $Υ(1S)$ with the emission of an $ω$ meson. The transitions of the $n=2$ triplet states provide a unique laboratory in which to study nonrelativistic quantum chromodynamics, as the kinematic threshold for production of an $ω$ and $Υ(1S)$ lies between the $J=0$ and $J=1$ states. A search for the $χ_{bJ}(3P)$ states is also reported.

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arXiv2021

Study of $e^+e^-\to Υ(\rm1S,2S)η$ and $e^+e^-\to Υ(\rm 1S)η^{\prime}$ at $\sqrt{s}=10.866$ GeV with the Belle detector

We report the first observation of the processes $e^+e^-\toΥ(\rm 1S,2S)η$ at $\sqrt{s}=10.866$~GeV with a $10.2σ$ and $16.5σ$ significance respectively. The measured Born cross sections are $σ(e^+e^- \to Υ(2S)η)=2.07 \pm 0.21 \pm 0.19$~pb, and $σ(e^+e^- \to Υ(\rm 1S)η)=0.42 \pm 0.08 \pm 0.04$~pb. We also set the upper limit on the cross section of the process $e^+e^- \to Υ(\rm 1S)η^{\prime}$ to be $σ(e^+e^- \to Υ(\rm 1S)η^{\prime})<0.035$~pb at $90\%$ CL. The results are obtained with the data sample collected with the Belle detector at the KEKB asymmetric-energy $e^+e^-$ collider in the energy range from $10.63$~GeV to $11.02$~GeV.

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arXiv2021

Rediscovery of $B^0\to J\mskip 1mu / ψ\mskip 2mu K^0_{\scriptscriptstyle L}$ at Belle II

We present preliminary results on the reconstruction of the $B^0\to J\mskip 1mu / ψ\mskip 2mu K^0_{\scriptscriptstyle L}$ decay, where $J\mskip 1mu / ψ\mskip 2mu\toμ^+μ^-$ or $e^+e^-$. Using a dataset corresponding to a luminosity of $62.8\pm0.6\mbox{fb}^{-1}$ collected by the Belle II experiment at the SuperKEKB asymmetric energy $e^+e^-$ collider, we measure a total of $267\pm21$ candidates with $J\mskip 1mu / ψ\mskip 2mu\toμ^+μ^-$ and $226\pm20$ with with $J\mskip 1mu / ψ\mskip 2mu\to e^+e^-$. The quoted errors are statistical only.

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arXiv2021

Measurements of Partial Branching Fractions of Inclusive $B \to X_u \, \ell^+\, ν_{\ell}$ Decays with Hadronic Tagging

We present measurements of partial branching fractions of inclusive semileptonic $B \to X_u \, \ell^+\, ν_{\ell}$ decays using the full Belle data set of 711 fb$^{-1}$ of integrated luminosity at the $Υ(4S)$ resonance and for $\ell = e, μ$. Inclusive semileptonic $B \to X_u \, \ell^+\, ν_{\ell}$ decays are CKM suppressed and measurements are complicated by the large background from CKM-favored $B \to X_c \, \ell^+\, ν_{\ell}$ transitions, which have a similar signature. Using machine learning techniques, we reduce this and other backgrounds effectively, whilst retaining access to a large fraction of the $B \to X_u \, \ell^+\, ν_{\ell}$ phase space and high signal efficiency. We measure partial branching fractions in three phase-space regions covering about $31\%$ to $86\%$ of the accessible $B \to X_u \, \ell^+\, ν_{\ell}$ phase space. The most inclusive measurement corresponds to the phase space with lepton energies of $E_\ell^B > 1 $ GeV, and we obtain $Δ\mathcal{B}(B \to X_u \ell^+ \, ν_\ell) = \left( 1.59 \pm 0.07 \pm 0.16 \right) \times 10^{-3}$ from a two-dimensional fit of the hadronic mass spectrum and the four-momentum-transfer squared distribution, with the uncertainties denoting the statistical and systematic error. We find $\left| V_{ub} \right| = \left( 4.10 \pm 0.09 \pm 0.22 \pm 0.15 \right) \times 10^{-3}$ from an average of four calculations for the partial decay rate with the third uncertainty denoting the average theory error. This value is higher but compatible with the determination from exclusive semileptonic decays within 1.3 standard deviations. In addition, we report charmless inclusive partial branching fractions separately for $B^+$ and $B^0$ mesons as well as for electron and muon final states. No isospin breaking or lepton flavor universality violating effects are observed.

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arXiv2021

Measurements of branching fractions and direct CP asymmetries in $B^{0}\to K^{+} π^{-}$, $B^+ \to K_{\rm S}^0π^+$ and $B^0 \to π^+π^-$ using 2019 and 2020 data

We report updated measurements of branching fractions ($\mathcal{B}$) and CP-violating charge asymmetries ($\mathcal{A_{\rm CP}}$) for charmless $B$ decays at Belle II, which operates on or near the $Υ$(4S) resonance at the SuperKEKB asymmetric energy $e^{+}e^{-}$ collider. We use samples of 2019 and 2020 data corresponding to 62.8 fb$^{-1}$ of integrated luminosity. The samples are analysed using two-dimensional fits in $ΔE$ and $M_{\it bc}$ to determine signal yields of approximately 568, 103, and 115 decays for the channels $B^0 \to K^+π^-$, $B^+ \to K_{\rm S}^0π^+$, and $B^0 \to π^+π^-$, respectively. Signal yields are corrected for efficiencies determined from simulation and control data samples to obtain branching fractions and CP-violating asymmetries for flavour-specific channels. The results are compatible with known determinations and contribute important information to an early assessment of Belle II detector performance.

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arXiv2021

Measurement of the time-integrated mixing probability $χ_d$ with a semileptonic double-tagging strategy and $34.6 {\rm fb}^{-1}$ of Belle II collision data

We present the first measurement of the time-integrated mixing probability $χ_d$ using Belle II data collected at a center-of-mass (CM) energy of 10.58 GeV, corresponding to the mass of the $Υ$(4S) resonance, with an integrated luminosity of $34.6 {\rm fb}^{-1}$ at the SuperKEKB $e^+ e^-$ collider. We reconstruct pairs of B mesons both of which decay to semileptonic final states. Using a novel methodology, we measure $χ_d = 0.187 \pm 0.010 \text{ (stat.)} \pm 0.019 \text{ (syst.)}$, which is compatible with existing indirect and direct determinations.

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arXiv2021

Measurement of the branching fractions of $B\toη' K$ decays using 2019/2020 Belle II data

This note describes the rediscovery of $B\toη' K$ decays in Belle II data, both in the charged and neutral final state: $B_0\toη' K_S$ and $B^\pm\toη' K^\pm$. The $η'$ is searched for in two decay modes: $η'\toηπ^+π^-$ with $η\toγγ$, and $η'\toργ$. The analysis uses data collected in 2019 and 2020 at the SuperKEKB asymmetric $e^+e^-$ collider, with an integrated luminosity of $62.8~fb^{-1}$, corresponding to $68.2$ million of $B\bar{B}$ pairs produced. The signal yield is obtained via an unbinned maximum likelihood fit to signal sensitive variables, obtaining branching ratios: $$\mathcal{B}\left(B^\pm\toη'K^\pm\right) = \left(63.4~^{+3.4}_{-3.3}\,(stat)\,\pm3.2\,(syst)\,\right) \times10^{-6} $$ $$\mathcal{B}\left(B_0\toη'K_S\right) = \left(59.9~^{+5.8}_{-5.5}\,(stat)\,\pm2.9\,(syst)\,\right) \times10^{-6} $$ which are consistent with world average.

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arXiv2021

Measurements of branching fractions and direct ${\it CP}$-violating asymmetries in $B^+ \to K^+ π^0~\mbox{and}~π^+ π^0$ decays using 2019 and 2020 Belle II data

We report measurements of branching fractions ($\mathcal B$) and direct ${\it CP}$-violating asymmetries ($\mathcal A_{\it CP}$) for the decays $B^+\to K^+π^0$ and $B^+ \to π^+π^0$ reconstructed with the Belle II detector in a sample of asymmetric-energy electron-positron collisions at the $Υ(4S)$ resonance corresponding to 62.8 $\text{fb}^{-1}$ of integrated luminosity. The results are $\mathcal{B}(B^+ \to K^+π^0) = [11.9 ^{+1.1}_{-1.0} (\rm stat)\pm 1.6(\rm syst)]\times 10^{-6}$, $\mathcal{B}(B^+ \to π^+π^0) = [5.5 ^{+1.0}_{-0.9} (\rm stat)\pm 0.7(\rm syst)]\times 10^{-6}$, $\mathcal A_{\it CP}(B^+ \to K^+π^0) = -0.09 \pm 0.09 (\rm stat)\pm 0.03(\rm syst)$, and $\mathcal A_{\it CP}(B^+ \to π^+π^0) = -0.04 \pm 0.17 (\rm stat)\pm 0.06(\rm syst)$. The results are consistent with previous measurements and show a detector performance comparable with early Belle performance.

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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

First search for direct $CP$-violating asymmetry in $B^0 \to K^0 π^0$ decays at Belle II

We report on the first measurement of the direct $CP$-violating asymmetry ($\mathcal{A}$) in the charmless decay $B^0 \to K^0π^0$ at Belle II and an updated measurement of its branching fraction ($\mathcal{B}$). We use a sample of electron-positron collisions collected in 2019 and 2020 at the $Υ(4S)$ resonance and corresponding to $62.8$ $\text{fb}^{-1}$ of integrated luminosity. We reconstruct and select about $50$ $B^0 \to K_S^0 π^0$ candidates, and we measure $\mathcal{A}_{K^0π^0} = -0.40_{-0.44}^{+0.46} (\text{stat}) \pm 0.04 (\text{syst})$ and $\mathcal{B}(B^0 \to K^0 π^0) = [8.5_{-1.6}^{+1.7} (\text{stat}) \pm 1.2 (\text{syst})] \times 10^{-6}$. This is the first measurement of $CP$ violation in $B^0 \to K^0π^0$ decays reported by Belle II. The results agree with previous determinations and show a detector performance comparable with the best Belle results.

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arXiv2021

Search for the dark photon in $B^0 \to A^{\prime} A^{\prime}$, $A^{\prime} \to e^+ e^-$, $μ^+ μ^-$, and $π^+ π^-$ decays at Belle

We present a search for the dark photon $A^{\prime}$ in the $B^0 \to A^{\prime} A^{\prime}$ decays, where $A^{\prime}$ subsequently decays to $e^+ e^-$, $μ^+ μ^-$, and $π^+ π^-$. The search is performed by analyzing $772 \times 10^6$ $B\overline{B}$ events collected by the Belle detector at the KEKB $e^+ e^-$ energy-asymmetric collider at the $Υ(4S)$ resonance. No signal is found in the dark photon mass range $0.01~\mathrm{GeV}/c^2 \le m_{A^{\prime}} \le 2.62~\mathrm{GeV}/c^2$, and we set upper limits of the branching fraction of $B^0 \to A^{\prime} A^{\prime}$ at the 90\% confidence level. The products of branching fractions, $\mathcal{B}(B^0 \to A^{\prime} A^{\prime}) \times \mathcal{B}(A^{\prime} \to e^+ e^-)^2$ and $\mathcal{B}(B^0 \to A^{\prime} A^{\prime}) \times \mathcal{B}(A^{\prime} \to μ^+ μ^-)^2$, have limits of the order of $10^{-8}$ depending on the $A^{\prime}$ mass. Furthermore, considering $A^{\prime}$ decay rate to each pair of charged particles, the upper limits of $\mathcal{B}(B^0 \to A^{\prime} A^{\prime})$ are of the order of $10^{-8}$-$10^{-5}$. From the upper limits of $\mathcal{B}(B^0 \to A^{\prime} A^{\prime})$, we obtain the Higgs portal coupling for each assumed dark photon and dark Higgs mass. The Higgs portal couplings are of the order of $10^{-2}$-$10^{-1}$ at $m_{h'} \simeq m_{B^0} \pm 40~\mathrm{MeV}/c^2$ and $10^{-1}$-$1$ at $m_{h'} \simeq m_{B^0} \pm 3~\mathrm{GeV}/c^2$.

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arXiv2021

Study of $B\to D^{(*)}h$ decays using $62.8~\mathrm{fb}^{-1}$ of Belle II data

We report measurements related to hadronic $B$ decays to final states that contain charm mesons. The analyses are performed on a $62.8~\mathrm{fb}^{-1}$ data set collected by the Belle II experiment at a center-of-mass energy corresponding to the mass of the $Υ(4S)$ resonance. The measurements reported are for the decay modes $B^-\to D^0 h^-$, $B^{-}\to D^{*0}h^-$, $\bar{B}^{0}\to D^{+} h^{-}$ and $\bar{B}^{0}\to D^{*+} h^{-}$, where $h=π$ or $K$. These modes are either signal or control channels for measurements related to the unitarity triangle angle $γ$ in direct or time-dependent $CP$-violation measurements. The reported observables are the ratios between the $B\to D^{(*)}K$ and $B\to D^{(*)}π$ decay rates, which are found to be in agreement with previous measurements.

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arXiv2021

Event Reconstruction in a Liquid Xenon Time Projection Chamber with an Optically-Open Field Cage

nEXO is a proposed tonne-scale neutrinoless double beta decay ($0νββ$) experiment using liquid ${}^{136}Xe$ (LXe) in a Time Projection Chamber (TPC) to read out ionization and scintillation signals. Between the field cage and the LXe vessel, a layer of LXe ("skin" LXe) is present, where no ionization signal is collected. Only scintillation photons are detected, owing to the lack of optical barrier around the field cage. In this work, we show that the light originating in the skin LXe region can be used to improve background discrimination by 5% over previous published estimates. This improvement comes from two elements. First, a fraction of the $γ$-ray background is removed by identifying light from interactions with an energy deposition in the skin LXe. Second, background from ${}^{222}Rn$ dissolved in the skin LXe can be efficiently rejected by tagging the $α$ decay in the ${}^{214}Bi-{}^{214}Po$ chain in the skin LXe.

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