arXiv subjects

Belle II Collaboration

Publications and source records attributed to Belle II Collaboration.

At least 19 recordsLinked to original sources

arXiv2026

Search for the lepton-flavor-violating decay $ \tau^{\pm} \to \mu^{\pm} \gamma$ at Belle II

We present a search for the lepton-flavor-violating decay $\tau^{\pm}\to\mu^{\pm}\gamma$ using a data sample that corresponds to an integrated luminosity of 428 fb$^{-1}$ recorded by the Belle II experiment at the SuperKEKB asymmetric-energy $e^{+}e^{-}$ collider. We employ a multivariate classifier to suppress the backgrounds from the Standard Model processes, and the signal extraction is performed using an extended maximum-likelihood fit. Since no significant excess over the expected background is observed, we set an upper limit on the branching fraction $\mathcal{B}(\tau^{\pm}\to\mu^{\pm}\gamma) < 9.5$ $ (12.2)\times10^{-8}$ at the 90\% (95\%) confidence level, using the CL${_s}$ technique.

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arXiv2026

Search for axion-like particles decaying to two photons at Belle II

Axion-like particles (ALPs) are predicted in many extensions of the Standard Model and provide a well-motivated portal between visible and hidden sectors through their coupling to photons. We search for ALPs produced in the process $e^{+}e^{-}\to\gamma a$, $a\to\gamma\gamma$, using a data sample corresponding to an integrated luminosity of $408~\mathrm{fb}^{-1}$ recorded by the Belle~II detector at the SuperKEKB $e^{+}e^{-}$ collider. Events containing three photons are used to reconstruct the ALP as a narrow peak in the di-photon invariant mass spectrum over the range $0.17 < m_{a} < 9.80~\mathrm{GeV}/c^{2}$. No significant excess above background is observed. We set 95\% confidence level upper limits on the production cross section and on the ALP-photon coupling $g_{a\gamma\gamma}$, reaching sensitivities at the level of $10^{-4}~\mathrm{GeV}^{-1}$. The limits are the most restrictive to date over nearly the entire mass range $0.17 < m_{a} < 5.00~\mathrm{GeV}/c^{2}$, and improve upon previous results by up to a factor 9.

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arXiv2026

Charged-lepton identification at Belle~II

Effective particle identification capabilities are a strategic priority for the physics program of the Belle~II experiment. We describe the algorithms used at Belle~II for identifying electrons and muons and separating them from charged hadrons. We present the performance obtained by the experiment during Run 1, which consists of 428 fb$^{-1}$ of data collected at the energy-asymmetric $e^+e^-$ collider SuperKEKB between 2019 and 2022 at center-of-mass energies near the mass of the $\Upsilon(4S)$.

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arXiv2026

Measurement of mixing-induced CP violation in the decay $B^0 \to \pi^0 \pi^0$

Measurements of charge-parity (CP) violation in quark transitions provide stringent tests of the current theory and sensitive probes of particles or interactions beyond it. We report the first measurement of mixing-induced CP violation in $B^0\to\pi^0\pi^0$ decays. The measurement uses 190 million $\Upsilon(4S)\to B^{0}\overline{B}{}^0$ events collected with the Belle II experiment at the SuperKEKB asymmetric-energy electron-positron collider. A new approach that exploits the quantum entanglement of the $B^{0}\overline{B}{}^0$ pair enables a decay-time-dependent analysis without reconstructing the $B^0 \to \pi^0\pi^0$ decay position. We measure the mixing-induced CP-violating coefficient to be $S_{\pi^{0}\pi^{0}} = 0.61^{+0.75}_{-0.79}\,(\mathrm{stat}) \pm 0.11\,(\mathrm{syst})$, attaining a precision that would require a data set twenty times as large with a conventional analysis. We also measure the direct CP-violating coefficient to be $C_{\pi^{0}\pi^{0}} = 0.05 \pm 0.28\,(\mathrm{stat}) \pm 0.07\,(\mathrm{syst})$. These results substantially improve the constraints from $B \to \pi\pi$ decays on the CP-violating phase $\phi_2$ of the quark-mixing matrix using far less data than had previously been assumed necessary.

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arXiv2026

Study of $e^+e^- \to \pi^+\pi^-\Upsilon(1D)$ at Belle II

The bottomonium spectrum, consisting of bound states of a $b$ quark and an anti-$b$ quark, provides an excellent laboratory for probing quantum chromodynamics in the non-perturbative regime. While $S$ and $P$-wave bottomonium states are well studied experimentally, information on $D$-wave states remains scarce. We search for $D$-wave bottomonium state via the decay of a vector bottomonium-like state $\Upsilon(10753)$ in the reaction $e^+e^- \to \pi^+\pi^- \Upsilon(1D)$, using $19.6~\mathrm{fb}^{-1}$ of data collected with the Belle II detector at center-of-mass energies $\sqrt{s} = 10.653, 10.701, 10.745$, and $10.805$~GeV, in the vicinity of the $\Upsilon(10753)$ resonance. No significant signals are observed. Upper limits at the 90% credibility level are set on the products of the cross sections and branching fractions, $\sigma[e^+e^- \to \pi^+\pi^- \Upsilon_2(1D)] \times \mathcal{B}[\Upsilon_2(1D) \to \gamma \chi_{b1}]$ and $\sigma[e^+e^- \to \pi^+\pi^- \Upsilon_3(1D)] \times \mathcal{B}[\Upsilon_3(1D) \to \gamma \chi_{b2}]$, at each center-of-mass energy.

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arXiv2026

Study of $e^{+}e^{-}\to h^{+}h^{-}J/\psi~(h=\pi,~K,~p)$ via initial-state radiation at Belle~II

Using a data sample of 427.9 fb$^{-1}$ collected by the Belle~II detector at or near the $\Upsilon(4S)$ and $\Upsilon(10753)$ resonances, the cross sections for $e^+e^-\to h^+h^-J/\psi$ $(h=\pi/K/p)$ at center-of-mass energies ranging from 3.8 GeV or the production threshold to 5.5/6.0/7.0 GeV have been measured via initial-state radiation. The cross sections for the processes $e^+e^-\to \pi^+\pi^-J/\psi$ and $e^+e^-\to K^+K^-J/\psi$ are consistent with previously published results. The cross sections for these channels obtained by combining with previous Belle results are also given. The process $e^+e^-\to p\bar p J/\psi$ is investigated for the first time. The yields are small and no significant structure is observed in the cross section versus energy. Searches for vector charmonium-like states in the $h^+h^-J/\psi$ systems, and for associated intermediate states in the $h^{\pm} J/\psi$ systems, are also presented.

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arXiv2025

Measurement of inclusive $B \to X_u \ell \nu$ partial branching fractions and $|V_{ub}|$ at Belle II

A sample of 365 fb$^{-1}$ of $e^+e^- \to \Upsilon(4S) \to B\bar{B}$ data collected by the Belle II experiment is used to measure the partial branching fractions of charmless semileptonic $B$ meson decays and determine the magnitude of the Cabibbo-Kobayashi-Maskawa matrix element $V_{ub}$. Events containing a signal electron or muon $\ell$ and a fully reconstructed hadronic $B$ decay that constrains the signal kinematics are selected, while the rest of the event defines the hadronic system $X_u$ associated with the signal. To discriminate the signal from the 50-times larger background originating from CKM-favored semileptonic $B$ decays, a template fit is performed in both signal and control regions after applying an optimized selection. The partial branching fraction measured for lepton energies greater than 1 GeV in the signal $B$ meson rest frame is $\Delta\mathcal{B}(B \to X_u \ell \nu) = (1.54 \pm 0.08 \, {\rm (stat.)} \pm 0.12 \, {\rm (syst.)}) \times 10^{-3}$. From this measurement, using the Gambino, Giordano, Ossola, Uraltsev theoretical framework, $|V_{ub}| = (4.01 \pm 0.19 ^{+0.07} _{-0.08}) \times 10^{-3}$ is determined, where the uncertainties are experimental and theoretical, respectively. This value is consistent with the world average obtained from previous inclusive measurements. Different theoretical predictions and partial branching fractions measured in other phase-space regions, defined by additional selections on the $X_u$ and leptonic system masses, are also used to determine $|V_{ub}|$.

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arXiv2025

First search for $B \rightarrow X_{s} \nu \bar{\nu}$ decays

We report the first search for the flavor-changing neutral-current decays $B \rightarrow X_{s} \nu \bar{\nu}$, where $X_{s}$ is a hadronic system with strangeness equal to 1, in data collected with the Belle~II detector at the SuperKEKB asymmetric-energy $e^+e^-$ collider. The data sample corresponds to an integrated luminosity of $365~\textrm{fb}^{-1}$ collected at the $\Upsilon(4S)$ resonance and $43~\textrm{fb}^{-1}$ collected at a center-of-mass energy $60~\textrm{MeV}$ below resonance for estimation of $e^+e^-\to q\bar{q}$ continuum background. One of the $B$ mesons from the $\Upsilon(4S) \to B\bar{B}$ decay is fully reconstructed in a hadronic decay mode. The $B \to X_s \nu \bar{\nu}$ decay is reconstructed with a sum-of-exclusives approach that uses 30 $X_s$ decay modes. This approach provides high sensitivity to the inclusive decay, despite the presence of two undetected neutrinos. The search is performed in three regions of the $X_{s}$ mass, chosen to separate contributions from prominent resonances. We do not observe a significant signal and set upper limits at 90\% confidence level on the partial branching fractions for the regions $0.0 < M_{X_{s}} < 0.6~\textrm{GeV}/c^{2}$, $0.6 < M_{X_{s}} < 1.0~\textrm{GeV}/c^{2}$, and $1.0~\textrm{GeV}/c^{2} < M_{X_{s}}$ of $2.2 \times 10^{-5}$, $9.3 \times 10^{-5}$, and $30.9 \times 10^{-5}$, respectively. Combining the three mass regions, we obtain the upper limit on the branching fraction, $B(B \to X_s \nu \bar{\nu}) < 3.3 \times 10^{-4}$.

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arXiv2025

Observation of the radiative decay $D_s (2317)^+ \to D_s^* \gamma$

We observe the radiative decay $D^{*}_{s0}(2317)^{+} \to D_{s}^{*+} \gamma$ for the first time, with a significance exceeding $10$ standard deviations. The signal is found in the continuum $e^+ e^- \to c\bar{c}$ process with the combined data samples of 980.4~$\rm fb^{-1}$ and 427.9~$\rm fb^{-1}$ collected by the Belle and Belle~II detectors operating at the KEKB and SuperKEKB asymmetric-energy $e^+e^-$ colliders, respectively. The branching fraction ratio ${\cal B}(D^{*}_{s0}(2317)^{+} \to D_{s}^{*+} \gamma)/{\cal B}(D^{*}_{s0}(2317)^{+} \to D_{s}^{+} \pi^{0})$ is measured to be $[7.13 \pm 0.70({\rm stat.}) \pm 0.23({\rm syst.})]\%$. This result provides significant new experimental input for the determination of the quark structure of the $D^{*}_{s0}(2317)^{+}$, which remains unknown.

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arXiv2025

Measurement of the $CP$ asymmetry in $D^0\to\pi^+\pi^-\pi^0$ decays at Belle II

We measure the time- and phase-space-integrated $CP$ asymmetry $A_{CP}$ in $D^0\to\pi^+\pi^-\pi^0$ decays reconstructed in $e^+e^-\to c\bar c$ events collected by the Belle II experiment from 2019 to 2022. This sample corresponds to an integrated luminosity of 428 fb$^{-1}$. We require $D^0$ mesons to be produced in $D^{*+}\to D^0\pi^+$ decays to determine their flavor at production. Control samples of $D^0\to K^-\pi^+$ decays are used to correct for reconstruction-induced asymmetries. The result, $A_{CP}(D^0\to\pi^+\pi^-\pi^0)=(0.29\pm0.27\pm0.13)\%$, where the first uncertainty is statistical and the second systematic, is the most precise result to date and is consistent with $CP$ conservation.

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arXiv2025

Search for $CP$ violation in $\Xi_c^+\to\Sigma^+h^+h^-$ and $\Lambda_c^+\to ph^+h^-$ at Belle II

We report decay-rate $CP$ asymmetries of the singly-Cabibbo-suppressed decays $\Xi_c^+\to\Sigma^+h^+h^-$ and $\Lambda_c^+\to ph^+h^-$, with $h=K,\pi$, measured using 428 fb$^{-1}$ of $e^+e^-$ collisions collected by the Belle II experiment at the SuperKEKB collider. The results, \begin{equation} A_{CP}(\Xi_c^+\to\Sigma^+K^+K^-) = (3.7\pm6.6\pm0.6)\%, \end{equation} \begin{equation} A_{CP}(\Xi_c^+\to\Sigma^+\pi^+\pi^-) = (9.5\pm6.8\pm0.5)\%, \end{equation} \begin{equation} A_{CP}(\Lambda_c^+\to pK^+K^-) = (3.9\pm1.7\pm0.7)\%, \end{equation} \begin{equation} A_{CP}(\Lambda_c^+\to p\pi^+\pi^-) = (0.3\pm1.0\pm0.2)\%, \end{equation} where the first uncertainties are statistical and the second systematic, agree with $CP$ symmetry. From these results we derive the sums \begin{equation} A_{CP}(\Xi_c^+\to\Sigma^+\pi^+\pi^-) \, + \, A_{CP}(\Lambda_c^+\to pK^+K^-) = (13.4 \pm 7.0\pm 0.9)\%, \end{equation} \begin{equation} A_{CP}(\Xi_c^+\to\Sigma^+K^+K^-) \, + \, A_{CP}(\Lambda_c^+\to p\pi^+\pi^-) = (\phantom{0}4.0 \pm 6.6\pm 0.7)\%, \end{equation} which are consistent with the $U$-spin symmetry prediction of zero. These are the first measurements of $CP$ asymmetries for individual hadronic three-body charmed-baryon decays.

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arXiv2025

Observation of $e^+e^-\to\eta\Upsilon(2S)$ and search for $e^+e^-\to\eta\Upsilon(1S),~\gamma X_b$ at $\sqrt{s}$ near 10.75 GeV

We present an analysis of the processes $e^{+}e^{-}\to\eta\Upsilon(1S)$, $\eta\Upsilon(2S)$, and $\gamma X_b$ with $X_b\to\pi^+\pi^-\chi_{bJ},~\chi_{bJ}\to\gamma\Upsilon(1S)$ $(J=1,~2)$ reconstructed from $\gamma\gamma\pi^+\pi^-\ell^+\ell^-~(\ell=e,~\mu)$ final states in $19.6\rm~\rm fb^{-1}$ of Belle II data collected at four energy points near the peak of the $\Upsilon(10753)$ resonance. Here, $X_b$ is a hypothetical bottomonium-sector partner of the $X(3872)$. In the process $e^{+}e^{-} \to \eta \Upsilon(2S)$, we observe a signal with a significance that exceeds $6.0\sigma$. Based on an analysis of the Born cross-section, the hypothesis that these events are associated solely with the production of the $\Upsilon(5S)$ or $\Upsilon(10753)$ resonances is rejected at the level of 3.6$\sigma$. This suggests that the observed events are more likely to arise from a new state near the $B^{*}\bar{B}^{*}$ threshold or from a substantial continuum production. No significant signal is observed for $e^{+}e^{-}\to\eta\Upsilon(1S)$ or $\gamma X_b$. Upper limits on the Born cross sections for the processes $e^{+}e^{-}\to\eta\Upsilon(1S)$ and $e^{+}e^{-}\to\gamma X_b$ with $X_b\to\pi^+\pi^-\chi_{bJ}$ are determined.

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arXiv2025

Search for $e^+ e^- \to \gamma\chi_{bJ}$ ($J$ = 0, 1, 2) near $\sqrt{s} = 10.746$ GeV at Belle II

We search for the $e^+ e^- \to \gamma \chi_{bJ}$ ($J$ = 0, 1, 2) processes at center-of-mass energies $\sqrt{s}$ = 10.653, 10.701, 10.746, and 10.804 GeV. These data were collected with the Belle II detector at the SuperKEKB collider and correspond to 3.5, 1.6, 9.8, and 4.7 fb$^{-1}$ of integrated luminosity, respectively. We set upper limits at the 90\% confidence level on the Born cross sections for $e^+ e^- \to \gamma \chi_{bJ}$ at each center-of-mass energy $\sqrt{s}$ near 10.746 GeV. The upper limits at 90\% confidence level on the Born cross sections for $e^+ e^- \to \gamma \chi_{b1}$ are significantly smaller than the corresponding measured values for $e^+e^-\to\omega\chi_{b1}$ and $e^+e^-\to\pi^+\pi^-\Upsilon(2S)$ at $\sqrt{s}$ = 10.746 GeV.

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arXiv2025

Search for the lepton-flavor-violating $\tau^{-} \rightarrow e^{\mp} \ell^{\pm} \ell^{\mp}$ decays at Belle II

We present the result of a search for the charged-lepton-flavor violating decays $\tau^- \rightarrow e^\mp \ell^\pm \ell^-$, where $\ell$ is a muon or an electron, using a data sample with an integrated luminosity of 428 fb$^{-1}$ recorded by the Belle II experiment at the SuperKEKB $e^+e^-$ collider. The selection of $e^+e^- \to\tau^+\tau^-$ events containing a signal candidate is based on an inclusive-tagging reconstruction and on a boosted decision tree to suppress background. Upper limits on the branching fractions between 1.3 and 2.5 $\times 10^{-8}$ are set at the 90% confidence level. These results are the most stringent bounds to date for four of the modes.

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arXiv2025

Model-agnostic likelihood for the reinterpretation of the $B^+\to K^+\nu\bar{\nu}$ measurement at Belle II

We recently measured the branching fraction of the $B^{+}\rightarrow K^{+}\nu\bar{\nu}$ decay using 362fb$^{-1}$ of on-resonance $e^+e^-$ collision data under the assumption of Standard Model kinematics, providing the first evidence for this decay. To facilitate future reinterpretations and maximize the scientific impact of this measurement, we publicly release the full analysis likelihood along with all necessary material required for reinterpretation under arbitrary theoretical models sensitive to this measurement. In this work, we demonstrate how the measurement can be reinterpreted within the framework of the Weak Effective Theory. Using a kinematic reweighting technique in combination with the published likelihood, we derive marginal posterior distributions for the Wilson coefficients, construct credible intervals, and assess the goodness of fit to the Belle II data. For the Weak Effective Theory Wilson coefficients, the posterior mode of the magnitudes $|C_\mathrm{VL}+C_\mathrm{VR}|$, $|C_\mathrm{SL}+C_\mathrm{SR}|$, and $|C_\mathrm{TL}|$ corresponds to the point ${(11.3, 0.0, 8.2)}$. The respective 95\% credible intervals are $[1.9, 16.2]$, $[0.0, 15.4]$, and $[0.0, 11.2]$.

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arXiv2025

Determination of $|V_{cb}|$ using $B\to D\ell\nu_\ell$ Decays at Belle II

We present a determination of the Cabibbo-Kobayashi-Maskawa matrix element $|V_{cb}|$ from the decay $B\to D\ell\nu_\ell$ using a $365~\mathrm{fb}^{-1}$ $e^+e^-\to\Upsilon(4S)\to B\bar B$ data sample recorded by the Belle II experiment at the SuperKEKB collider. The semileptonic decay of one $B$ meson is reconstructed in the modes $B^0\to D^-(\to K^+\pi^-\pi^-)\ell^+\nu_\ell$ and $B^+\to \bar D^0(\to K^+\pi^-)\ell^+\nu_\ell$, where $\ell$ denotes either an electron or a muon. Charge conjugation is implied. The second $B$ meson in the $\Upsilon(4S)$ event is not reconstructed explicitly. Using an inclusive reconstruction of the unobserved neutrino momentum, we determine the recoil variable $w=v_B\cdot v_D$, where $v_B$ and $v_D$ are the 4-velocities of the $B$ and $D$ mesons. We measure the total decay branching fractions to be $\mathcal{B}(B^0\to D^-\ell^+\nu_\ell)=(2.06 \pm 0.05\,(\mathrm{stat.}) \pm 0.10\,(\mathrm{sys.}))\%$ and $\mathcal{B}(B^+\to\bar D^0\ell^+\nu_\ell)=(2.31 \pm 0.04\,(\mathrm{stat.}) \pm 0.09\,(\mathrm{sys.}))\%$. We probe lepton flavor universality by measuring $\mathcal{B}(B\to De\nu_e)/\mathcal{B}(B\to D\mu\nu_\mu)=1.020 \pm 0.020\,(\mathrm{stat.})\pm 0.022\,(\mathrm{sys.})$. Fitting the partial decay branching fraction as a function of $w$ and using the average of lattice QCD calculations of the $B\to D$ form factor, we obtain $ |V_{cb}|=(39.2\pm 0.4\,(\mathrm{stat.}) \pm 0.6\,(\mathrm{sys.}) \pm 0.5\,(\mathrm{th.})) \times 10^{-3}$.

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arXiv2025

Measurement of the CP asymmetry in $D^+ \to \pi^+ \pi^0$ decays at Belle II

We measure the CP asymmetry in $D^+ \to \pi^+ \pi^0$ decays reconstructed in $e^+ e^-$ collisions at the Belle II experiment using a data set corresponding to an integrated luminosity of 428 fb$^{-1}$. A control sample of $D^+ \to \pi^+ K_{S}$ decays is used to correct for detection and production asymmetries. The result, $A_{CP}(D^+ \to \pi^+\pi^0) =(-1.8 \pm 0.9 \pm 0.1)\%$, where the first uncertainty is statistical and the second systematic, is the most precise determination to date. It agrees with the prediction of CP symmetry from the standard model, and with results of previous measurements.

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arXiv2025

Charged-hadron identification at Belle II

The Belle II experiment's ability to identify particles critically affects the sensitivity of its measurements. We describe Belle II's algorithms for identifying charged particles and evaluate their performance in separating pions, kaons, and protons using 426 fb$^{-1}$ of data collected at the energy-asymmetric $e^+e^-$ collider SuperKEKB in 2019--2022 at center-of-mass energies at and near the mass of the $\Upsilon(4S)$.

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