arXiv subjects

J. Becker

Publications and source records attributed to J. Becker.

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.

hep-ex↗

arXiv2026

Search for the $\boldsymbol{B^0 \to K^0_{\rm S} \tau^+ \tau^-}$ decay

We present the first search for $B^0 \to K^0_{\rm S} \tau^+\tau^-$ decays. We look for signal decays in $B^0\bar B^0$ events produced in asymmetric-energy electron-positron collisions. This work uses samples from the Belle and Belle~II detectors, comprising 1.16 billion $\Upsilon(4S)$ events. In $\Upsilon(4S)\to B^0\bar{B}^0$ decays, the non-signal $\bar{B}^0$ meson is fully reconstructed in a hadronic channel. For the signal $B^0$ meson, $\tau$-lepton decays into final states with a single charged particle are selected. A multivariate classifier is used to combine several discriminating inputs into a single fit observable. We observe no evidence for the signal and set an upper limit on the branching fraction $\mathcal{B}(B^0\to K^0_{\rm S} \tau^+\tau^-) < 8.3 \times 10^{-4}$ at the 90\% confidence level. Combining this with the recent measurement of the isospin-partner decay $B^+\to K^+\tau^+\tau^-$, we determine an upper limit $\mathcal{B}(B\to K\tau^+\tau^-) < 5.4\times10^{-4}$ at the 90\% confidence level.

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arXiv2026

Commissioning and Low Latency Operation of the Graph Neural Network Electromagnetic Calorimeter Trigger at the Belle II Experiment

We present the commissioning and operation of the Graph Neural Network Electromagnetic Calorimeter Trigger Module (GNN-ETM) of the Belle II experiment at the SuperKEKB collider. The GNN-ETM processes calorimeter trigger cells as graph nodes to perform clustering and feature extraction. We fully integrate the system with the successive stages of the first-level trigger, develop slow-control drivers, and add online monitoring capabilities. We optimise the existing FPGA-based architecture through hardware-algorithm co-design, achieving an overall system latency of 1.053 us. Our hardware implementation is validated through register-transfer-level simulations, achieving bit-accurate agreement with the offline reference model. Online monitoring enables the measurement of instantaneous trigger rates, providing a quantitative basis for trigger-level performance studies. In summary, we report on the GNN-ETM as a fully operational, low-latency trigger module with online control and monitoring capabilities, compatible with the latency requirements of the Belle II first-level trigger system.

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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

First measurement of the masses of the $\Upsilon_1(1D)$ and $\Upsilon_3(1D)$ states and the energy dependence of the cross sections for $e^+e^-\to\Upsilon_J(1D)\eta$ and $e^+e^-\to\Upsilon_J(1D)\pi^+\pi^-$

We study the processes $e^+e^-\to\Upsilon_J(1D)\eta$ and $e^+e^-\to\Upsilon_J(1D)\pi^+\pi^-$ at center-of-mass energies $\sqrt{s}$=(10.73 -- 11.02) GeV using a $142.5\,\mathrm{fb}^{-1}$ data sample, including 122~fb$^{-1}$ near the $\Upsilon$(10860) peak ($\sqrt{s}$ = 10.866 GeV), collected with the Belle detector at the KEKB asymmetric-energy $e^+e^-$ collider. From the peak sample, the products of Born cross section times branching fraction are obtained for $\sigma_{\rm Born}(e^+e^-\to\Upsilon_J(1D)\eta)$ or $\sigma_{\rm Born}(e^+e^-\to\Upsilon_J(1D)\pi^+\pi^-)$ and ${\cal B}(\Upsilon_J(1D)\to\chi_{b1}\gamma)$ or ${\cal B}(\Upsilon_J(1D)\to\chi_{b2}\gamma)$ for each $\Upsilon_J(1D)$ state. The corresponding branching fractions for $\Upsilon(10860)$ decays are also obtained. The significances of the $\Upsilon_1(1D)$, $\Upsilon_2(1D)$, and $\Upsilon_3(1D)$ signals are 4.8$\sigma$, ${>}10\sigma$, and 3.0$\sigma$, respectively, including systematic uncertainties. The mass for $\Upsilon_2(1D)$ is measured to be $(10167.0\pm 1.0\pm 0.2)$ MeV/$c^2$, where the first and second uncertainties are statistical and systematic. The mass splittings $\Delta m_{12}=m(\Upsilon_2(1D))-m(\Upsilon_1(1D))$ and $\Delta m_{23}=m(\Upsilon_3(1D))-m(\Upsilon_2(1D))$ are $(11.8\pm1.5\pm0.4)$ MeV/$c^2$ and $(7.6\pm2.4\pm0.6)$ MeV/$c^2$, respectively.~We determine the energy dependence of the cross sections for $e^+e^-\to\Upsilon_J(1D)\eta$ and $e^+e^-\to\Upsilon_J(1D)\pi^+\pi^-$ for the $\Upsilon_1(1D)$, $\Upsilon_2(1D)$, and $\Upsilon_3(1D)$ states, combined.

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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

First evidence of $X(3872)\to\pi^0\chi_{c0}(1P)$ and search for $X(3915)\to\pi^0\chi_{c1}(1P)$

We search for the pionic transitions $X(3872)\to\pi^0\chi_{cJ}(1P)$ $(J = 0,~1,~2)$ and $X(3915)\to\pi^0\chi_{c1}$ in $B^+\to \pi^0\chi_{cJ}K^+$ decays using the Belle and Belle~II data samples collected at the $\Upsilon(4S)$ resonance, corresponding to integrated luminosities of $711~\mathrm{fb}^{-1}$ and $492~\mathrm{fb}^{-1}$, respectively. We report the first evidence for the decay $X(3872)\to\pi^0\chi_{c0}$ with a significance of $3.4\sigma$, including systematic uncertainties. We measure the product of branching fractions ${\cal B}(B^+\to X(3872)K^+)\times{\cal B}(X(3872)\to\pi^0\chi_{c0})=(20.0\pm6.8\pm2.3)\times10^{-6}$ and the branching fraction ratio ${\cal B}(X(3872)\to\pi^0\chi_{c0})/{\cal B}(X(3872)\to\pi^+\pi^-J/\psi)=2.3\pm0.8\pm0.4$, where the first and second uncertainties are statistical and systematic, respectively. The upper limits at 90\% credibility on the products of branching fractions for the $\pi^0\chi_{c1}$ and $\pi^0\chi_{c2}$ modes are $7.5\times10^{-6}$ and $15.3\times10^{-6}$, respectively. The corresponding upper limits on the branching fraction ratios relative to the $\pi^+\pi^-J/\psi$ decay are $0.9$ and $1.8$. The measured branching fractions for $X(3872)\to\pi^0\chi_{cJ}$ are consistent with several theoretical predictions based on the hadronic molecular interpretation of the $X(3872)$. No significant signal is seen for the $X(3915)\to\pi^0\chi_{c1}$ decay, and we set the 90\% credibility upper limit of ${\cal B}(B^+\to X(3915)K^+)\times{\cal B}(X(3915)\to\pi^0\chi_{c1})<6.6\times10^{-6}$, while the decays for $J=0$ and 2 are forbidden by parity conservation.

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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

Observation of the decays $B^{+} \to \Sigma_{c}(2455)^{++} \bar{\Xi}_{c}^{\prime-}$ and $B^{0} \to \Sigma_{c}(2455)^{0} \bar{\Xi}_{c}^{\prime0}$

We report the first observation of the decays $B^{+} \to \Sigma_{c}(2455)^{++} \bar{\Xi}_{c}^{\prime-}$ and $B^{0} \to \Sigma_{c}(2455)^{0} \bar{\Xi}_{c}^{\prime0}$, with significances of $6.4\,\sigma$ and $5.3\, \sigma$, respectively, including systematic uncertainties. This analysis is based on data samples containing $771.6 \times 10^{6}$ $\Upsilon(4S)$ decays collected with the Belle detector at the KEKB collider and $520.6 \times 10^{6}$ $\Upsilon(4S)$ decays collected with the Belle~II detector at the SuperKEKB collider. The branching fractions are measured to be $\mathcal{B}(B^+ \to \Sigma_c(2455)^{++} \bar{\Xi}_c^{\prime -}) = (1.68 \pm 0.31 \pm 0.12^{+1.49}_{-0.54}) \times 10^{-3}$ and $\mathcal{B}(B^0 \to \Sigma_c(2455)^{0} \bar{\Xi}_c^{\prime 0}) = (1.28 \pm 0.32 \pm 0.10^{+0.30}_{-0.21}) \times 10^{-3}$, where the first and second uncertainties are statistical and systematic, respectively, and the third arises from the uncertainties in the absolute branching fractions of $\bar{\Xi}_{c}^{-}$ and $\bar{\Xi}_{c}^{0}$ decays. This result represents the first observation of $B$-meson decays into a pair of charmed baryon-antibaryon states belonging to the same $SU(3)$ flavor sextet.

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arXiv2026

Measurement of time-dependent $CP$ violation parameters in $B^{0} \to K_{S}^{0} \pi^{0} \gamma$ decays at Belle and Belle II

We perform a measurement of time-dependent $CP$ violation parameters in $B^{0} \to K_{S}^{0} \pi^{0} \gamma$ decays using a dataset of approximately $772 \times 10^6$ and $521 \times 10^6$ $\Upsilon(4S)$ decays collected by the Belle and Belle II experiments, respectively. The measured parameters for the combined dataset in the $K^{*0}(892)$ dominated region ($M_{K_{S}^{0} \pi^{0}} \in [0.8,1.0] \mathrm{GeV}/c^2$) are $S = 0.09 \pm 0.16 \pm 0.02$ and $C = -0.09 \pm 0.08 \pm 0.04$. For the non-$K^{*0}(892)$ region ($M_{K_{S}^{0} \pi^{0}} \in [1.0,1.8] \mathrm{GeV}/c^2$), the corresponding values are $S = -0.32 \pm 0.33 \pm 0.09$ and $C = -0.07 \pm 0.17 \pm 0.08$. The first quoted uncertainties are statistical, while the second ones are systematic. These results are consistent with Standard Model predictions and more precise than previous measurements.

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arXiv2026

Demonstrating CBM Capabilities by $\Lambda$ Baryon Reconstruction in Ni+Ni Collisions with the mCBM Experiment at SIS18 of GSI/FAIR

The Compressed Baryonic Matter (CBM) experiment at the upcoming Facility for Antiproton and Ion Research (FAIR) is a high-rate fixed-target experiment designed to investigate nuclear matter at extreme baryon densities in relativistic nucleus-nucleus collisions. To enable high-statistics measurements of rare probes, CBM is designed to operate at event rates up to 10 MHz. This necessitates the development of fast and radiation-tolerant detectors, self-triggered front-end electronics, a free-streaming data acquisition architecture, and real-time event reconstruction capabilities. Prototype versions and pre-series productions of the CBM detector systems have been deployed in the mini-CBM demonstrator setup mCBM - an experimental precursor comprising sub-components of all major CBM systems, installed at the SIS18 facility of GSI/FAIR within the FAIR Phase-0 program. In 2024, Ni+Ni collisions at a kinetic beam energy of 1.93 AGeV and an average interaction rate of about 250 kHz were successfully recorded. This dataset enables a detailed evaluation of the operational performance of the detector systems as well as the complete CBM data chain, while the reconstruction of rare $\Lambda$ baryons serves as a natural benchmark. This paper presents the first results on $\Lambda$ signal reconstruction with the mCBM experiment, demonstrating the readiness of the detector technologies and the data chain for the upcoming full-scale CBM experiment.

physics.ins-det↗

arXiv2026

Search for $\Xi^0p$, $\Omega^- p$, and $\Omega^- n$ dibaryons in $\Upsilon(1S)$ and $\Upsilon(2S)$ decays at Belle

We search for $\Xi^0p$, $\Omega^-p$, and $\Omega^-n$ dibaryon states in $\Upsilon(1S)$ and $\Upsilon(2S)$ decays, probing mass regions near the corresponding baryon-pair thresholds. Multistrange baryon-baryon interactions are relevant to neutron-star matter but remain largely unconstrained. Experimental and theoretical studies supporting attractive $\Xi N$ and $\Omega N$ interactions, where $N$ denotes a nucleon, motivate searches for weakly bound states. We use samples of $102$ million $\Upsilon(1S)$ and $158$ million $\Upsilon(2S)$ decays collected with the Belle detector at the KEKB asymmetric-energy $e^+e^-$ collider. No significant signals are observed, and the first $90$% confidence-level upper limits are set on the branching fractions of $\Upsilon(1S)$ and $\Upsilon(2S)$ decays to $\Xi^0p$, $\Omega^-p$, and $\Omega^-n$ dibaryon states, at the level of $O(10^{-7})$-$O(10^{-6})$, depending on the channel and the assumed mass difference from the corresponding baryon-pair threshold.

hep-ex↗

arXiv2026

Search for the decay $B^+ \rightarrow K^+\tau^+\tau^-$ using data from the Belle and Belle II experiments

We report a search for the rare decay $B^{+} \rightarrow K^{+} \tau^{+} \tau^{-}$ using $1.2 \times 10^9$ $\Upsilon(4S)$ mesons produced near threshold in electron-positron collisions and collected by the Belle and Belle~II experiments. We fully reconstruct the hadronic decay of one $B$ meson produced in the $\Upsilon(4S)\rightarrow B^{+} B^{-}$ decay, and search for $B^{\pm}\rightarrow K^{\pm} \tau^{+}\tau^{-}$ candidates among the remaining collision products, reconstructing a charged kaon and leptonic decays of the $\tau$ leptons. We optimize the selection for best sensitivity and look for an excess over background at low values of the residual energy detected in the calorimeter after full event reconstruction. We observe no significant excess and set the limit $\mathcal{B}(B^{+}\rightarrow K^{+}\tau^{+}\tau^{-})< 0.56\times 10^{-3}$ at the 90% confidence level, improving on the only previous result by a factor of four.

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arXiv2026

Searches for charged-lepton-flavor violation in $\chi_{bJ}(1P)$ decays

We report the first searches for charged-lepton-flavor violation in decays of $\chi_{bJ}(1P)$ ($J=0, 1,$ and $2$) to a pair of charged leptons using 158 million $\Upsilon(2S)$ decays collected with the Belle detector in $e^+e^-$ collisions at the KEKB collider. No significant signal is observed, and we set upper limits on the branching fractions for $\chi_{bJ}(1P)$ decays to $e^\pm\mu^\mp$ at the level of $10^{-6}$ and to $e^\pm\tau^\mp$ or $\mu^\pm\tau^\mp$ at the level of $10^{-5}$. Limits on $\chi_{b0}(1P)$ decays are translated into bounds on the corresponding Wilson coefficients of scalar operators that mediate charged-lepton-flavor violation.

hep-ex↗

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.

hep-ex↗

arXiv2026

Real-time graph neural networks on FPGAs for the Belle II electromagnetic calorimeter

We present the development and evaluation of a real-time Graph Neural Network-based trigger module for the electromagnetic calorimeter of the Belle~II experiment at the SuperKEKB collider. The algorithm processes calorimeter trigger cells as graph nodes to perform clustering, feature extraction, and per-cluster signal classification with deterministic latency. The model predicts cluster positions and energies and provides a signal classification score, enabling a more flexible clustering strategy than the baseline trigger algorithm. Implemented on an FPGA and integrated into the Belle~II trigger readout infrastructure for synchronous operation, the system sustains the MHz trigger throughput with an end-to-end latency of $3.168\,\mu$s. The performance is evaluated on simulated events and collision data. The energy resolution is comparable to the baseline trigger, while the position resolution for high-energy clusters improves by up to 18% in the central detector region. Cluster purity increases by up to 20% at low energies for isolated clusters, and cluster efficiency improves by up to 20% for overlapping clusters. The signal classifier enables additional background suppression at fixed signal retention. These results demonstrate a first step towards GNN-based real-time reconstruction on FPGAs in a collider trigger. While the end-to-end latency exceeds the trigger decision budget, the system already sustains full operational conditions with 100% uptime.

physics.ins-det↗

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.

hep-ex↗

arXiv2026

Study of $B^+ \to \mu^+ \nu_\mu$ decays at Belle and Belle II

We report a measurement of the branching fraction for the leptonic decay $B^+\to\mu^+\nu_\mu$. This work presents the first $B^+\to\mu^+\nu_\mu$ result using Belle~II data, an updated Belle measurement that supersedes the previous result, and their combination, which yields the most precise search to date. The analysis is based on $1076\,\mathrm{fb}^{-1}$ of $e^+e^-$ collision data collected at a center-of-mass energy of $10.58\,\mathrm{GeV}$ with the Belle and Belle~II detectors at the KEKB and SuperKEKB colliders, respectively. We measure $\mathcal{B}(B^+\to\mu^+\nu_\mu)=(4.4\pm1.9\pm 1.0)\times10^{-7}$, where the first uncertainty is statistical and the second systematic. The observed significance relative to the background-only hypothesis is 2.4 standard deviations. We set a 90\% confidence level upper limit of $\mathcal{B}(B^+\to\mu^+\nu_\mu)<6.7\times10^{-7}$ using a frequentist approach and a 90\% credibility level upper limit of $\mathcal{B}(B^+\to\mu^+\nu_\mu)<7.2\times 10^{-7}$ using a Bayesian approach. These are the most stringent limits to date. The result is interpreted as an exclusion region in the parameter space of type~II and type~III two-Higgs-doublet models. We search for stable sterile neutrinos with masses $m_N\in[0,1.5]\,\mathrm{GeV}$. No signal is observed, and the resulting exclusion on the squared mixing parameter $|U_{\mu N}|^2$ provides improvement over previous limits. We report a measurement of the partial branching fraction of semileptonic $B\to X_u\ell\nu_\ell$ decays with $p_\mu^B>2.2\,\mathrm{GeV}$, obtaining $\Delta\mathcal{B}(B\to X_u\ell\nu_\ell)=(2.72\pm0.05\pm0.29)\times10^{-4}$. We present a model-dependent study of weak annihilation decays using the muon momentum spectrum. We observe a signal of 2.4 standard deviations above the background-only hypothesis in regions where the distribution resembles that of $B\to X_u\ell\nu_\ell$ decays.

hep-ex↗