arXiv subjects

G. de Marino

Publications and source records attributed to G. de Marino.

At least 19 recordsLinked to original sources

arXiv2026

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

We report a search for the rare decay $B^{+} \rightarrow K^{+} τ^{+} τ^{-}$ using $1.2 \times 10^9$ $Υ(4S)$ 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 $Υ(4S)\rightarrow B^{+} B^{-}$ decay, and search for $B^{\pm}\rightarrow K^{\pm} τ^{+}τ^{-}$ candidates among the remaining collision products, reconstructing a charged kaon and leptonic decays of the $τ$ 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 $\mathcal{B}(B^{+}\rightarrow K^{+}τ^{+}τ^{-})< 0.56\times 10^{-3}$ at the 90% confidence level, improving on the only previous result by a factor of four.

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arXiv2026

Test of lepton flavor universality with $\bar{B} \rightarrow D^{()} τ^{-} \barν_τ$ and $\bar{B} \rightarrow D^{()} \ell^{-} \barν_{\ell}$ decays at Belle II

We test lepton flavor universality with a measurement of the branching-fraction ratios $R(D^{(*)}) \equiv \mathcal{B}(\bar{B} \rightarrow D^{(*)} τ^{-} \barν_τ)/\mathcal{B}(\bar{B} \rightarrow D^{(*)} \ell^{-} \barν_{\ell})$, where $\ell$ denotes an electron or muon. The analysis uses $387\times 10^6$ $Υ(\mathrm{4S})$ decays collected with the Belle II detector in energy-asymmetric $e^+e^-$ collisions. One $B$ meson is fully reconstructed in a hadronic decay mode, while the other is reconstructed either in $\bar{B}\rightarrow D^{(*)}τ^{-}\barν_τ$, with $τ^- \rightarrow \ell^- \barν_{\ell}ν_τ$, or in $\bar{B}\rightarrow D^{(*)}\ell^{-}\barν_{\ell}$. We extract the signal from the distributions of the residual calorimeter energy and the squared mass of the undetected particles, obtaining $R(D^{*}) = 0.242 \pm0.019(\mathrm{stat}) \pm0.016(\mathrm{syst})$ and $R(D) = 0.439 \pm 0.055(\mathrm{stat}) \pm 0.046(\mathrm{syst})$. These results are consistent with both the standard model predictions and previous measurements, and constitute the most precise determination of $R(D^{(*)})$ with hadronic tagging.

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arXiv2026

Search for the lepton-flavor-violating decay $ τ^{\pm} \to μ^{\pm} γ$ at Belle II

We present a search for the lepton-flavor-violating decay $τ^{\pm}\toμ^{\pm}γ$ 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 $\mathcal{B}(τ^{\pm}\toμ^{\pm}γ) < 9.5$ $ (12.2)\times10^{-8}$ at the 90\% (95\%) confidence level, using the CL${_s}$ technique.

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arXiv2026

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

We present the first search for $B^0 \to K^0_{\rm S} τ^+τ^-$ 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 $Υ(4S)$ events. In $Υ(4S)\to B^0\bar{B}^0$ decays, the non-signal $\bar{B}^0$ meson is fully reconstructed in a hadronic channel. For the signal $B^0$ meson, $τ$-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 $\mathcal{B}(B^0\to K^0_{\rm S} τ^+τ^-) < 8.3 \times 10^{-4}$ at the 90\% confidence level. Combining this with the recent measurement of the isospin-partner decay $B^+\to K^+τ^+τ^-$, we determine an upper limit $\mathcal{B}(B\to Kτ^+τ^-) < 5.4\times10^{-4}$ at the 90\% confidence level.

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arXiv2026

Study of $B^+ \to μ^+ ν_μ$ decays at Belle and Belle II

We report a measurement of the branching fraction for the leptonic decay $B^+\toμ^+ν_μ$. This work presents the first $B^+\toμ^+ν_μ$ 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 $\mathcal{B}(B^+\toμ^+ν_μ)=(4.4\pm1.9\pm 1.0)\times10^{-7}$, 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 $\mathcal{B}(B^+\toμ^+ν_μ)<6.7\times10^{-7}$ using a frequentist approach and a 90\% credibility level upper limit of $\mathcal{B}(B^+\toμ^+ν_μ)<7.2\times 10^{-7}$ 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 $|U_{μN}|^2$ provides improvement over previous limits. We report a measurement of the partial branching fraction of semileptonic $B\to X_u\ellν_\ell$ decays with $p_μ^B>2.2\,\mathrm{GeV}$, obtaining $Δ\mathcal{B}(B\to X_u\ellν_\ell)=(2.72\pm0.05\pm0.29)\times10^{-4}$. 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 $B\to X_u\ellν_\ell$ decays.

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arXiv2026

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

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 $B^0\toπ^0π^0$ decays. The measurement uses 190 million $Υ(4S)\to B^{0}\overline{B}{}^0$ 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 $B^0 \to π^0π^0$ decay position. We measure the mixing-induced CP-violating coefficient to be $S_{π^{0}π^{0}} = 0.61^{+0.75}_{-0.79}\,(\mathrm{stat}) \pm 0.11\,(\mathrm{syst})$, 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 $C_{π^{0}π^{0}} = 0.05 \pm 0.28\,(\mathrm{stat}) \pm 0.07\,(\mathrm{syst})$. These results substantially improve the constraints from $B \to ππ$ decays on the CP-violating phase $ϕ_2$ of the quark-mixing matrix using far less data than had previously been assumed necessary.

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arXiv2026

First search for $B \rightarrow X_{s} ν\barν$ decays

We report the first search for the flavor-changing neutral-current decays $B \rightarrow X_{s} ν\barν$, 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 $Υ(4S)$ 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 $Υ(4S) \to B\bar{B}$ decay is fully reconstructed in a hadronic decay mode. The $B \to X_s ν\barν$ 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, $B(B \to X_s ν\barν) < 3.3 \times 10^{-4}$.

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arXiv2026

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

We perform a measurement of time-dependent $CP$ violation parameters in $B^{0} \to K_{S}^{0} π^{0} γ$ decays using a dataset of approximately $772 \times 10^6$ and $521 \times 10^6$ $Υ(4S)$ decays collected by the Belle and Belle II experiments, respectively. The measured parameters for the combined dataset in the $K^{*0}(892)$ dominated region ($M_{K_{S}^{0} π^{0}} \in [0.8,1.0] \mathrm{GeV}/c^2$) 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 ($M_{K_{S}^{0} π^{0}} \in [1.0,1.8] \mathrm{GeV}/c^2$), 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

A search for feebly-interacting particles in $B$ decays with missing energy at Belle

We present a search for an invisible hidden-sector particle $ X_{\rm{inv}}$, produced in $B^0 \to \bar{D}^0 X_{\rm{inv}}$ and $B^\pm\to h X_{\rm{inv}}$ decays, where $h = π^\pm$, $K^\pm$, $D_{s}^\pm$, $p^\pm$. The search is performed using $e^+ e^-$ collision data recorded with the Belle detector, corresponding to 711~fb$^{-1}$. No significant signal is observed. We set 90\% confidence level upper limits ranging between $10^{-4}$ and $10^{-6}$ on the branching fraction $\mathcal{B}(B \to h X_{\rm{inv}})$ as a function of $m_{ X_{\rm{inv}}}$. Corresponding limits are set on $\mathcal{B}(B \to hX)\times\mathcal{B}(X\toγγ)$ for lifetimes $cτ_X$ between $10~μ$m and 10 m. Many of these limits are the first direct constraints on their respective decays. Our results provide the most stringent exclusion limits to date on the branching fractions for all search channels, and exclude previously unexplored regions of parameter space relevant to several new physics models.

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arXiv2025

Search for a Dark Higgs Boson Produced in Association with Inelastic Dark Matter at the Belle II Experiment

Inelastic dark matter models that have two dark matter particles and a massive dark photon can reproduce the observed relic dark matter density without violating cosmological limits. The mass splitting between the two dark matter particles $χ_{1}$ and $χ_{2}$, with $m(χ_{2}) > m(χ_{1})$, is induced by a dark Higgs field and a corresponding dark Higgs boson $h^{\prime}$. We present a search for dark matter in events with two vertices, at least one of which must be displaced from the interaction region, and missing energy. Using a $365\,\mbox{fb}^{-1}$ data sample collected at Belle II, which operates at the SuperKEKB $e^+e^-$ collider, we observe no evidence for a signal. We set upper limits on the product of the production cross section $σ\left(e^+e^- \to h^\prime χ_1 χ_2\right)$, and the product of branching fractions $\mathcal{B}\left(χ_2\toχ_1 e^+ e^-\right)\times\mathcal{B}\left(h^\prime\to x^+x^-\right)$, where $x^+x^-$ indicates $μ^+μ^-, π^+π^-$, or $K^+K^-$, as functions of $h^{\prime}$ mass and lifetime at the level of $10^{-1}\,\mbox{fb}$. We set model-dependent upper limits on the dark Higgs mixing angle at the level of $10^{-5}$ and on the dark photon kinetic mixing parameter at the level of $10^{-3}$. This is the first search for dark Higgs bosons in association with inelastic dark matter.

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arXiv2025

Observation of $e^+e^-\toηΥ(2S)$ and search for $e^+e^-\toηΥ(1S),~γX_b$ at $\sqrt{s}$ near 10.75 GeV

We present an analysis of the processes $e^{+}e^{-}\toηΥ(1S)$, $ηΥ(2S)$, and $γX_b$ with $X_b\toπ^+π^-χ_{bJ},~χ_{bJ}\toγΥ(1S)$ $(J=1,~2)$ reconstructed from $γγπ^+π^-\ell^+\ell^-~(\ell=e,~μ)$ final states in $19.6\rm~\rm fb^{-1}$ of Belle II data collected at four energy points near the peak of the $Υ(10753)$ resonance. Here, $X_b$ is a hypothetical bottomonium-sector partner of the $X(3872)$. In the process $e^{+}e^{-} \to ηΥ(2S)$, we observe a signal with a significance that exceeds $6.0σ$. Based on an analysis of the Born cross-section, the hypothesis that these events are associated solely with the production of the $Υ(5S)$ or $Υ(10753)$ resonances is rejected at the level of 3.6$σ$. 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 $e^{+}e^{-}\toηΥ(1S)$ or $γX_b$. Upper limits on the Born cross sections for the processes $e^{+}e^{-}\toηΥ(1S)$ and $e^{+}e^{-}\toγX_b$ with $X_b\toπ^+π^-χ_{bJ}$ are determined.

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arXiv2025

Model-agnostic likelihood for the reinterpretation of the $B^+\to K^+ν\barν$ measurement at Belle II

We recently measured the branching fraction of the $B^{+}\rightarrow K^{+}ν\barν$ 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

Measurements of the mass difference $m(B^0)-m(B^+)$ and the energy dependence of the cross-section ratio $σ(e^+e^-\to B^0\bar{B}^0) / σ(e^+e^-\to B^+B^-)$ at Belle and Belle II

Using data samples collected by the Belle and Belle II experiments at the $Υ(4S)$ resonance with integrated luminosities of 571 fb$^{-1}$ and 365 fb$^{-1}$, respectively, we measure the pseudoscalar $B$-meson mass difference to be $m(B^0)-m(B^+) = (0.495\pm0.024\pm0.005)$ MeV/c$^2$. The results are based on a simultaneous fit to the variable $\tilde{M}_{bc}$, which is related to the $B$ momentum, for $B^0$ and $B^+$ candidates; and to the energy dependence of ${\cal R}=σ(e^+e^-\to B^0\bar{B}^0) / σ(e^+e^-\to B^+B^-)$, which is measured using changes in the average center-of-mass energy over the data taking periods. The phase-space hypothesis ${\cal R}=(p_{B^0}/p_{B^+})^3$, upon which previous measurements rely, is strongly disfavored by our fit; the measured mass-difference value for the phase-space hypothesis also differs significantly from our measurement. We constrain ${\cal R}$ in a broader energy range than covered by the direct measurement and extract the energy dependence of ${\cal R}$ in the range from the $B\bar{B}$ threshold up to 10.59 GeV. We interpret the results using a phenomenological model and constrain the parameters of the $B\bar{B}$ potential in the isovector channel.

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arXiv2025

Measurement of the $ D^{0}\rightarrow K^{-}π^{+}e^{+}e^{-} $ branching fraction and search for $ D^{0}\rightarrow π^{+}π^{-}e^{+}e^{-} $ and $D^{0}\rightarrow K^{+}K^{-}e^{+}e^{-} $ decays at Belle

We present a study of the rare charm meson decays $ D^{0}\rightarrow K^{+}K^{-}e^{+}e^{-} $, $ π^{+}π^{-}e^{+}e^{-} $, and $ K^{-}π^{+}e^{+}e^{-} $ using a 942 fb$^{-1}$ data set collected by the Belle detector at the KEKB asymmetric-energy $ e^{+}e^{-} $ collider. We use $ D^{0} $ candidates identified by the charge of the pion in $ D^{*} \rightarrow D^{0} π$ decays and normalize the branching fractions to $ D^{0} \rightarrow K^{-}π^{+}π^{-}π^{+} $ decays. The branching fraction for decay $ D^{0} \rightarrow K^{-}π^{+}e^{+}e^{-} $ is measured to be (39.6 $\pm$ 4.5 (stat) $\pm$ 2.9 (syst)) $\times$ $10^{-7}$, with the dielectron mass in the $ ρ/ω$ mass region $ 675 < m_{ee} < 875 $ MeV$/c^{2}$. We also search for $ D^{0}\rightarrow h^{-} h^{(\prime)+}e^{+}e^{-} $ ($ h^{(\prime)}=K,\,π$) decays with the dielectron mass near the $η$ and $ϕ$ resonances, and away from these resonances for the $ K^{+}K^{-}e^{+}e^{-} $ and $ π^{+}π^{-}e^{+}e^{-} $ modes. For these modes, we find no significant signals and set 90$\%$ confidence level upper limits on their branching fractions at the $\mathcal{O}$(10$^{-7}$) level.

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arXiv2025

Measurement of the time-integrated $CP$ asymmetry in $D^0 \to K^0_{\rm S} K^0_{\rm S}$ decays using opposite-side flavor tagging at Belle and Belle II

We measure the time-integrated $CP$ asymmetry in $D^0 \to K^0_{\rm S} K^0_{\rm S}$ decays reconstructed in $e^+e^-\to c{\overline c}$ events collected by the Belle and Belle II experiments. The corresponding data samples have integrated luminosities of 980 and 428 fb${}^{-1}$, respectively. To infer the flavor of the $D^0$ meson, we exploit the correlation between the flavor of the reconstructed decay and the electric charges of particles reconstructed in the rest of the $e^+e^-\to c{\overline c}$ event. This results in a sample which is independent from any other previously used at Belle or Belle II. The result, $A_{CP}(D^0 \to K^0_{\rm S} K^0_{\rm S}) = (1.3 \pm 2.0 \pm 0.2)\%$, where the first uncertainty is statistical and the second systematic, is consistent with previous determinations and with $CP$ symmetry.

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arXiv2025

Search for lepton flavor-violating decay modes $B^0 \to K^{\ast 0}τ^\pm\ell^\mp$ ($\ell = e,μ$) with hadronic B-tagging at Belle and Belle II

We present the results of a search for the charged-lepton-flavor violating decays $B^0 \rightarrow K^{*0}τ^\pm \ell^{\mp}$, where $\ell^{\mp}$ is either an electron or a muon. The results are based on 365 fb$^{-1}$ and 711 fb$^{-1}$ datasets collected with the Belle II and Belle detectors, respectively. We use an exclusive hadronic $B$-tagging technique, and search for a signal decay in the system recoiling against a fully reconstructed $B$ meson. We find no evidence for $B^0 \rightarrow K^{*0}τ^\pm \ell^{\mp}$ decays and set upper limits on the branching fractions in the range of $(2.9-6.4)\times10^{-5}$ at 90% confidence level.

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arXiv2025

Measurement of $C\!P$ violation in $B^{0}\rightarrow K_{S}^{0}π^{0}$ decays at Belle II

We report a measurement of the $C\!P$-violating parameters $A$ and $S$ in $B^{0}\to K_{S}^{0} π^{0}$ decays at Belle II using a sample of $387\times 10^{6}$ $B\bar{B}$ events recorded in $e^{+}e^{-}$ collisions at a center-of-mass energy corresponding to the $Υ(4S)$ resonance. These parameters are determined by fitting the proper decay-time distribution of a sample of 415 signal events. We obtain $A = 0.04^{+0.15}_{-0.14}\pm 0.05$ and $S = 0.75^{+0.20}_{-0.23}\pm 0.04$, where the first uncertainties are statistical and the second are systematic.

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arXiv2025

Search for lepton-flavor-violating $τ^- \to \ell^- K_s^0$ decays at Belle and Belle II

We present the results of a search for charged-lepton-flavor violating decays $τ^{-} \rightarrow \ell^{-}K_{S}^{0}$, where $\ell^{-}$ is either an electron or a muon. We combine $e^+e^-$ data samples recorded by the Belle II experiment at the SuperKEKB collider (428 fb$^{-1}$) with samples recorded by the Belle experiment at the KEKB collider (980 fb$^{-1}$) to obtain a sample of 1.3 billion $e^+e^-\toτ^+τ^-$ events. We observe 0 and 1 events and set $90\%$ confidence level upper limits of $0.8 \times 10^{-8}$ and $1.2 \times 10^{-8}$ on the branching fractions of the decay modes $τ^{-} \rightarrow e^{-}K_{S}^{0}$ and $τ^{-} \rightarrow μ^{-}K_{S}^{0}$, respectively. These are the most stringent upper limits to date.

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