We analyze the potential of rare charm decays as probes of new physics at a high-luminosity flavor facility operating at the Z pole, such as the FCC-ee or CEPC. In particular, we identify clean null-test observables in D-0 -> pi(+)pi(-) nu(nu) over bar and in polarized Lambda(+)(c) -> pl(+)l(-) decays with l = e, mu. Complementarity with the LHC and HL-LHC flavor programs arises from the characteristic features of a Tera- Z environment: the capability to study missing-energy modes and charm production with significant polarization. We improve the theoretical description of D-0 -> pi(+)pi(-) nu(nu) over bar decays and work out the phenomenology of polarization-induced null-test observables in Lambda(+)(c) -> pl(+)l(-) decays. In regions of dilepton mass near the phi resonance, polarization asymmetries can reach O(5%) for muons and O(14%) for electrons times the Lambda(+)(c) polarization. We also point out synergies between the dineutrino and the dilepton modes using the SMEFT framework of heavy new physics. Using the IDEA detector concept at FCC-ee, we find in simulation studies that dineutrino branching fractions as low as similar to 2 x 10(-7) can be probed, which reaches well into the parameter space of new physics, and also allows for discrimination of lepton flavor structures. Furthermore, the measurement of asymmetries in Lambda(+)(c) -> p mu(+)mu(-) at O(1%) will be possible. Similar sensitivities are expected for dielectron final states, although robust predictions will require further dedicated studies.

We present an analysis of the processes e(+)e(-) -> eta(sic)(1S), eta(sic)(2S), and gamma X-b with X-b -> pi(+) pi(-) chi(bJ), chi(bJ) -> gamma(sic)(1S) (J = 1, 2) reconstructed from gamma gamma pi(+)pi(-) l(+)l(-) (l = e; mu) final states in 19.6 fb(-1) of Belle II data collected at four energy points near the peak of the (sic)(10753) resonance. Here, X-b is a hypothetical bottomonium-sector partner of the X(3872). In the process e(+)e(-) (2S), we observe a signal with a significance that exceeds 6.0 sigma when all energy points are combined. A signal with a significance of 4.2 sigma is observed at a center-of-mass energy of 10.653 GeV, close to the B*(B) over bar* threshold. At this energy, we measure sigma(eta(sic)(2S)) /sigma(pi(+)pi(-)(2S)> 23 at the 90% confidence level, indicating a strong violation of heavy-quark spin symmetry. No significant signal is observed for e(+)e(-) -> eta(sic)(1S) or gamma X-b. Upper limits on the Born cross sections for the processes e(+)e(-) ->eta(sic)(1S) and e(+)e(-) ->gamma X-b with X-b -> pi(+)pi(-) chi(bJ) are determined.

We report the first search for the flavor-changing neutral-current decays B→X_{s}νν[over ¯], 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  fb^{-1} collected at the ϒ(4S) resonance and 43  fb^{-1} collected at a center-of-mass energy 60 MeV below resonance for estimation of e^{+}e^{-}→qq[over ¯] continuum background. One of the B mesons from the ϒ(4S)→BB[over ¯] decay is fully reconstructed in a hadronic decay mode. The B→X_{s}νν[over ¯] 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, M_{X_{s}}^{true}, chosen to separate contributions from K, K^{*}(892), and heavier strange final states. 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}}^{true}<0.6  GeV/c^{2}, 0.6<M_{X_{s}}^{true}<1.0  GeV/c^{2}, and 1.0  GeV/c^{2}<M_{X_{s}}^{true} of 2.2×10^{-5}, 9.3×10^{-5}, and 30.9×10^{-5}, respectively. Combining the three mass regions, we obtain the upper limit on the branching fraction, B(B→X_{s}νν[over ¯])<3.3×10^{-4}.

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^-, a, using a data sample corresponding to an integrated luminosity of 408 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 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_aγγ, reaching sensitivities at the level of 10^-4 GeV^-1. The limits are the most restrictive to date over nearly the entire mass range 0.17 < m_a < 5.00 GeV/c^2, and improve upon previous results by up to a factor 9.

We report the first search for the flavor-changing neutral-current decays B -> X-s nu(nu) over bar, 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 fb(-1) collected at the Upsilon(4S) resonance and 43 fb(-1) collected at a center-of-mass energy 60 MeV below resonance for estimation of e(+)e(-) -> q (q) over bar continuum background. One of the B mesons from the Upsilon(4S) -> B (B) over bar decay is fully reconstructed in a hadronic decay mode. The B -> X-s nu(nu) over bar 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, M-Xs(true), chosen to separate contributions from K, K+(892), and heavier strange final states. 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-Xs(true) < 0.6 GeV/c(2), 0.6 < M-Xs(true) < 1.0 GeV/c(2), and 1.0 GeV/c(2) < M-Xs(true) of 2.2 x 10(-5), 9.3 x 10(-5), and 30.9 x 10(-5), respectively. Combining the three mass regions, we obtain the upper limit on the branching fraction, B(B -> X-s nu(nu) over bar) < 3.3 x 10(-4).

We report the first searches for charged-lepton-flavor violation in decays of χ_{bJ}(1P) (J=0, 1, and 2) to a pair of charged leptons using 158 million ϒ(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 χ_{bJ}(1P) decays to e^{±}μ^{∓} at the level of 10^{-6} and to e^{±}τ^{∓} or μ^{±}τ^{∓} at the level of 10^{-5}. Limits on χ_{b0}(1P) decays are translated into bounds on the corresponding Wilson coefficients of scalar operators that mediate charged-lepton-flavor violation.

We search for the pionic transitions X(3872)^0χ_cJ(1P) (J = 0, 1, 2) and X(3915)^0χ_c1 in B^+→ π^0χ_cJK^+ decays using the Belle and Belle II data samples collected at the Υ(4S) resonance, corresponding to integrated luminosities of 711 fb^-1 and 492 fb^-1, respectively. We report the first evidence for the decay X(3872)^0χ_c0 with a significance of 3.4σ, including systematic uncertainties. We measure the product of branching fractions B(B^+→ X(3872)K^+)× B(X(3872)^0χ_c0)=(20.0±6.8±2.3)×10^-6 and the branching fraction ratio B(X(3872)^0χ_c0)/ B(X(3872)^+π^-J/ψ)=2.3±0.8±0.4, 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 π^0χ_c1 and π^0χ_c2 modes are 7.5×10^-6 and 15.3×10^-6, respectively. The corresponding upper limits on the branching fraction ratios relative to the π^+π^-J/ψ decay are 0.9 and 1.8. The measured branching fractions for X(3872)^0χ_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)^0χ_c1 decay, and we set the 90% credibility upper limit of B(B^+→ X(3915)K^+)× B(X(3915)^0χ_c1)<6.6×10^-6, while the decays for J=0 and 2 are forbidden by parity conservation.

We report decay-rate CP asymmetries of the singly-Cabibbo-suppressed decays Xi(+)(c) -> Sigma(+) h(+) h(-) and Lambda(+)(c) -> 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, A(CP) ( Xi(+)(c) -> Sigma(+) K+ K-) = (3.7 +/- 6.6 +/- 0.6)%, A(CP) ( Xi(+)(c) -> Sigma(+) pi(+) pi(-)) = (9.5 +/- 6.8 +/- 0.5)%, A(CP) (Lambda(+)(c) -> pK(+) K-) = (3.9 +/- 1.7 +/- 0.7)%, A(CP) (Lambda(+)(c) -> p pi(+) pi(-)) = (0.3 +/- 1.0 +/- 0.2)%, where the first uncertainties are statistical and the second systematic, agree with CP symmetry. From these results we derive the sums A(CP) ( Xi(+)(c) -> Sigma(+) pi(+) pi(-)) + A(CP) (Lambda(+)(c) -> pK(+) K-) = (13.4 +/- 7.0 +/- 0.9)%, A(CP) ( Xi(+)(c) -> Sigma(+) K+ K-) + A(CP) (Lambda(+)(c) -> p pi(+) pi(-)) = (4.0 +/- 6.6 +/- 0.7)%, 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.

We search for Ξ^0p, Ω^-p, and Ω^-n dibaryon states in Υ(1S) and Υ(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 ΞN and ΩN interactions, where N denotes a nucleon, motivate searches for weakly bound states. We use samples of 102 million Υ(1S) and 158 million Υ(2S) decays collected with the Belle detector at the KEKB asymmetric-energy e^+e^- collider. No significant signals are observed, and the first 90

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^0^0π^0 decays. The measurement uses 190 million Υ(4S)→ B^0B^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^0B^0 pair enables a decay-time-dependent analysis without reconstructing the B^0 → π^0π^0 decay position. We measure the mixing-induced CP-violating coefficient to be S_π^0π^0 = 0.61^+0.75_-0.79 (stat) ± 0.11 (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^0π^0 = 0.05 ± 0.28 (stat) ± 0.07 (syst). These results substantially improve the constraints from B → ππ decays on the CP-violating phase ϕ_2 of the quark-mixing matrix using far less data than had previously been assumed necessary.

We present a search for the lepton-flavor-violating decay τ^±^±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 ℬ(τ^±^±γ) < 9.5 (12.2)×10^-8 at the 90% (95%) confidence level, using the CL_s technique.

We present the first search for B^0 → K^0_ S τ^+τ^- decays. We look for signal decays in B^0B̅^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)→ B^0B̅^0 decays, the non-signal B̅^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 ℬ(B^0→ K^0_ S τ^+τ^-) < 8.3 × 10^-4 at the 90% confidence level. Combining this with the recent measurement of the isospin-partner decay B^+→ K^+τ^+τ^-, we determine an upper limit ℬ(B→ Kτ^+τ^-) < 5.4×10^-4 at the 90% confidence level.

We report a measurement of the branching fraction for the leptonic decay B^+^+ν_μ. This work presents the first B^+^+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 fb^-1 of e^+e^- collision data collected at a center-of-mass energy of 10.58 GeV with the Belle and Belle II detectors at the KEKB and SuperKEKB colliders, respectively. We measure ℬ(B^+^+ν_μ)=(4.4±1.9± 1.0)×10^-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 ℬ(B^+^+ν_μ)<6.7×10^-7 using a frequentist approach and a 90% credibility level upper limit of ℬ(B^+^+ν_μ)<7.2× 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∈[0,1.5] 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→ X_u_ℓ decays with p_μ^B>2.2 GeV, obtaining Δℬ(B→ X_u_ℓ)=(2.72±0.05±0.29)×10^-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→ X_u_ℓ decays.

We report the first observation of the radiative decay D_{s0}^{*}(2317)^{+}→D_{s}^{*+}γ with a statistical significance exceeding 10 standard deviations. The signal is observed in the continuum e^{+}e^{-}→cc[over ¯] process, using combined data samples of 980.4  fb^{-1} from Belle and 427.9  fb^{-1} from Belle II, collected at the KEKB and SuperKEKB asymmetric-energy e^{+}e^{-} colliders, respectively. The branching fraction ratio B[D_{s0}^{*}(2317)^{+}→D_{s}^{*+}γ]/B[D_{s0}^{*}(2317)^{+}→D_{s}^{+}π^{0}] is measured to be [7.13±0.70(stat)±0.26(syst)]%. This result provides crucial discrimination between theoretical models of the D_{s0}^{*}(2317)^{+} structure.

We study the processes χbJ ω and χbJ (π+π−π0)non−ω (J = 0, 1, 2) at center-of-mass energies √(s) from 10.73 to 11.02 GeV using a 142.5 fb−1 data sample collected with the Belle detector at the KEKB asymmetric-energy e+e− collider; and at √(s)∼ 10.75 GeV using a 19.8 fb−1 sample collected with Belle II at SuperKEKB. We find that the Υ(10753) state decays into χbJ ω but not into χbJ (π+π−π0)non−ω, while the Υ(10860) state, in contrast, decays into χbJ (π+π−π0)non−ω but not into χbJ ω. The mass and width of the Υ(10753) state are measured to be (10756.1 ± 3.4(stat.) ± 2.7(syst.)) MeV/c2 and (32.2 ± 11.3(stat.) ± 14.9(syst.)) MeV. The products of the partial width to e+e− and branching fractions for Υ(10753) → χb1 ω and Υ(10753) → χb2 ω are (1.57 ± 0.27(stat.) ± 0.22(syst.)) eV and (1.39 ± 0.41(stat.) ± 0.33(syst.)) eV.

We search for the e(+)e(-) -> gamma chi(bJ) (J = 0, 1, 2) processes at center-of-mass energies root 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(-) -> gamma chi(bJ) at each center-of-mass energy root s near 10.746 GeV. The upper limits at 90% confidence level on the Born cross sections for e(+)e(-) -> gamma chi(b1) are significantly smaller than the corresponding measured values fore e(+)e(-) -> omega chi(b1) and e(+)e(-) -> pi(+)pi(-) Upsilon(2S) at root s = 10.746 GeV.

We measure the time-integrated CP asymmetry, A _CP , in D^0 → K^0_S K^0_S decays reconstructed in e^+e^-→ cc̅ events collected by the Belle and Belle II experiments with the data sample corresponding to an integrated luminosities of 980 fb^-1 and 427 fb^-1 , respectively. The D^0 decays are required to come from the flavor-conserving D^*+→ D^0π ^+ decay ( D^*+ tag) to determine the charm flavor at production time. A control sample of D^*+ -tagged D^0→ K^+ K^- is used to correct for production and detection asymmetries. The result, A_CP(D^0→ K^0_S K^0_S ) = (-1.4 ± 1.3(stat) ± 0.1(syst))% , is consistent with previous determinations and with CP symmetry.

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(B0) − m(B+) = (0.495 ± 0.024 ± 0.005) MeV/c2. The results are based on a simultaneous fit to the variable M_bc , which is related to the B momentum, for B0 and B+ candidates; and to the energy dependence of ℛ=σ (e^+e^-→B^0B^0) / σ (e^+e^-→B^+B^-) , which is measured using changes in the average center-of-mass energy over the data taking periods. The phase-space hypothesis 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 ℛ in a broader energy range than covered by the direct measurement and extract the energy dependence of ℛ in the range from the BB threshold up to 10.59 GeV. We interpret the results using a phenomenological model and constrain the parameters of the BB potential in the isovector channel.

We perform a measurement of time-dependent CP violation parameters in B^0→ K_S^0 π^0 γ decays using a dataset of approximately 772 × 10^6 and 521 × 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∈ [0.8,1.0] GeV/c^2) are S = 0.09 ± 0.16 ± 0.02 and C = -0.09 ± 0.08 ± 0.04. For the non-K^*0(892) region (M_K_S^0 π^0∈ [1.0,1.8] GeV/c^2), the corresponding values are S = -0.32 ± 0.33 ± 0.09 and C = -0.07 ± 0.17 ± 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.