Giacomo De Pietro

We study the processes e + e − → ϒ J ( 1 D ) η and e + e − → ϒ J ( 1 D ) π + π − at center-of-mass energies s = ( 10.73 − 11.02 ) GeV using a 142.5 fb − 1 data sample, including 122 fb − 1 near the ϒ ( 10860 ) peak ( 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 σ Born ( e + e − → ϒ J ( 1 D ) η ) or σ Born ( e + e − → ϒ J ( 1 D ) π + π − ) and B ( ϒ J ( 1 D ) → χ b 1 γ ) or B ( ϒ J ( 1 D ) → χ b 2 γ ) for each ϒ J ( 1 D ) state. The corresponding branching fractions for ϒ ( 10860 ) decays are also obtained. The significances of the ϒ 1 ( 1 D ) , ϒ 2 ( 1 D ) , and ϒ 3 ( 1 D ) signals are 4.8 σ , > 10 σ , and 3.0 σ , respectively, including systematic uncertainties. The mass for ϒ 2 ( 1 D ) is measured to be ( 10167.0 ± 1.0 ± 0.2 ) MeV / c 2 , where the first and second uncertainties are statistical and systematic. The mass splittings Δ m 12 = m ( ϒ 2 ( 1 D ) ) − m ( ϒ 1 ( 1 D ) ) and Δ m 23 = m ( ϒ 3 ( 1 D ) ) − m ( ϒ 2 ( 1 D ) ) are ( 11.8 ± 1.5 ± 0.4 ) MeV / c 2 and ( 7.6 ± 2.4 ± 0.6 ) MeV / c 2 , respectively. We determine the energy dependence of the cross sections for e + e − → ϒ J ( 1 D ) η and e + e − → ϒ J ( 1 D ) π + π − for the ϒ 1 ( 1 D ) , ϒ 2 ( 1 D ) , and ϒ 3 ( 1 D ) states, combined.

We report the first observation of the decays B + → Σ c ( 2455 ) + + Ξ ¯ c ′ − and B 0 → Σ c ( 2455 ) 0 Ξ ¯ c ′ 0 , with significances of 6.4 σ and 5.3 σ , respectively, including systematic uncertainties. This analysis is based on data samples containing 771.6 × 10 6 ϒ ( 4 S ) decays collected with the Belle detector at the KEKB collider and 520.6 × 10 6 ϒ ( 4 S ) decays collected with the Belle II detector at the SuperKEKB collider. The branching fractions are measured to be B ( B + → Σ c ( 2455 ) + + Ξ ¯ c ′ − ) = ( 1.69 ± 0.31 ± 0.1 2 − 0.56 + 1.39 ) × 10 − 3 and B ( B 0 → Σ c ( 2455 ) 0 Ξ ¯ c ′ 0 ) = ( 1.28 ± 0.32 ± 0.1 0 − 0.22 + 0.26 ) × 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 Ξ ¯ c − and Ξ ¯ 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 S U ( 3 ) flavor sextet.

We report the first searches for charged-lepton-flavor violation in decays of χ b J ( 1 P ) ( J = 0 , 1, and 2) to a pair of charged leptons using 158 million ϒ ( 2 S ) 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 b J ( 1 P ) 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 b 0 ( 1 P ) decays are translated into bounds on the corresponding Wilson coefficients of scalar operators that mediate charged-lepton-flavor violation.

Using a data sample of 427.9 fb − 1 collected by the Belle II detector at or near the ϒ ( 4 S ) and ϒ ( 10753 ) resonances, the cross sections for e + e − → h + h − J / ψ ( h = π / 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 − → π + π − J / ψ and e + e − → K + K − J / ψ 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 − → p p ¯ J / ψ 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 charmoniumlike states in the h + h − J / ψ systems, and for associated intermediate states in the h ± J / ψ systems, are also presented.

We report a measurement of the branching fraction for the leptonic decay B + → μ + ν μ . This work presents the first B + → μ + 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 + → μ + 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 + → μ + ν μ ) < 6.7 × 10 − 7 using a frequentist approach and a 90% credibility level upper limit of B ( B + → μ + 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 μ N | 2 provides improvement over previous limits. We report a measurement of the partial branching fraction of semileptonic B → X u ℓ ν ℓ decays with p μ B > 2.2 GeV , obtaining Δ B ( B → X u ℓ ν 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 ν ℓ decays.

We report the first observation of the radiative decay D s 0 * ( 2317 ) + → D s * + γ with a statistical significance exceeding 10 standard deviations. The signal is observed in the continuum e + e − → c c ¯ 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 s 0 * ( 2317 ) + → D s * + γ ] / B [ D s 0 * ( 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 s 0 * ( 2317 ) + structure.

We present a search for an invisible hidden-sector particle X inv , produced in B 0 → D ¯ 0 X inv and B ± → h X inv decays, where h = π ± , K ± , D s ± , p ± . 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 B ( B → h X inv ) as a function of m X inv . Corresponding limits are set on B ( B → h X ) × B ( X → γ γ ) for lifetimes c τ 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.

We report the first search for the flavor-changing neutral-current decays B → X s ν ν ¯ , 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 ( 4 S ) resonance and 43 fb − 1 collected at a center-of-mass energy 60 MeV below resonance for estimation of e + e − → q q ¯ continuum background. One of the B mesons from the ϒ ( 4 S ) → B B ¯ decay is fully reconstructed in a hadronic decay mode. The B → X s ν ν ¯ 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 ν ν ¯ ) < 3.3 × 10 − 4 .