Racha Cheaib

Results are presented on χ b 1 , 2 ( 2 P ) → ω ϒ ( 1 S ) transitions from e + e − → ϒ ( 3 S ) → γ χ b 1 , 2 ( 2 P ) decays. The data were collected with the detector at the PEP-II asymmetric-energy e + e − collider at SLAC. The integrated luminosity of the data sample is 28.0 fb − 1 , corresponding to 121.3 × 10 6 ϒ ( 3 S ) decays. Signals of χ b 1 , 2 ( 2 P ) are observed over a negligible background. Improved precision measurements of branching fractions are obtained. First measurements of the χ b 1 , 2 ( 2 P ) angular distributions are performed. No evidence is found for the presence of a χ b 0 ( 2 P ) → ω ϒ ( 1 S ) decay mode.

A bstract: We present the result of a search for the charged-lepton-flavor violating decays τ ⁻ → e ^∓ ℓ ^± ℓ ⁻ , where ℓ 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 ⁻ → τ ⁺ τ ⁻ 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 × 10 ^{− 8} are set at the 90% confidence level. These results are the most stringent bounds to date for four of the modes.

We present a study of the rare charm meson decays D 0 → K + K − e + e − , π + π − e + e − , and K − π + e + e − using a 942 fb − 1 dataset collected by the Belle detector at the KEKB asymmetric-energy e + e − collider. We identify D 0 candidates via the charge of the pion from D * + → D 0 π + decays and normalize the branching fractions to D 0 → K − π + π − π + decays. The branching fraction for decay D 0 → K − π + e + e − is measured to be ( 39.6 ± 4.5 ( stat ) ± 2.9 ( syst ) ) × 10 − 7 , with the dielectron mass in the ρ / ω mass region 675 < m e e < 875 MeV / c 2 . We also search for D 0 → h − h ( ′ ) + e + e − ( h ( ′ ) = 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 O ( 10 − 7 ) level.

We present a search for the rare flavor-changing neutral-current decay B 0 → K * 0 τ + τ − with data collected by the Belle II experiment at the SuperKEKB electron-positron collider. The analysis uses a 365 fb − 1 data sample recorded at the center-of-mass energy of the ϒ ( 4 S ) resonance. One of the B mesons produced in the ϒ ( 4 S ) → B 0 B ¯ 0 process is fully reconstructed in a hadronic decay mode, while its companion B meson is required to decay into a K * 0 and two τ leptons of opposite charge. The τ leptons are reconstructed in final states with a single electron, muon, charged pion or charged ρ meson, and additional neutrinos. We set an upper limit on the branching fraction of B ( B 0 → K * 0 τ + τ − ) < 1.8 × 10 − 3 at the 90% confidence level, which is the most stringent constraint reported to date.

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 ′ . 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 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 ( e + e − → h ′ χ 1 χ 2 ) , and the product of branching fractions B ( χ 2 → χ 1 e + e − ) × B ( h ′ → x + x − ) , where x + x − indicates μ + μ − , π + π − , or K + K − , as functions of h ′ mass and lifetime at the level of 10 − 1 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.

We report measurements of the ratios of branching fractions R ( D ( * ) + ) = B ( B ¯ 0 → D ( * ) + τ − ν ¯ τ ) / B ( B ¯ 0 → D ( * ) + ℓ − ν ¯ ℓ ) , where ℓ denotes either an electron or a muon. These ratios test the universality of the charged-current weak interaction. The results are based on a 365 fb − 1 data sample collected with the Belle II detector at the SuperKEKB e + e − collider, which operates at a center-of-mass energy corresponding to the ϒ ( 4 S ) resonance, just above the threshold for B B ¯ production. Signal candidates are reconstructed by selecting events in which the companion B meson from the ϒ ( 4 S ) → B B ¯ decay is identified in semileptonic modes. The τ lepton is reconstructed via its leptonic decays. We obtain R ( D + ) = 0.418 − 0.073 + 0.075 ( stat ) − 0.056 + 0.049 ( syst ) and R ( D * + ) = 0.306 − 0.033 + 0.035 ( stat ) − 0.018 + 0.016 ( syst ) , which are consistent with world average values. Accounting for the correlation between them, these values differ from the Standard Model expectation by a collective significance of 1.7 standard deviations.

Using data samples of 102 million ϒ ( 1 S ) events and 158 million ϒ ( 2 S ) events collected by the Belle detector at the KEKB asymmetric-energy e + e − collider, we search for [ u d s c c ¯ ] pentaquark states decaying to J / ψ Λ . Using the first observations of ϒ ( 1 S , 2 S ) inclusive decays to J / ψ Λ , we find evidence of the P c c ¯ s ( 4459 ) 0 state with a local significance of 3.3 standard deviations, including statistical and systematic uncertainties. We measure the mass and width of the P c c ¯ s ( 4459 ) 0 to be ( 4471.7 ± 4.8 ± 0.6 ) MeV / c 2 and ( 22 ± 13 ± 3 ) MeV , respectively. The branching fractions for P c c ¯ s ( 4459 ) 0 production are measured to be B [ ϒ ( 1 S ) → P c c ¯ s ( 4459 ) 0 / P ¯ c c ¯ s ( 4459 ) 0 + anything ] = ( 3.5 ± 2.0 ± 0.2 ) × 10 − 6 and B [ ϒ ( 2 S ) → P c c ¯ s ( 4459 ) 0 / P ¯ c c ¯ s ( 4459 ) 0 + anything ] = ( 2.9 ± 1.7 ± 0.4 ) × 10 − 6 . The inclusive branching fractions of ϒ ( 1 S , 2 S ) → J / ψ Λ / Λ ¯ are measured to be B [ ϒ ( 1 S ) → J / ψ Λ / Λ ¯ + anything ] = ( 36.9 ± 5.3 ± 2.4 ) × 10 − 6 and B [ ϒ ( 2 S ) → J / ψ Λ / Λ ¯ + anything ] = ( 22.3 ± 5.7 ± 3.1 ) × 10 − 6 . We measure the visible cross section σ ( e + e − → J / ψ Λ / Λ ¯ + anything ) = ( 90 ± 14 ± 6 ) fb for the continuum production at s = 10.52 GeV . In all cases, the first uncertainties are statistical and the second are systematic.