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Bilokin, S. (ORCID:0000000300176260)

Publications and source records attributed to Bilokin, S. (ORCID:0000000300176260).

11 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

Measurement of branching fractions, C P asymmetry, and isospin asymmetry for B β†’ ρ Ξ³ decays using Belle and Belle II data

We present measurements of B + β†’ ρ + Ξ³ and B 0 β†’ ρ 0 Ξ³ decays using a combined data sample of 772 Γ— 10 6 B B Β― pairs collected by the Belle experiment and 387 Γ— 10 6 B B Β― pairs collected by the Belle II experiment in e + e βˆ’ collisions at the Ο’ ( 4 S ) resonance. After an optimized selection, a simultaneous fit to the Belle and Belle II datasets yields 114 Β± 12 B + β†’ ρ + Ξ³ and 99 Β± 12 B 0 β†’ ρ 0 Ξ³ decays. The measured branching fractions are ( 13.1 βˆ’ 1.9 βˆ’ 1.2 + 2.0 + 1.3 ) Γ— 10 βˆ’ 7 and ( 7.6 Β± 1.3 βˆ’ 0.8 + 1.0 ) Γ— 10 βˆ’ 7 for B + β†’ ρ + Ξ³ and B 0 β†’ ρ 0 Ξ³ decays, respectively, where the first uncertainty is statistical and the second is systematic. We also measure the isospin asymmetry A I ( B β†’ ρ Ξ³ ) = ( 10.9 βˆ’ 11.7 βˆ’ 7.3 + 11.2 + 7.8 ) % and the direct C P asymmetry A C P ( B + β†’ ρ + Ξ³ ) = ( βˆ’ 8.2 Β± 15.2 βˆ’ 1.3 + 2.0 ) % . Published by the American Physical Society 2025

Adachi, I. (ORCID:0000000322870173)β†—
DOE OSTI2024

Test of lepton flavor universality with a measurement of R ( D * ) using hadronic B tagging at the Belle II experiment

The ratio of branching fractions R ( D * ) = B ( B Β― β†’ D * Ο„ βˆ’ Ξ½ Β― Ο„ ) / B ( B Β― β†’ D * β„“ βˆ’ Ξ½ Β― β„“ ) , where β„“ is an electron or muon, is measured using a Belle II data sample with an integrated luminosity of 189 fb βˆ’ 1 at the SuperKEKB asymmetric-energy e + e βˆ’ collider. Data is collected at the Ο’ ( 4 S ) resonance, and one B meson in the Ο’ ( 4 S ) β†’ B B Β― decay is fully reconstructed in hadronic decay modes. The accompanying signal B meson is reconstructed as B Β― β†’ D * Ο„ βˆ’ Ξ½ Β― Ο„ using leptonic Ο„ decays. The normalization decay, B Β― β†’ D * β„“ βˆ’ Ξ½ Β― β„“ , produces the same observable final-state particles. The ratio of branching fractions is extracted in a simultaneous fit to two signal-discriminating variables in both channels and yields R ( D * ) = 0.262 βˆ’ 0.039 + 0.041 ( stat ) βˆ’ 0.032 + 0.035 ( syst ) . This result is consistent with the current world average and with Standard Model predictions. Published by the American Physical Society 2024

Adachi, I. (ORCID:0000000322870173)β†—
DOE OSTI2024

New graph-neural-network flavor tagger for Belle II and measurement of sin 2β’πœ™ 1 in 𝐡 0 β†’ 𝐽/πœ“β’πΎ$^0_ S$ decays

We present GFlaT, a new algorithm that uses a graph-neural-network to determine the flavor of neutral 𝐡 mesons produced in ϒ⁑(4⁒𝑆) decays. It improves previous algorithms by using the information from all charged final-state particles and the relations between them. We evaluate its performance using 𝐡 decays to flavor-specific hadronic final states reconstructed in a 362 fb βˆ’1 sample of electron-positron collisions collected at the ϒ⁑(4⁒𝑆) resonance with the Belle II detector at the SuperKEKB collider. We achieve an effective tagging efficiency of (37.40 Β± 0.43 Β± 0.36%), where the first uncertainty is statistical and the second systematic, which is 18% better than the previous Belle II algorithm. Demonstrating the algorithm, we use 𝐡 0 →𝐽/πœ“β’πΎ$^0_ S$ decays to measure the mixing-induced and direct 𝐢⁒𝑃 violation parameters, 𝑆 = (0.724 Β± 0.035 Β± 0.009) and 𝐢 = (βˆ’0.035 Β± 0.026 Β± 0.029).

CP violation↗
DOE OSTI2024

Search for a ΞΌ + ΞΌ βˆ’ resonance in four-muon final states at Belle II

We report on a search for a resonance X decaying to a pair of muons in e + e βˆ’ β†’ ΞΌ + ΞΌ βˆ’ X events in the 0.212 – 9.000 GeV / c 2 mass range, using 178 fb βˆ’ 1 of data collected by the Belle II experiment at the SuperKEKB collider at a center of mass energy of 10.58 GeV. The analysis probes two different models of X beyond the standard model: a Z β€² vector boson in the L ΞΌ βˆ’ L Ο„ model and a muonphilic scalar. We observe no evidence for a signal and set exclusion limits at the 90% confidence level on the products of cross section and branching fraction for these processes, ranging from 0.046 fb to 0.97 fb for the L ΞΌ βˆ’ L Ο„ model and from 0.055 fb to 1.3 fb for the muonphilic scalar model. For masses below 6 GeV / c 2 , the corresponding constraints on the couplings of these processes to the standard model range from 0.0008 to 0.039 for the L ΞΌ βˆ’ L Ο„ model and from 0.0018 to 0.040 for the muonphilic scalar model. These are the first constraints on the muonphilic scalar from a dedicated search. Published by the American Physical Society 2024

Astronomy & Astrophysics↗
DOE OSTI2024

First Measurement of R ( X Ο„ / β„“ ) as an Inclusive Test of the b β†’ c Ο„ Ξ½ Anomaly

We measure the tau-to-light-lepton ratio of inclusive B -meson branching fractions R ( X Ο„ / β„“ ) ≑ B ( B β†’ X Ο„ Ξ½ ) / B ( B β†’ X β„“ Ξ½ ) , where β„“ indicates an electron or muon, and thereby test the universality of charged-current weak interactions. We select events that have one fully reconstructed B meson and a charged lepton candidate from 189 fb βˆ’ 1 of electron-positron collision data collected with the Belle II detector. We find R ( X Ο„ / β„“ ) = 0.228 Β± 0.016 ( stat ) Β± 0.036 ( syst ) , in agreement with standard-model expectations. This is the first direct measurement of R ( X Ο„ / β„“ ) . Published by the American Physical Society 2024

Physics↗
DOE OSTI2024

Search for the e + e βˆ’ β†’ Ξ· b ( 1 S ) Ο‰ and e + e βˆ’ β†’ Ο‡ b 0 ( 1 P ) Ο‰ processes at s = 10.745 GeV

We search for the e + e βˆ’ β†’ Ξ· b ( 1 S ) Ο‰ and e + e βˆ’ β†’ Ο‡ b 0 ( 1 P ) Ο‰ processes at a center-of-mass energy of 10.745 GeV, which is close to the peak of the Ο’ ( 10753 ) state. We use data collected by the Belle II experiment during a special run, corresponding to an integrated luminosity of 9.8 fb βˆ’ 1 . We reconstruct Ο‰ β†’ Ο€ + Ο€ βˆ’ Ο€ 0 decays and use the Ο‰ meson’s recoil mass to search for the signals. We do not find evidence for either process, and set upper limits on the corresponding Born-level cross sections of 2.5 pb and 7.8 pb, respectively, at the 90% confidence level. The Ο‡ b 0 ( 1 P ) Ο‰ limit is the result of a combination of this analysis and a previous search using full reconstruction. Published by the American Physical Society 2024

Astronomy & Astrophysics↗
DOE OSTI2023

Search for a Ο„ + Ο„ - Resonance in e + e - β†’ ΞΌ + ΞΌ - Ο„ + Ο„ - Events with the Belle II Experiment

We report the first search for a nonstandard-model resonance decaying into $Ο„$ pairs in $e^+e^-$ β†’ $ΞΌ^+ΞΌ^-Ο„^+Ο„^-$ events in the 3.6–10 GeV/c 2 mass range. We use a 62.8 fb -1 sample of $e^+e^-$ collisions collected at a center-of-mass energy of 10.58 GeV by the Belle II experiment at the SuperKEKB collider. The analysis probes three different models predicting a spin-1 particle coupling only to the heavier lepton families, a Higgs-like spin-0 particle that couples preferentially to charged leptons (leptophilic scalar), and an axionlike particle, respectively. We observe no evidence for a signal and set exclusion limits at 90% confidence level on the product of cross section and branching fraction into $Ο„$ pairs, ranging from 0.7 to 24 fb, and on the couplings of these processes. We obtain world-leading constraints on the couplings for the leptophilic scalar model for masses above 6.5 GeV/c 2 and for the axionlike particle model over the entire mass range.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2023

Test of Light-Lepton Universality in the Rates of Inclusive Semileptonic B-Meson Decays at Belle II

We present the first measurement of the ratio of branching fractions of inclusive semileptonic B -meson decays, R ( X e / ΞΌ ) = B ( B β†’ X e Ξ½ ) / B ( B β†’ X ΞΌ Ξ½ ) , a precision test of electron-muon universality, using data corresponding to 189 fb - 1 from electron-positron collisions collected with the Belle II detector. In events where the partner B meson is fully reconstructed, we use fits to the lepton momentum spectra above 1.3 GeV / c to obtain R ( X e / ΞΌ ) = 1.007 Β± 0.009 ( stat ) Β± 0.019 ( syst ) , which is the most precise lepton-universality test of its kind and agrees with the standard-model expectation.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2023

Measurement of the branching fraction and CP asymmetry of B 0 β†’Ο€ 0 ⁒π 0 decays using 198Γ—1⁒0 6 $B\overline{B}$ pairs in Belle II data

We report measurements of the branching fraction and C P asymmetry in B 0 β†’ Ο€ 0 Ο€ 0 decays reconstructed at Belle II in an electron-positron collision sample containing 198 Γ— 10 6 B B Β― pairs. We measure a branching fraction B ( B 0 β†’ Ο€ 0 Ο€ 0 ) = ( 1.38 Β± 0.27 Β± 0.22 ) Γ— 10 - 6 and a C P asymmetry A C P ( B 0 β†’ Ο€ 0 Ο€ 0 ) = - 0.14 Β± 0.46 Β± 0.07 , where the first uncertainty is statistical and the second is systematic.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2023

Measurement of the B 0 lifetime and flavor-oscillation frequency using hadronic decays reconstructed in 2019–2021 Belle II data

We measure the B0 lifetime and flavor-oscillation frequency using B 0 β†’ D(*) – Ο€ + decays collected by the Belle II experiment in asymmetric-energy e + ⁒e – collisions produced by the SuperKEKB collider operating at the Ξ₯⁑(4⁒S) resonance. We fit the decay-time distribution of signal decays, where the initial flavor is determined by identifying the flavor of the other B meson in the event. The results, based on 33000 signal decays reconstructed in a data sample corresponding to 190 fb –1 , are Ο„ B 0 = (1.499 Β± 0.013 Β± 0.008) ps, Δ⁒m d = (0.516 Β± 0.008 Β± 0.005) ps –1 , where the first uncertainties are statistical and the second are systematic. These results are consistent with the world-average values.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2023

Search for Lepton-Flavor-Violating Ο„ Decays to a Lepton and an Invisible Boson at Belle II

We search for lepton-flavor-violating $Ο„^– \rightarrow e^–α$ $Ο„^– \rightarrow ΞΌ^–α$ decays, where Ξ± is an invisible spin-0 boson. The search uses electron-positron collisions at 10.58 GeV center-of-mass energy with an integrated luminosity of 62.8 fb –1 , produced by the SuperKEKB collider and collected with the Belle II detector. We search for an excess in the lepton-energy spectrum of the known $Ο„^–\rightarrow e^–\bar{v}_{e}v_Ο„$ decays. We report 95% confidence-level upper limits on the branching-fraction ratio $\mathscr{B}(Ο„^–\rightarrow e^–α) \ / \mathscr{B}(Ο„^–\rightarrow e^– \bar{v}_{e}v_Ο„)$ in the range (1.1-9.7) Γ— 10 –3 and on $\mathscr{B}(Ο„^–\rightarrow ΞΌ^–α) \ / \mathscr{B}(Ο„^–\rightarrow ΞΌ^– \bar{v}_{ΞΌ}v_Ο„)$ in the range (0.7-12.2)Γ—10 –3 for Ξ± masses between 0 and 1.6 GeV/c 2 . These results provide the most stringent bounds on invisible boson production from Ο„ decays.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗