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Das, S. (ORCID:000000016857966X)

Publications and source records attributed to Das, S. (ORCID:000000016857966X).

At least 19 records
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DOE OSTI2025

Charged-hadron identification at Belle II

The Belle II experiment’s ability to identify particles critically affects the sensitivity of its measurements. We describe Belle II’s algorithms for identifying charged particles and evaluate their performance in separating pions, kaons, and protons using $426\,\text {fb}^{-1}$ of data collected at the energy-asymmetric $e^{+}e^{-}$ collider SuperKEKB in 2019–2022 at center-of-mass energies at and near the mass of the $\Upsilon$ (4S).

Adachi, I. (ORCID:0000000322870173)β†—
DOE OSTI2025

Measurement of the 𝐢⁒𝑃 asymmetry in 𝐷 + β†’ πœ‹ + β’πœ‹ 0 decays at Belle II

We measure the 𝐢⁒𝑃 asymmetry in 𝐷 + β†’ πœ‹ + β’πœ‹ 0 decays reconstructed in 𝑒 + ⁒𝑒 βˆ’ collisions at the Belle II experiment using a dataset corresponding to an integrated luminosity of 428 fb βˆ’1 . A control sample of 𝐷 + β†’ πœ‹ + ⁒𝐾$^0_𝑆$ decays is used to correct for detection and production asymmetries. The result 𝐴 𝐢⁒𝑃 ⁑(𝐷 + β†’ πœ‹ + β’πœ‹ 0 ) = (βˆ’1.8 Β± 0.9 Β± 0.1)%, where the first uncertainty is statistical and the second systematic, is the most precise determination to date. It agrees with the prediction of 𝐢⁒𝑃 symmetry from the standard model and with results of previous measurements.

CP violation↗
DOE OSTI2025

Model-independent measurement of 𝐷 0 βˆ’$\overline{D}$ 0 mixing parameters in 𝐷 0 β†’ 𝐾$^{0}_{𝑆}$β’πœ‹ + β’πœ‹ βˆ’ decays at Belle and Belle II

We perform a model-independent measurement of the 𝐷 0 βˆ’$\overline{D}$ 0 mixing parameters using samples of 𝑒 + ⁒𝑒 βˆ’ -collision data collected by the Belle and Belle II experiments that have integrated luminosities of 951 fb βˆ’1 and 408 fb βˆ’1 , respectively. Approximately 2.05 Γ— 10 6 neutral 𝐷 mesons are reconstructed in the 𝐷 0 β†’ 𝐾$^{0}_{S}$β’πœ‹ + β’πœ‹ βˆ’ channel, with the neutral 𝐷 flavor tagged by the charge of the pion in the 𝐷* + β†’ 𝐷 0⁒ πœ‹ + decay. Assuming charge-parity symmetry, the mixing parameters are measured to be π‘₯ = (4.0 Β±1.7 Β±0.4) Γ— 10 βˆ’3 and 𝑦 = (2.9 Β±1.4 Β±0.3) Γ— 10 βˆ’3 , where the first uncertainties are statistical and the second systematic. The results are consistent with previous determinations.

Charmed mesons↗
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 OSTI2025

Production cross sections of light and charmed mesons in 𝑒 + ⁒𝑒 βˆ’ annihilation near 10.58 GeV

We report measurements of production cross sections for 𝜌 + , 𝜌 0 , πœ”, 𝐾* + , 𝐾* 0 , πœ™, πœ‚, 𝐾$^0_𝑆$, 𝑓 0 ⁑(980), 𝐷 + , 𝐷 0 , 𝐷$^+_𝑠$, 𝐷* + , 𝐷* 0 , and 𝐷*$^+_𝑠$ in 𝑒 + ⁒𝑒 βˆ’ collisions at a center-of-mass energy near 10.58 GeV. The data were recorded by the Belle experiment, consisting of 571 fb βˆ’1 at 10.58 GeV and 74 fb βˆ’1 at 10.52 GeV. Production cross sections are extracted as a function of the fractional hadron momentum π‘₯ 𝑝 . The measurements are compared to pythia Monte Carlo generator predictions with various fragmentation settings, including those that have increased fragmentation into vector mesons over pseudoscalar mesons. The cross sections measured for light hadrons are consistent with no additional increase of vector over pseudoscalar mesons. The charmed-meson cross sections are compared to earlier measurementsβ€”when availableβ€”including older Belle results, which they supersede. They are in agreement before application of an improved initial-state radiation correction procedure that causes slight changes in their π‘₯ 𝑝 shapes.

fragmentation functions↗
DOE OSTI2025

Measurement of the time-integrated C P asymmetry in D 0 β†’ K S 0 K S 0 decays using Belle and Belle II data

We measure the time-integrated C P asymmetry in D 0 β†’ K S 0 K S 0 decays reconstructed in e + e βˆ’ β†’ c c Β― events collected by the Belle and Belle II experiments. The corresponding data samples have integrated luminosities of 980 and 428 fb βˆ’ 1 , respectively. The D 0 decays are required to originate from the D * + β†’ D 0 Ο€ + decay, which determines the charm flavor at production time. A control sample of D 0 β†’ K + K βˆ’ decays is used to correct for production and detection asymmetries. The result, ( βˆ’ 1.4 Β± 1.3 ( stat ) Β± 0.1 ( syst ) ) % , is consistent with previous determinations and with C P symmetry. 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

Angular analysis of B β†’ K * e + e βˆ’ in the low- q 2 region with new electron identification at Belle

We perform an angular analysis of the B β†’ K * e + e βˆ’ decay for the dielectron mass squared, q 2 , range of 0.0008 – 1.1200 GeV 2 / c 4 using the full Belle dataset in the K * 0 β†’ K + Ο€ βˆ’ and K * + β†’ K S 0 Ο€ + channels, incorporating new methods of electron identification to improve the statistical power of the dataset. This analysis is sensitive to contributions from right-handed currents from physics beyond the Standard Model by constraining the Wilson coefficients C 7 ( β€² ) . We perform a fit to the B β†’ K * e + e βˆ’ differential decay rate and measure the imaginary component of the transversality amplitude to be A T Im = βˆ’ 1.27 Β± 0.52 Β± 0.12 , and the K * transverse asymmetry to be A T ( 2 ) = 0.52 Β± 0.53 Β± 0.11 , with F L and A T Re fixed to the Standard Model values. The resulting constraints on the value of C 7 β€² are consistent with the Standard Model within a 2 Οƒ confidence interval. Published by the American Physical Society 2024

Ferlewicz, D. (ORCID:0000000243741234)β†—
DOE OSTI2024

Measurement of Angular Coefficients of B Β― β†’ D * β„“ Ξ½ Β― β„“ : Implications for | V c b | and Tests of Lepton Flavor Universality

We measure the complete set of angular coefficients J i for exclusive B Β― β†’ D * β„“ Ξ½ Β― β„“ decays ( β„“ = e , ΞΌ ). Our analysis uses the full 711 fb βˆ’ 1 Belle dataset with hadronic tag-side reconstruction. The results allow us to extract the form factors describing the B Β― β†’ D * transition and the Cabibbo-Kobayashi-Maskawa matrix element | V cb | . Using recent lattice QCD calculations for the hadronic form factors, we find | V cb | = ( 40.7 Β± 0.7 ) Γ— 10 βˆ’ 3 using the Boyd-Grinstein-Lebed parametrization, compatible with determinations from inclusive semileptonic decays. We search for lepton flavor universality violation as a function of the hadronic recoil parameter w and investigate the differences of the electron and muon angular distributions. We find no deviation from standard model expectations. Published by the American Physical Society 2024

Physics↗
DOE OSTI2024

Search for lepton-flavor-violating Ο„ – β†’ ΞΌ – ΞΌ + ΞΌ – decays at Belle II

We present the result of a search for the charged-lepton-flavor violating decay Ο„ – β†’ ΞΌ – ΞΌ + ΞΌ – using a 424 fb –1 sample of data recorded by the Belle II experiment at the SuperKEKB e + e – collider. The selection of e + e – β†’ Ο„ + Ο„ – events is based on an inclusive reconstruction of the non-signal tau decay, and on a boosted decision tree to suppress background. We observe one signal candidate, which is compatible with the expectation from background processes. We set a 90% confidence level upper limit of 1.9 Γ— 10 –8 on the branching fraction of the Ο„ – β†’ ΞΌ – ΞΌ + ΞΌ – decay, which is the most stringent bound to date.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2024

Search for Rare b β†’ d β„“ + β„“ βˆ’ Transitions at Belle

We present the results of a search for the b β†’ d β„“ + β„“ βˆ’ flavor-changing neutral-current rare decays B + , 0 β†’ ( Ξ· , Ο‰ , Ο€ + , 0 , ρ + , 0 ) e + e βˆ’ and B + , 0 β†’ ( Ξ· , Ο‰ , Ο€ 0 , ρ + ) ΞΌ + ΞΌ βˆ’ using a 711 fb βˆ’ 1 data sample that contains 772 Γ— 10 6 B B Β― events. The data were collected at the Ο’ ( 4 S ) resonance with the Belle detector at the KEKB asymmetric-energy e + e βˆ’ collider. We find no evidence for signal and set upper limits on branching fractions at the 90% confidence level in the range ( 3.8 – 47 ) Γ— 10 βˆ’ 8 depending on the decay channel. The obtained limits are the world’s best results. This is the first search for the channels B + , 0 β†’ ( Ο‰ , ρ + , 0 ) e + e βˆ’ and B + , 0 β†’ ( Ο‰ , ρ + ) ΞΌ + ΞΌ βˆ’ . Published by the American Physical Society 2024

Physics↗
DOE OSTI2024

Measurements of the Branching Fraction, Polarization, and C P Asymmetry for the Decay B 0 β†’ Ο‰ Ο‰

We present a comprehensive study of B 0 β†’ Ο‰ Ο‰ decays using 772 Γ— 10 6 B B Β― pairs collected with the Belle detector at the KEKB e + e βˆ’ collider. This process is a suppressed charmless decay into two vector mesons and can exhibit interesting polarization and C P violation. The decay is observed for the first time with a significance of 7.9 standard deviations. We measure a branching fraction B = ( 1.53 Β± 0.29 Β± 0.17 ) Γ— 10 βˆ’ 6 , a fraction of longitudinal polarization f L = 0.87 Β± 0.13 Β± 0.13 , and a time-integrated C P asymmetry A C P = βˆ’ 0.44 Β± 0.43 Β± 0.11 , where the first uncertainties listed are statistical and the second are systematic. This is the first observation of B 0 β†’ Ο‰ Ο‰ and the first measurements of f L and A C P for this decay. Published by the American Physical Society 2024

Physics↗
DOE OSTI2024

Search for Baryon-Number-Violating Processes in B βˆ’ Decays to the Ξ Β― c 0 Ξ› Β― c βˆ’ Final State

We report the results of the first search for B βˆ’ decays to the Ξ Β― c 0 Ξ› Β― c βˆ’ final state using 711 fb βˆ’ 1 of data collected at the Ο’ ( 4 S ) resonance with the Belle detector at the KEKB asymmetric-energy e + e βˆ’ collider. The results are interpreted in terms of both direct baryon-number-violating B βˆ’ decay and Ξ c 0 βˆ’ Ξ Β― c 0 oscillations which follow the standard model decay B βˆ’ β†’ Ξ c 0 Ξ› Β― c βˆ’ . We observe no evidence for baryon number violation and set the 95% confidence-level upper limits on the ratio of baryon-number-violating and standard model branching fractions B ( B βˆ’ β†’ Ξ Β― c 0 Ξ› Β― c βˆ’ ) / B ( B βˆ’ β†’ Ξ c 0 Ξ› Β― c βˆ’ ) to be < 2.7 % and on the effective angular frequency of mixing Ο‰ in Ξ c 0 βˆ’ Ξ Β― c 0 oscillations to be < 0.76 ps βˆ’ 1 (equivalent to Ο„ mix > 1.3 ps ). Published by the American Physical Society 2024

Physics↗
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

Study of Ξ₯(10753) decays to Ο€+Ο€βˆ’Ξ₯(nS) final states at Belle II

Abstract We present an analysis of the processe + e βˆ’ β†’Ο€ + Ο€ βˆ’ Ξ₯(nS) (wheren= 1, 2, or 3) reconstructed in 19.6 fb βˆ’1 of Belle II data during a special run of the SuperKEKB collider at four energy points near the peak of the Ξ₯(10753) resonance. By analyzing the mass distribution of theΟ€ + Ο€ βˆ’ Ξ₯(nS) system and the Born cross sections of thee + e βˆ’ β†’Ο€ + Ο€ βˆ’ Ξ₯(nS) process, we report the first observation of Ξ₯(10753) decays to theΟ€ + Ο€ βˆ’ Ξ₯(1S) andΟ€ + Ο€ βˆ’ Ξ₯(2S) final states, and find no evidence for decays toΟ€ + Ο€ βˆ’ Ξ₯(3S). Possible intermediate states in theΟ€ + Ο€ βˆ’ Ξ₯(1S,2S) transitions are also investigated, and no evidence for decays proceeding via the$$ {\pi}^{\mp }{Z}_b^{\pm } $$ Ο€ βˆ“ Z b Β± orf 0 (980)Ξ₯(nS) intermediate states is found. We measure Born cross sections for thee + e βˆ’ β†’Ο€ + Ο€ βˆ’ Ξ₯(nS) process that, combined with results from Belle, obtain the mass and width of Ξ₯(10753) to be (10756.6 Β± 2.7 Β± 0.9) MeV/c 2 and (29.0 Β± 8.8 Β± 1.2) MeV, respectively. The relative ratios of the Born cross sections at the Ξ₯(10753) resonance peak are also reported for the first time.

Physics↗
DOE OSTI2024

Measurement of C P asymmetries in B 0 β†’ K S 0 K S 0 K S 0 decays at Belle II

We report a measurement of decay-time-dependent charge-parity ( C P ) asymmetries in B 0 β†’ K S 0 K S 0 K S 0 decays. We use 387 Γ— 10 6 B B Β― pairs collected at the Ο’ ( 4 S ) resonance with the Belle II detector at the SuperKEKB asymmetric-energy electron-positron collider. We reconstruct 220 signal events and extract the C P -violating parameters S and C from a fit to the distribution of the decay-time difference between the two B mesons. The resulting confidence region is consistent with previous measurements in B 0 β†’ K S 0 K S 0 K S 0 and B 0 β†’ ( c c Β― ) K 0 decays and with predictions based on the standard model. Published by the American Physical Society 2024

Astronomy & Astrophysics↗
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

Search for a heavy neutral lepton that mixes predominantly with the tau neutrino

We report a search for a heavy neutral lepton (HNL) that mixes predominantly with Ξ½ Ο„ . The search utilizes data collected with the Belle detector at the KEKB asymmetric energy e + e βˆ’ collider. The data sample was collected at and just below the center-of-mass energies of the Ο’ ( 4 S ) and Ο’ ( 5 S ) resonances and has an integrated luminosity of 915 fb βˆ’ 1 , corresponding to ( 836 Β± 12 ) Γ— 10 6 e + e βˆ’ β†’ Ο„ + Ο„ βˆ’ events. We search for production of the HNL (denoted N ) in the decay Ο„ βˆ’ β†’ Ο€ βˆ’ N followed by its decay via N β†’ ΞΌ + ΞΌ βˆ’ Ξ½ Ο„ . The search focuses on the parameter-space region in which the HNL is long-lived, so that the ΞΌ + ΞΌ βˆ’ originate from a common vertex that is significantly displaced from the collision point of the KEKB beams. Consistent with the expected background yield, one event is observed in the data sample after application of all the event-selection criteria. We report limits on the mixing parameter of the HNL with the Ο„ neutrino as a function of the HNL mass. Published by the American Physical Society 2024

Astronomy & Astrophysics↗