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We measure the tau-to-light-lepton ratio of inclusive B-meson branching fractions R(X_{τ/â})≡B(BâXτν)/B(BâXâν), where â 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^{-1} of electron-positron collision data collected with the Belle II detector. We find R(X_{τ/â})=0.228±0.016(stat)±0.036(syst), in agreement with standard-model expectations. This is the first direct measurement of R(X_{τ/â}).
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We study the processes e^{+}e^{-}âωχ_{bJ}(1P) (J=0, 1, or 2) using samples at center-of-mass energies sqrt[s]=10.701, 10.745, and 10.805 GeV, corresponding to 1.6, 9.8, and 4.7 fb^{-1} of integrated luminosity, respectively. These data were collected with the Belle II detector during special operations of the SuperKEKB collider above the Ï(4S) resonance. We report the first observation of ωχ_{bJ}(1P) signals at sqrt[s]=10.745 GeV. By combining Belle II data with Belle results at sqrt[s]=10.867 GeV, we find energy dependencies of the Born cross sections for e^{+}e^{-}âωχ_{b1,b2}(1P) to be consistent with the shape of the Ï(10753) state. These data indicate that the internal structures of the Ï(10753) and Ï(10860) states may differ. Including data at sqrt[s]=10.653 GeV, we also search for the bottomonium equivalent of the X(3872) state decaying into ωÏ(1S). No significant signal is observed for masses between 10.45 and 10.65 GeV/c^{2}.
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An absolute measurement of the Λ_{c}^{+} lifetime is reported using Λ_{c}^{+}âpK^{-}π^{+} decays in events reconstructed from data collected by the Belle II experiment at the SuperKEKB asymmetric-energy electron-positron collider. The total integrated luminosity of the data sample, which was collected at center-of-mass energies at or near the Ï(4S) resonance, is 207.2 fb^{-1}. The result, τ(Λ_{c}^{+})=203.20±0.89±0.77 fs, where the first uncertainty is statistical and the second systematic, is the most precise measurement to date and is consistent with previous determinations.
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The dark photon A^{'} and the dark Higgs boson h^{'} are hypothetical particles predicted in many dark sector models. We search for the simultaneous production of A^{'} and h^{'} in the dark Higgsstrahlung process e^{+}e^{-}âA^{'}h^{'} with A^{'}âµ^{+}µ^{-} and h^{'} invisible in electron-positron collisions at a center-of-mass energy of 10.58 GeV in data collected by the Belle II experiment in 2019. With an integrated luminosity of 8.34 fb^{-1}, we observe no evidence for signal. We obtain exclusion limits at 90% Bayesian credibility in the range of 1.7-5.0 fb on the cross section and in the range of 1.7×10^{-8}-200×10^{-8} on the effective coupling ϵ^{2}×α_{D} for the A^{'} mass in the range of 4.0 GeV/c^{2}
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We search for lepton-flavor-violating τ^{-}âe^{-}α and τ^{-}âµ^{-}α decays, where α 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 τ^{-}âe^{-}ν[over ¯]_{e}ν_{τ} and τ^{-}âµ^{-}ν[over ¯]_{µ}ν_{τ} decays. We report 95% confidence-level upper limits on the branching-fraction ratio B(τ^{-}âe^{-}α)/B(τ^{-}âe^{-}ν[over ¯]_{e}ν_{τ}) in the range (1.1-9.7)×10^{-3} and on B(τ^{-}âµ^{-}α)/B(τ^{-}âµ^{-}ν[over ¯]_{µ}ν_{τ}) in the range (0.7-12.2)×10^{-3} for α masses between 0 and 1.6 GeV/c^{2}. These results provide the most stringent bounds on invisible boson production from τ decays.
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The L_{µ}-L_{τ} extension of the standard model predicts the existence of a lepton-flavor-universality-violating Z^{'} boson that couples only to the heavier lepton families. We search for such a Z^{'} through its invisible decay in the process e^{+}e^{-}âµ^{+}µ^{-}Z^{'}. We use a sample of electron-positron collisions at a center-of-mass energy of 10.58 GeV collected by the Belle II experiment in 2019-2020, corresponding to an integrated luminosity of 79.7 fb^{-1}. We find no excess over the expected standard-model background. We set 90%-confidence-level upper limits on the cross section for this process as well as on the coupling of the model, which ranges from 3×10^{-3} at low Z^{'} masses to 1 at Z^{'} masses of 8 GeV/c^{2}.
Asunto(s)
Mesones , Humanos , ElectronesRESUMEN
We report the first search for a nonstandard-model resonance decaying into τ pairs in e^{+}e^{-}âµ^{+}µ^{-}τ^{+}τ^{-} 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 τ 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.
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We measure the lifetime of the D_{s}^{+} meson using a data sample of 207 fb^{-1} collected by the Belle II experiment running at the SuperKEKB asymmetric-energy e^{+}e^{-} collider. The lifetime is determined by fitting the decay-time distribution of a sample of 116×10^{3} D_{s}^{+}âÏπ^{+} decays. Our result is τ_{D_{s}^{+}}=(499.5±1.7±0.9) fs, where the first uncertainty is statistical and the second is systematic. This result is significantly more precise than previous measurements.
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We present the first comprehensive tests of the universality of the light leptons in the angular distributions of semileptonic B^{0}-meson decays to charged spin-1 charmed mesons. We measure five angular-asymmetry observables as functions of the decay recoil that are sensitive to lepton-universality-violating contributions. We use events where one neutral B is fully reconstructed in Ï(4S)âBB[over ¯] decays in data corresponding to 189 fb^{-1} integrated luminosity from electron-positron collisions collected with the Belle II detector. We find no significant deviation from the standard model expectations.
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The first simultaneous determination of the absolute value of the Cabibbo-Kobayashi-Maskawa matrix element V_{ub} using inclusive and exclusive decays is performed with the full Belle data set at the Ï(4S) resonance, corresponding to an integrated luminosity of 711 fb^{-1}. We analyze collision events in which one B meson is fully reconstructed in hadronic modes. This allows for the reconstruction of the hadronic X_{u} system of the semileptonic bâuâν[over ¯]_{â} decay. We separate exclusive Bâπâν[over ¯]_{â} decays from other inclusive BâX_{u}âν[over ¯]_{â} and backgrounds with a two-dimensional fit that utilizes the number of charged pions in the X_{u} system and the four-momentum transfer q^{2} between the B and X_{u} systems. Combining our measurement with information from lattice QCD and QCD calculations of the inclusive partial rate as well as external experimental information on the shape of the Bâπâν[over ¯]_{â} form factor, we determine |V_{ub}^{excl}|=(3.78±0.23±0.16±0.14)×10^{-3} and |V_{ub}^{incl}|=(3.88±0.20±0.31±0.09)×10^{-3}, respectively, with the uncertainties being the statistical error, systematic errors, and theory errors. The ratio of |V_{ub}^{excl}|/|V_{ub}^{incl}|=0.97±0.12 is compatible with unity.
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We present the first measurement of the ratio of branching fractions of inclusive semileptonic B-meson decays, R(X_{e/µ})=B(BâXeν)/B(BâXµν), 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/µ})=1.007±0.009(stat)±0.019(syst), which is the most precise lepton-universality test of its kind and agrees with the standard-model expectation.
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The first measurements of differential branching fractions of inclusive semileptonic BâX_{u}â^{+}ν_{â} decays are performed using the full Belle data set of 711 fb^{-1} of integrated luminosity at the Ï(4S) resonance and for â=e, µ. With the availability of these measurements, new avenues for future shape-function model-independent determinations of the Cabibbo-Kobayashi-Maskawa matrix element |V_{ub}| can be pursued to gain new insights in the existing tension with respect to exclusive determinations. The differential branching fractions are reported as a function of the lepton energy, the four-momentum-transfer squared, light-cone momenta, the hadronic mass, and the hadronic mass squared. They are obtained by subtracting the backgrounds from semileptonic BâX_{c}â^{+}ν_{â} decays and other processes, and corrected for resolution and acceptance effects.
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We report a measurement of the D^{0} and D^{+} lifetimes using D^{0}âK^{-}π^{+} and D^{+}âK^{-}π^{+}π^{+} decays reconstructed in e^{+}e^{-}âcc[over ¯] data recorded by the Belle II experiment at the SuperKEKB asymmetric-energy e^{+}e^{-} collider. The data, collected at center-of-mass energies at or near the Ï(4S) resonance, correspond to an integrated luminosity of 72 fb^{-1}. The results, τ(D^{0})=410.5±1.1(stat)±0.8(syst) fs and τ(D^{+})=1030.4±4.7(stat)±3.1(syst) fs, are the most precise to date and are consistent with previous determinations.
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A search for the flavor-changing neutral-current decay B^{+}âK^{+}νν[over ¯] is performed at the Belle II experiment at the SuperKEKB asymmetric energy electron-positron collider. The data sample corresponds to an integrated luminosity of 63 fb^{-1} collected at the Ï(4S) resonance and a sample of 9 fb^{-1} collected at an energy 60 MeV below the resonance. Because the measurable decay signature involves only a single charged kaon, a novel measurement approach is used that exploits not only the properties of the B^{+}âK^{+}νν[over ¯] decay, but also the inclusive properties of the other B meson in the Ï(4S)âBB[over ¯] event, to suppress the background from other B meson decays and light-quark pair production. This inclusive tagging approach offers a higher signal efficiency compared to previous searches. No significant signal is observed. An upper limit on the branching fraction of B^{+}âK^{+}νν[over ¯] of 4.1×10^{-5} is set at the 90% confidence level.
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We present a search for the direct production of a light pseudoscalar a decaying into two photons with the Belle II detector at the SuperKEKB collider. We search for the process e^{+}e^{-}âγa, aâγγ in the mass range 0.2
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Theories beyond the standard model often predict the existence of an additional neutral boson, the Z^{'}. Using data collected by the Belle II experiment during 2018 at the SuperKEKB collider, we perform the first searches for the invisible decay of a Z^{'} in the process e^{+}e^{-}âµ^{+}µ^{-}Z^{'} and of a lepton-flavor-violating Z^{'} in e^{+}e^{-}âe^{±}µ^{∓}Z^{'}. We do not find any excess of events and set 90% credibility level upper limits on the cross sections of these processes. We translate the former, in the framework of an L_{µ}-L_{τ} theory, into upper limits on the Z^{'} coupling constant at the level of 5×10^{-2}-1 for M_{Z^{'}}≤6 GeV/c^{2}.
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We present first evidence that the cosine of the CP-violating weak phase 2ß is positive, and hence exclude trigonometric multifold solutions of the Cabibbo-Kobayashi-Maskawa (CKM) Unitarity Triangle using a time-dependent Dalitz plot analysis of B^{0}âD^{(*)}h^{0} with DâK_{S}^{0}π^{+}π^{-} decays, where h^{0}∈{π^{0},η,ω} denotes a light unflavored and neutral hadron. The measurement is performed combining the final data sets of the BABAR and Belle experiments collected at the Ï(4S) resonance at the asymmetric-energy B factories PEP-II at SLAC and KEKB at KEK, respectively. The data samples contain (471±3)×10^{6}BB[over ¯] pairs recorded by the BABAR detector and (772±11)×10^{6}BB[over ¯] pairs recorded by the Belle detector. The results of the measurement are sin2ß=0.80±0.14(stat)±0.06(syst)±0.03(model) and cos2ß=0.91±0.22(stat)±0.09(syst)±0.07(model). The result for the direct measurement of the angle ß of the CKM Unitarity Triangle is ß=[22.5±4.4(stat)±1.2(syst)±0.6(model)]°. The measurement assumes no direct CP violation in B^{0}âD^{(*)}h^{0} decays. The quoted model uncertainties are due to the composition of the D^{0}âK_{S}^{0}π^{+}π^{-} decay amplitude model, which is newly established by performing a Dalitz plot amplitude analysis using a high-statistics e^{+}e^{-}âcc[over ¯] data sample. CP violation is observed in B^{0}âD^{(*)}h^{0} decays at the level of 5.1 standard deviations. The significance for cos2ß>0 is 3.7 standard deviations. The trigonometric multifold solution π/2-ß=(68.1±0.7)° is excluded at the level of 7.3 standard deviations. The measurement resolves an ambiguity in the determination of the apex of the CKM Unitarity Triangle.
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We report a measurement of the time-dependent CP asymmetry of B[over ¯]^{0}âD_{CP}^{(*)}h^{0} decays, where the light neutral hadron h^{0} is a π^{0}, η, or ω meson, and the neutral D meson is reconstructed in the CP eigenstates K^{+}K^{-}, K_{S}^{0}π^{0}, or K_{S}^{0}ω. The measurement is performed combining the final data samples collected at the Ï(4S) resonance by the BABAR and Belle experiments at the asymmetric-energy B factories PEP-II at SLAC and KEKB at KEK, respectively. The data samples contain (471±3)×10^{6} BB[over ¯] pairs recorded by the BABAR detector and (772±11)×10^{6} BB[over ¯] pairs recorded by the Belle detector. We measure the CP asymmetry parameters -η_{f}S=+0.66±0.10(stat)±0.06(syst) and C=-0.02±0.07(stat)±0.03(syst). These results correspond to the first observation of CP violation in B[over ¯]^{0}âD_{CP}^{(*)}h^{0} decays. The hypothesis of no mixing-induced CP violation is excluded in these decays at the level of 5.4 standard deviations.