Search Results (316 total)

We report a high-statistics measurement of the branching fraction for τ^-→π^-π⁰ν_τ and the invariant mass spectrum of the produced π^-π⁰ system using 72.2  fb^-1 of data recorded with the Belle detector at the KEKB asymmetric-energy e⁺e^- collider. The branching fraction obtained is (25.24±0.01±0.39)%, where the first error is statistical and the second is systematic. The unfolded π^-π⁰ mass spectrum is used to determine resonance parameters for the ρ(770), ρ^′(1450), and ρ^′′(1700) mesons. We also use this spectrum to estimate the hadronic (2π) contribution to the anomalous magnetic moment of the muon (a_μ^ππ). Our result for a_μ^ππ integrated over the mass range sqrt[s]=2m_π-1.8  GeV/c² is a_μ^ππ=(523.5±1.5(exp⁡)±2.6(Br)±2.5(isospin))×10^-10, where the first error is due to the experimental uncertainties, the second is due to the uncertainties in the branching fractions, and the third is due to the uncertainties in the isospin-violating corrections.

We report a study of the suppressed B meson decay B^-→DK^- followed by D→K⁺π^-, where D indicates a D⁰ or D̅ ⁰ state. The two decay paths interfere and provide information on the CP-violating angle ϕ₃. We use a data sample containing 657×10⁶ BB̅ pairs recorded at the Υ(4S) resonance with the Belle detector at the KEKB asymmetric-energy e⁺e^- storage ring. We do not find significant evidence for the mode B^-→DK^-, D→K⁺π^-, and set an upper limit of r_B<0.19, where r_B is the magnitude of the ratio of amplitudes |A(B^-→D̅ ⁰K^-)/A(B^-→D⁰K^-)|. The decay B^-→Dπ^-, D→K⁺π^- is also analyzed as a reference, for which we observe a signal with 6.6σ significance, and measure the charge asymmetry A_Dπ to be -0.02_-0.16^+0.15(stat)±0.04(syst). In addition, the ratio B(B^-→D⁰K^-)/B(B^-→D⁰π^-) is measured to be [6.77±0.23(stat)±0.30(syst)]×10^-2.

We report a measurement of the exclusive e⁺e^-→Λ_c⁺Λ_c^- cross section as a function of center-of-mass energy near the Λ_c⁺Λ_c^- threshold. A clear peak with a significance of 8.2σ is observed in the Λ_c⁺Λ_c^- invariant mass distribution just above threshold. With an assumption of a resonance origin for the observed peak, a mass and width of M=[4634_-7⁺⁸(stat)_-8⁺⁵(syst)]  MeV/c² and Γ_tot=[92_-24⁺⁴⁰(stat)_-21⁺¹⁰(syst)]  MeV are determined. The analysis is based on a study of events with initial-state-radiation photons in a data sample collected with the Belle detector at the Υ(4S) resonance and nearby continuum with an integrated luminosity of 695  fb^-1 at the KEKB asymmetric-energy e⁺e^- collider.

We report the first observation of two resonancelike structures in the π⁺χ_c1 invariant mass distribution near 4.1  GeV/c² in exclusive B̅ ⁰→K^-π⁺χ_c1 decays. From a Dalitz plot analysis in which the π⁺χ_c1 mass structures are represented by Breit-Wigner resonance amplitudes, we determine masses and widths of: M₁=(4051±14_-41⁺²⁰)  MeV/c², Γ₁=(82_-17-22⁺²¹⁺⁴⁷)  MeV, M₂=(4248_-29-35^+44+180)  MeV/c², and Γ₂=(177_-39-61^+54+316)  MeV; and product branching fractions of B(B̅ ⁰→K^-Z_1,2⁺)×B(Z_1,2⁺→π⁺χ_c1)=(3.0_-0.8-1.6^+1.5+3.7)×10^-5 and (4.0_-0.9-0.5^+2.3+19.7)×10^-5, respectively. Here the first uncertainty is statistical, the second is systematic. The significance of each of the π⁺χ_c1 structures exceeds 5σ, including the systematic uncertainty from various fit models. This analysis is based on 657×10⁶ BB̅ events collected at the Υ(4S) resonance with the Belle detector at the KEKB asymmetric-energy e⁺e^- collider.

We report the first observation of B^±→ψ(2S)π^±, a Cabibbo- and color-suppressed decay. This analysis is based on 657×10⁶ BB̅ events collected at the Υ(4S) resonance with the Belle detector at the KEKB energy-asymmetric e⁺e^- collider. The measured branching fraction is (2.44±0.22±0.20)×10^-5 and the charge asymmetry is A=0.022±0.085±0.016. The ratio of the branching fractions B(B^±→ψ(2S)π^±)/B(B^±→ψ(2S)K^±)=(3.99±0.36±0.17)% is also determined.

We report new measurements of the decays B⁺→ρ⁺γ, B⁰→ρ⁰γ, and B⁰→ωγ using a data sample of 657×10⁶ B meson pairs accumulated with the Belle detector at the KEKB e⁺e^- collider. We measure branching fractions B(B⁺→ρ⁺γ)=(8.7_-2.7-1.1^+2.9+0.9)×10^-7, B(B⁰→ρ⁰γ)=(7.8_-1.6-1.0^+1.7+0.9)×10^-7, and B(B⁰→ωγ)=(4.0_-1.7^+1.9±1.3)×10^-7. We also report the isospin asymmetry Δ(ργ)=-0.48_-0.19-0.09^+0.21+0.08 and the first measurement of the direct CP-violating asymmetry A_CP(B⁺→ρ⁺γ)=-0.11±0.32±0.09, where the first and second errors are statistical and systematic, respectively.

We present a high-statistics measurement of differential cross sections and the total cross section for the process γγ→π⁰π⁰ in the kinematic range 0.6  GeV≤W≤4.0  GeV and |cos⁡θ^*|≤0.8, where W and θ^* are the energy and pion scattering angle, respectively, in the γγ center-of-mass system. Differential cross sections are fitted to obtain information on S, D₀, D₂, G₀, and G₂ waves. The G waves are important above W≃1.6  GeV. The general behavior of partial waves is studied by fitting differential cross sections in a simple parametrization where amplitudes contain resonant contributions and smooth background. The D₂ wave is dominated by the f₂(1270) meson whose parameters are consistent with the current world averages. The D₀ wave contains a f₂(1270) component, whose fraction is fitted. For the S wave, the f₀(980) parameters are found to be consistent with the values determined from our recent π⁺π^- data. In addition to the f₀(980), the S wave prefers to have another resonance-like contribution whose parameters are obtained.

Using the previous Belle measurement of the inclusive photon energy in B→X_sγ decays, we determine the first and second moments of this spectrum for minimum photon energies in the B meson rest frame ranging from 1.8 to 2.3 GeV. Combining these measurements with recent Belle data on the lepton energy and hadronic mass moments in B→X_cℓν decays, we perform fits to theoretical expressions derived in the 1S and kinetic mass schemes and extract the magnitude of the Cabibbo-Kobayashi-Maskawa matrix element V_cb, the b-quark mass, and other nonperturbative parameters. In the 1S scheme analysis we find |V_cb|=(41.56±0.68(fit)±0.08(τ_B))×10^-3 and m_b^1S=(4.723±0.055)  GeV. In the kinetic scheme, we obtain |V_cb|=(41.58±0.69(fit)±0.08(τ_B)±0.58(th))×10^-3 and m_b^kin=(4.543±0.075)  GeV.

The Collins effect connects transverse quark spin with a measurable azimuthal asymmetry in the yield of hadronic fragments around the quark’s momentum vector. Using two different reconstruction methods we measure statistically significant azimuthal asymmetries for charged pion pairs in e⁺e^- annihilation at center-of-mass energies of 10.52 GeV and 10.58 GeV, which can be attributed to the fragmentation of primordial quarks with transverse spin components. The measurement was performed using a data set of 547  fb^-1 collected by the Belle detector at KEKB improving the statistics of the previously published results by nearly a factor of 20.

We report a search for the decay B⁰→J/ψϕ, using a sample of 657×10⁶ BB̅ pairs collected with the Belle detector at the Υ(4S) resonance. No statistically significant signal is found and an upper limit for the branching fraction is determined to be B(B⁰→J/ψϕ)<9.4×10^-7 at 90% confidence level.

We present a search for the decays B⁺→π⁺ℓ⁺ℓ^- and B⁰→π⁰ℓ⁺ℓ^-, where ℓ⁺ℓ^- is either a μ⁺μ^- or e⁺e^- pair, with a data sample of 657×10⁶ BB̅ pairs collected with the Belle detector at the KEKB e⁺e^- collider. Signal events are reconstructed from a charged or a neutral pion candidate and a pair of oppositely charged electrons or muons. No significant signal is observed and we set an upper limit on the isospin-averaged branching fraction B(B→πℓ⁺ℓ^-)<6.2×10^-8 at the 90% confidence level.

Using a 492  fb^-1 data sample collected near the Υ(4S) resonance with the Belle detector at the KEKB asymmetric-energy e⁺e^- collider, we observe the decay B⁰→pp̅ K^*0 with a branching fraction of (1.18_-0.25^+0.29(stat)±0.11(syst))×10^-6. We study the decay dynamics of B⁰→pp̅ K^*0 and compare with B⁺→pp̅ K^*+. The K^*0 meson is found to be almost 100% polarized (with a fraction of (101±13±3)% in the helicity zero state), while the K^*+ meson has a (32±17±9)% fraction in the helicity zero state. The direct CP asymmetries for B⁰→pp̅ K^*0 and B⁺→pp̅ K^*+ are measured to be -0.08±0.20±0.02 and -0.01±0.19±0.02, respectively. In addition, we report improved measurements of the branching fractions B(B⁺→pp̅ K^*+)=(3.38_-0.60^+0.73±0.39)×10^-6 and B(B⁰→pp̅ K⁰)=(2.51_-0.29^+0.35±0.21)×10^-6, which supersede our previous measurements.

We present a measurement of the branching fraction B(D_s⁺→μ⁺ν_μ) using a 548  fb^-1 data sample collected by the Belle experiment at the KEKB e⁺e^- collider. The D_s momentum is determined by reconstruction of the system recoiling against DKγX in events of the type e⁺e^-→D_s^*DKX, D_s^*→D_sγ, where X represents additional pions or photons from fragmentation. This full-reconstruction method provides high resolution in the neutrino momentum and thus good background separation, equivalent to that achieved by experiments at the tau-charm factories. We obtain the branching fraction B(D_s⁺→μ⁺ν_μ)=[6.44±0.76(stat)±0.57(syst)]×10^-3, implying a D_s decay constant of f_Ds=[275±16(stat)±12(syst)]  MeV.

A search for mixing in the neutral D meson system has been performed using semileptonic D⁰→K^(*)-e⁺ν and D⁰→K^(*)-μ⁺ν decays. Neutral D mesons from D^*+→D⁰π_s⁺ decays are used and the flavor at production is tagged by the charge of the slow pion. The measurement is performed using 492  fb^-1 of data recorded by the Belle detector. From the yield of right-sign and wrong-sign decays arising from nonmixed and mixed events, respectively, we measure the ratio of the time-integrated mixing rate to the unmixed rate to be R_M=(1.3±2.2±2.0)×10^-4. This corresponds to an upper limit of R_M<6.1×10^-4 at the 90% C.L.

We report a study of the processes e⁺e^-→J/ψD^(*)D̅ ^(*). In J/ψD^*D̅ ^* we observe a significant enhancement in the D^*D̅ ^* invariant mass spectrum, which we interpret as a new charmoniumlike state and denote X(4160). The X(4160) parameters are M=(4156_-20⁺²⁵±15)  MeV/c² and Γ=(139_-61⁺¹¹¹±21)  MeV. We also report a new measurement of the X(3940) mass and width: M=(3942_-6⁺⁷±6)  MeV/c² and Γ=(37_-15⁺²⁶±8)  MeV. The analysis is based on a 693  fb^-1 data sample recorded near the Υ(4S) resonance by the Belle detector at the KEKB asymmetric-energy collider.

We report improved measurements of time-dependent CP violation parameters for B⁰(B̅ ⁰)→ψ(2S)K_S⁰. This analysis is based on a data sample of 657×10⁶ BB̅ pairs collected at the Υ(4S) resonance with the Belle detector at the KEKB energy-asymmetric e⁺e^- collider. We fully reconstruct one neutral B meson in the ψ(2S)K_S⁰ CP-eigenstate decay channel, and the flavor of the accompanying B meson is identified to be either B⁰ or B̅ ⁰ from its decay products. CP violation parameters are obtained from the asymmetries in the distributions of the proper-time intervals between the two B decays: S_ψ(2S)KS⁰=+0.72±0.09(stat)±0.03(syst), A_ψ(2S)KS⁰=+0.04±0.07(stat)±0.05(syst). These results are in agreement with results from measurements of B⁰→J/ψK⁰.

We report a study of semileptonic B decays to P-wave D^** mesons. Semileptonic decay to a D₂^* meson is observed for the first time and its product branching ratio is measured to be B(B⁺→D̅ ₂^*0ℓ⁺ν)×B(D̅ ₂^*0→D^-π⁺)=0.22±0.03(stat.)±0.04(syst.)%. The result is obtained using fully reconstructed B tags from a data sample that contains 657×10⁶BB̅ pairs collected at the Υ(4S) resonance with the Belle detector at the KEKB asymmetric-energy e⁺e^- collider.

We report an improved measurement of the B⁺→D⁺D̅ ⁰ and B⁰→D⁰D̅ ⁰ decays based on 657×10⁶ BB̅ events collected with the Belle detector at KEKB. We measure the branching fraction and charge asymmetry for the B⁺→D⁺D̅ ⁰ decay: B(B⁺→D⁺D̅ ⁰)=(3.85±0.31±0.38)×10^-4 and A_CP(B⁺→D⁺D̅ ⁰)=0.00±0.08±0.02, where the first error is statistical and the second is systematic. We also set the upper limit for the B⁰→D⁰D̅ ⁰ decay: B(B⁰→D⁰D̅ ⁰)<0.43×10^-4 at 90% CL.

A distinct peak is observed in the π^±ψ^′ invariant mass distribution near 4.43 GeV in B→Kπ^±ψ^′ decays. A fit using a Breit-Wigner resonance shape yields a peak mass and width of M=4433±4(stat)±2(syst)  MeV and Γ=45_-13⁺¹⁸(stat)_-13⁺³⁰(syst)  MeV. The product branching fraction is determined to be B(B⁰→K^∓Z^±(4430))×B(Z^±(4430)→π^±ψ^′)=(4.1±1.0(stat)±1.4(syst))×10^-5, where Z^±(4430) is used to denote the observed structure. The statistical significance of the observed peak is 6.5σ. These results are obtained from a 605  fb^-1 data sample that contains 657×10⁶ BB̅ pairs collected near the Υ(4S) resonance with the Belle detector at the KEKB asymmetric energy e⁺e^- collider.

We present the results of a time-dependent Dalitz plot analysis of B⁰→π⁺π^-π⁰ decays based on a 414  fb^-1 data sample that contains 449×10⁶BB̅ pairs. The data were collected on the Υ(4S) resonance with the Belle detector at the KEKB asymmetric energy e⁺e^- collider. Combining our analysis with information on charged B decay modes, we perform a full Dalitz and isospin analysis and obtain a constraint on the quark mixing angle ϕ₂, 68°<ϕ₂<95° at the 68.3% confidence level for the ϕ₂ solution consistent with the standard model. A large standard-model–disfavored region also remains. The branching fractions for the decay processes B⁰→ρ^±(770)π^∓ and B⁰→ρ⁰(770)π⁰ are measured to be (22.6±1.1[stat]±4.4[syst])×10^-6 and (3.0±0.5[stat]±0.7[syst])×10^-6, respectively. These are the first branching fraction measurements of the process B⁰→ρ(770)π with the lowest resonance ρ(770) explicitly separated from the radial excitations.

We report improved measurements of time-dependent CP violation parameters for B⁰(B̅ ⁰)→J/ψπ⁰ decay. This analysis is based on 535×10⁶ BB̅ pairs accumulated at the Υ(4S) resonance with the Belle detector at the KEKB asymmetric-energy e⁺e^- collider. From the distribution of proper time intervals between the two B decays, we obtain the following CP violation parameters S_J/ψπ⁰=-0.65±0.21(stat)±0.05(syst)  and  A_J/ψπ⁰=+0.08±0.16(stat)±0.05(syst),which are consistent with standard model expectations.

We report a measurement of the CP-violating parameters in B⁰→K_S⁰K_S⁰ decays based on a data sample of 657×10⁶ BB̅ pairs collected at the Υ(4S) resonance with the Belle detector at the KEKB asymmetric-energy e⁺e^- collider. In this Letter, one neutral B meson is fully reconstructed in the B⁰→K_S⁰K_S⁰ decay mode, and the flavor of the accompanying B meson is identified by its decay products. The CP-violating parameters are measured from the asymmetry in the distributions of the proper-time interval between the two B decays: S_KS⁰KS⁰=-0.38_-0.77^+0.69(stat)±0.09(syst) and A_KS⁰KS⁰=-0.38±0.38(stat)±0.05(syst).

We search for the radiative penguin decays B_s⁰→ϕγ and B_s⁰→γγ in a 23.6  fb^-1 data sample collected at the Υ(5S) resonance with the Belle detector at the KEKB e⁺e^- asymmetric-energy collider. We observe for the first time a radiative penguin decay of the B_s⁰ meson in the B_s⁰→ϕγ mode and we measure B(B_s⁰→ϕγ)=(57_-15⁺¹⁸(stat)_-11⁺¹²(syst))×10^-6. No significant B_s⁰→γγ signal is observed and we set a 90% confidence level upper limit of B(B_s⁰→γγ)<8.7×10^-6.

We report the first observation of e⁺e^-→Υ(1S)π⁺π^-, Υ(2S)π⁺π^-, and first evidence for e⁺e^-→Υ(3S)π⁺π^-, Υ(1S)K⁺K^-, near the peak of the Υ(5S) resonance at sqrt[s]∼10.87  GeV. The results are based on a data sample of 21.7  fb^-1 collected with the Belle detector at the KEKB e⁺e^- collider. Attributing the signals to the Υ(5S) resonance, the partial widths Γ(Υ(5S)→Υ(1S)π⁺π^-)=0.59±0.04(stat)±0.09(syst)  MeV and Γ(Υ(5S)→Υ(2S)π⁺π^-)=0.85±0.07(stat)±0.16(syst)  MeV are obtained from the observed cross sections. These values exceed by more than 2 orders of magnitude the previously measured partial widths for dipion transitions between lower Υ resonances.