Search Results (304 total)

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.

The infrared phonon spectrum of the spinel CdCr₂O₄ is measured as a function of temperature from 6 to 300 K. The triply degenerate Cr phonons soften in the paramagnetic phase as temperature is lowered below 100 K and then split into a singlet and doublet in the low temperature antiferromagnetic phase, which is tetragonally distorted to relieve the geometric frustration in the pyrochlore lattice of ions. The phonon splitting is inconsistent with the simple increase (decrease) in the force constants due to decreasing (increasing) bond lengths in the tetragonal phase. Rather they correspond to changes in the force constants due to the magnetic order in the antiferromagnetic state. The phonon splitting in this system is opposite of that observed earlier in ZnCr₂O₄ as predicted by theory. The magnitude of the splitting gives a measure of the spin-phonon coupling strength, which is smaller than in the case of ZnCr₂O₄.

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.

We report the observation of a new D_sJ meson produced in B⁺→D̅ ⁰D_sJ→D̅ ⁰D⁰K⁺. This state has a mass of M=2708±9_-10⁺¹¹  MeV/c², a width Γ=108±23_-31⁺³⁶  MeV/c² and a 1^- spin-parity. The statistical significance of this observation is 8.4σ. The results are based on an analysis of 449×10⁶ BB̅ events collected at the Υ(4S) resonance with the Belle detector at the KEKB asymmetric-energy e⁺e^- collider.

We search for the doubly charmed baryonic decay B̅ ⁰→Λ_c⁺Λ̅ _c^-, in a data sample of 520×10⁶ BB̅ events accumulated at the Υ(4S) resonance with the Belle detector at the KEKB asymmetric-energy e⁺e^- collider. We find no significant signal and set an upper limit of B(B̅ ⁰→Λ_c⁺Λ̅ _c^-)<6.2×10^-5 at 90% confidence level. The result is significantly below a naive extrapolation from B(B^-→Ξ_c⁰Λ̅ _c^-) assuming a simple Cabibbo-suppression factor of |V_cd/V_cs|². The small branching fraction may be attributed to a suppression due to the large momentum of the baryonic decay products, which has been observed in other charmed baryonic two-body B decays.

We consider ν_μ→ν_τ oscillations in the context of the mass varying neutrino (MaVaN) model, where the neutrino mass can vary depending on the electron density along the flight path of the neutrino. Our analysis assumes a mechanism with dependence only upon the electron density, hence ordinary matter density, of the medium through which the neutrino travels. Fully-contained, partially-contained and upward-going muon atmospheric neutrino data from the Super-Kamiokande detector, taken from the entire SK-I period of 1489 live days, are compared to MaVaN model predictions. We find that, for the case of 2-flavor oscillations, and for the specific models tested, oscillation independent of electron density is favored over density dependence. Assuming maximal mixing, the best-fit case and the density-independent case do not differ significantly.

Using 462  fb^-1 of e⁺e^- annihilation data recorded by the Belle detector, we report the first observation of the decay D_s1(2536)⁺→D⁺π^-K⁺. The ratio of branching fractions B(D_s1(2536)⁺→D⁺π^-K⁺) / B(D_s1(2536)⁺→D^*+K⁰) is measured to be (3.27±0.18±0.37)%. We also study the angular distributions in the D_s1(2536)⁺→D^*+K_S⁰ decay and measure the ratio of D- and S-wave amplitudes. The S-wave dominates, with a partial width of Γ_S/Γ_total=0.72±0.05±0.01.

We report measurements of the exclusive cross section for e⁺e^-→D⁰D^-π⁺ over the center-of-mass energy range 4.0 GeV to 5.0 GeV with initial-state radiation and the first observation of the decay ψ(4415)→D⁰D^-π⁺. From a study of the resonant substructure in ψ(4415) decay we conclude that the ψ(4415)→D⁰D^-π⁺ decay is dominated by ψ(4415)→DD̅ ₂^*(2460). We obtain B(ψ(4415)→D⁰D^-π_nonresonant⁺)/B(ψ(4415)→DD̅ ₂^*(2460)→D⁰D^-π⁺)<0.22 at 90% C.L. The analysis is based on a data sample collected with the Belle detector with an integrated luminosity of 673  fb^-1.

We report inelastic light scattering experiments on the single-layered manganite LaSrMnO₄. The distinct features observed are the appearance of numerous symmetry-forbidden phonon peaks, with a strong temperature dependence of their intensity and frequency, and a double-peak structure of the activated high-frequency in-plane modes. These observations are ascribed to the conspicuous dynamics of an admixed d_x²−y² orbital present in the background of the crystal field stabilized d_3z²−r² orbitals. Our study suggests a significant competition between the kinetic energy gain due to the resulting orbital canting and the strong crystal field splitting.

We report discovery of collinear-magnetism-driven ferroelectricity in the Ising chain magnet Ca₃Co_2-xMn_xO₆ (x≈0.96). Neutron diffraction shows that Co²⁺ and Mn⁴⁺ ions alternating along the chains exhibit an up-up-down-down (↑↑↓↓) magnetic order. The ferroelectricity results from the inversion symmetry breaking in the ↑↑↓↓ spin chain with an alternating charge order. Unlike in spiral magnetoelectrics where antisymmetric exchange coupling is active, the symmetry breaking in Ca₃(Co,Mn)₂O₆ occurs through exchange striction associated with symmetric superexchange.

We used a continuously rotating torsion balance instrument to measure the acceleration difference of beryllium and titanium test bodies towards sources at a variety of distances. Our result Δa_N,Be-Ti=(0.6±3.1)×10^-15  m/s² improves limits on equivalence-principle violations with ranges from 1 m to ∞ by an order of magnitude. The Eötvös parameter is η_Earth,Be-Ti=(0.3±1.8)×10^-13. By analyzing our data for accelerations towards the center of the Milky Way we find equal attractions of Be and Ti towards galactic dark matter, yielding η_DM,Be-Ti=(-4±7)×10^-5. Space-fixed differential accelerations in any direction are limited to less than 8.8×10^-15  m/s² with 95% confidence.

We report the first measurement of CP-violation parameters in B⁰→ρ⁰γ decays based on a data sample of 657×10⁶BB̅ pairs collected with the Belle detector at the KEKB asymmetric-energy e⁺e^- collider. We obtain the time-dependent and direct CP-violating parameters, S_ρ⁰γ=-0.83±0.65(stat)±0.18(syst) and A_ρ⁰γ=-0.44±0.49(stat)±0.14(syst), respectively.

The process e⁺e^-→K⁺K^-J/ψ is observed for the first time via initial-state radiation. The cross section of e⁺e^-→K⁺K^-J/ψ for center-of-mass energies between threshold and 6.0 GeV is measured using 673  fb^-1 of data collected with the Belle detector on and off the Υ(4S) resonance. No significant signal for Y(4260)→K⁺K^-J/ψ is observed, and we determine B(Y(4260)→K⁺K^-J/ψ)Γ(Y(4260)→e⁺e^-)<1.2  eV/c² at a 90% confidence level. We also find evidence for e⁺e^-→K_S⁰K_S⁰J/ψ in the same data sample.