Search Results (477 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.

Azimuthal angle (Δϕ) correlations are presented for a broad range of transverse momentum (0.4<p_T<10 GeV/c) and centrality (0–92%) selections for charged hadrons from dijets in Au+Au collisions at sqrt[s_NN]=200 GeV. With increasing p_T, the away-side Δϕ distribution evolves from a broad and relatively flat shape to a concave shape, then to a convex shape. Comparisons with p+p data suggest that the away-side distribution can be divided into a partially suppressed “head” region centered at Δϕ~π, and an enhanced “shoulder” region centered at Δϕ~π±1.1. The p_T spectrum for the associated hadrons in the head region softens toward central collisions. The spectral slope for the shoulder region is independent of centrality and trigger p_T. The properties of the near-side distributions are also modified relative to those in p+p collisions, reflected by the broadening of the jet shape in Δϕ and Δη, and an enhancement of the per-trigger yield. However, these modifications seem to be limited to p_T≲4 GeV/c, above which both the hadron pair shape and per-trigger yield become similar to p+p collisions. These observations suggest that both the away- and near-side distributions contain a jet fragmentation component which dominates for p_T≳5 GeV/c and a medium-induced component which is important for p_T≲4 GeV/c. We also quantify the role of jets at intermediate and low p_T through the yield of jet-induced pairs in comparison with binary scaled p+p pair yield. The yield of jet-induced pairs is suppressed at high pair proxy energy (sum of the p_T magnitudes of the two hadrons) and is enhanced at low pair proxy energy. The former is consistent with jet quenching; the latter is consistent with the enhancement of soft hadron pairs due to transport of lost energy to lower p_T.

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.

The PHENIX experiment has measured the suppression of semi-inclusive single high-transverse-momentum π⁰'s in Au+Au collisions at sqrt[s_NN]=200 GeV. The present understanding of this suppression is in terms of energy loss of the parent (fragmenting) parton in a dense color-charge medium. We have performed a quantitative comparison between various parton energy-loss models and our experimental data. The statistical point-to-point uncorrelated as well as correlated systematic uncertainties are taken into account in the comparison. We detail this methodology and the resulting constraint on the model parameters, such as the initial color-charge density dN^g/dy, the medium transport coefficient 〈q-^ 〉, or the initial energy-loss parameter ε₀. We find that high-transverse-momentum π⁰ suppression in Au+Au collisions has sufficient precision to constrain these model-dependent parameters at the ±20–25% (one standard deviation) level. These constraints include only the experimental uncertainties, and further studies are needed to compute the corresponding theoretical uncertainties.

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.

A three-dimensional correlation function obtained from midrapidity, low p_T, pion pairs in central Au+Au collisions at sqrt[s_NN]=200  GeV is studied. The extracted model-independent source function indicates a long range tail in the directions of the pion pair transverse momentum (out) and the beam (long). A proper breakup time τ₀∼9  fm/c and a mean proper emission duration Δτ∼2  fm/c, leading to sizable emission time differences (⟨|Δt_LCM|⟩≈12  fm/c), are required to allow models to be successfully matched to these tails. The model comparisons also suggest an outside-in “burning” of the emission source reminiscent of many hydrodynamical models.

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.

Ce 4d-4f resonant angle-resolved photoemission spectroscopy was carried out to study the electronic structure of strongly correlated Ce 4f electrons in a quasi-two-dimensional nonmagnetic heavy-fermion system CeCoGe_1.2Si_0.8. For the first time, dispersive coherent peaks of an f state crossing the Fermi level, the so-called Kondo resonance, are directly observed together with the hybridized conduction band. Moreover, the experimental band dispersion is quantitatively in good agreement with a simple hybridization-band picture based on the periodic Anderson model. The obtained physical quantities, i.e., coherent temperature, Kondo temperature, and mass enhancement, are comparable to the results of thermodynamic measurements. These results manifest an itinerant nature of Ce 4f electrons in heavy-fermion systems and clarify their microscopic hybridization mechanism.

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.

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 present a new analysis of J/ψ production yields in deuteron-gold collisions at sqrt[s_NN]=200 GeV using data taken from the PHENIX experiment in 2003 and previously published in S. S. Adler [Phys. Rev. Lett 96, 012304 (2006)]. The high statistics proton-proton J/ψ data taken in 2005 are used to improve the baseline measurement and thus construct updated cold nuclear matter modification factors (R_dAu). A suppression of J/ψ in cold nuclear matter is observed as one goes forward in rapidity (in the deuteron-going direction), corresponding to a region more sensitive to initial-state low-x gluons in the gold nucleus. The measured nuclear modification factors are compared to theoretical calculations of nuclear shadowing to which a J/ψ (or precursor) breakup cross section is added. Breakup cross sections of σ_breakup=2.8_-1.4^+1.7 (2.2_-1.5^+1.6) mb are obtained by fitting these calculations to the data using two different models of nuclear shadowing. These breakup cross-section values are consistent within large uncertainties with the 4.2±0.5 mb determined at lower collision energies. Projecting this range of cold nuclear matter effects to copper-copper and gold-gold collisions reveals that the current constraints are not sufficient to firmly quantify the additional hot nuclear matter effect.

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 present transverse momentum (p_T) spectra of charged hadrons measured in deuteron-gold and nucleon-gold collisions at sqrt[s_NN]=200 GeV for four centrality classes. Nucleon-gold collisions were selected by tagging events in which a spectator nucleon was observed in one of two forward rapidity detectors. The spectra and yields were investigated as a function of the number of binary nucleon-nucleon collisions, ν, suffered by deuteron nucleons. A comparison of charged particle yields to those in p+p collisions show that yield per nucleon-nucleon collision saturates with ν for high momentum particles. We also present the charged hadron to neutral pion ratios as a function of p_T.

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.