Search Results (695 total)

We present a study of the decays B^0,+→J/ψωK^0,+ using 383×10⁶ BB̅ events obtained with the BABAR detector at PEP-II. We observe Y(3940)→J/ψω, with mass 3914.6_-3.4^+3.8(stat)±2.0(syst)  MeV/c², and width 34_-8⁺¹²(stat)±5(syst)  MeV. The ratio of B⁰ and B⁺ decay to YK is 0.27_-0.23^+0.28(stat)_-0.01^+0.04(syst), and the relevant B⁰ and B⁺ branching fractions are reported.

Ab initio calculations reveal a universal scaling law on how the electronic structure of boron nitride (BN) nanoribbons and nanotubes is modified by a transverse electric field. Due to the structural symmetry difference, the energy gap of zigzag BN ribbons can be reduced or increased by the electric field depending on the sign of the field, while that of the armchair ones is always reduced. However, the linear giant Stark effect coefficients of zigzag and armchair BN nanoribbons, as well as those of BN nanotubes, are found to obey a unified scaling law where the coefficient increases linearly with the ribbon width or the tube diameter with a slope of 1.0. The mechanism of the scaling law is identified using a general model, which may be applicable to other semiconducting nanostructures.

The quantum adiabatic theorem plays an important role in quantum mechanics. However, counterexamples were produced recently, indicating that their transition probabilities do not converge as predicted by the adiabatic theorem [K. P. Marzlin , Phys. Rev. Lett. 93, 160408 (2004); D. M. Tong , Phys. Rev. Lett. 95, 110407 (2005)]. For a special class of Hamiltonians, we examine the standard criterion for adiabatic evolution experimentally and theoretically, as well as three newly suggested adiabatic conditions. We show that the standard criterion is neither sufficient nor necessary.

The absence of a missing moment inertia in clean solid ⁴He suggests that the minimal experimentally relevant model is the one in which disorder induces superfluidity in a bosonic lattice. To this end, we explore the relevance of the disordered Bose-Hubbard model in this context. We posit that a clean array of ⁴He atoms is a self-generated Mott insulator; that is, the ⁴He atoms constitute the lattice as well as the “charge carriers.” With this assumption, we are able to interpret the textbook defect-driven supersolids as excitations of either the lower or the upper Hubbard bands. In the experiments at hand, disorder induces the closing of the Mott gap through the generation of midgap localized states at the chemical potential. Depending on the magnitude of the disorder, we find that the destruction of the Mott state takes place for d+z>4 either through a Bose-glass phase (strong disorder) or through a direct transition to a superfluid (weak disorder). For d+z<4, disorder is always relevant. The critical value of the disorder that separates these two regimes is shown to be a function of the boson filling, interaction, and the momentum cutoff. We apply our work to the experimentally observed enhancement ³He impurities have on the onset temperature for the missing moment of inertia. We find quantitative agreement with experimental trends.

For sessile droplets partially wetting a solid surface, it has been observed experimentally that the value of the contact angle depends on the contact line curvature and this dependence has been attributed to tension in the contact line. But previous analyses of these observations have neglected adsorption at the solid-liquid interface and its effect on the surface tension of this interface. We show that if this adsorption is taken into account the relation between the contact angle and contact line curvature is completely accounted for without introducing line tension. Further, from the observed relation between the contact angle and contact line curvature, the adsorption at the solid-liquid interface can be determined, as can the surface tensions of the solid-liquid and solid-vapor interfaces.

We report a new STAR measurement of the longitudinal double-spin asymmetry A_LL for inclusive jet production at midrapidity in polarized p+p collisions at a center-of-mass energy of sqrt[s]=200  GeV. The data, which cover jet transverse momenta 5<p_T<30  GeV/c, are substantially more precise than previous measurements. They provide significant new constraints on the gluon spin contribution to the nucleon spin through the comparison to predictions derived from one global fit to polarized deep-inelastic scattering measurements.

We present the first spin alignment measurements for the K^*0(892) and ϕ(1020) vector mesons produced at midrapidity with transverse momenta up to 5 GeV/c at sqrt[s_NN]=200 GeV at RHIC. The diagonal spin-density matrix elements with respect to the reaction plane in Au+Au collisions are ρ₀₀=0.32±0.04 (stat) ± 0.09 (syst) for the K^*0 (0.8<p_T<5.0 GeV/c) and ρ₀₀=0.34±0.02 (stat) ± 0.03 (syst) for the ϕ (0.4<p_T<5.0 GeV/c) and are constant with transverse momentum and collision centrality. The data are consistent with the unpolarized expectation of 1/3 and thus no evidence is found for the transfer of the orbital angular momentum of the colliding system to the vector-meson spins. Spin alignments for K^*0 and ϕ in Au+Au collisions were also measured with respect to the particle's production plane. The ϕ result, ρ₀₀=0.41±0.02 (stat) ± 0.04 (syst), is consistent with that in p+p collisions, ρ₀₀=0.39±0.03 (stat) ± 0.06 (syst), also measured in this work. The measurements thus constrain the possible size of polarization phenomena in the production dynamics of vector mesons.

The τ^-→ηπ^-π⁺π^-ν_τ decay with the η→γγ mode is studied using 384  fb^-1 of data collected by the BABAR detector. The branching fraction is measured to be (1.60±0.05±0.11)×10^-4. It is found that τ^-→f₁(1285)π^-ν_τ→ηπ^-π⁺π^-ν_τ is the dominant decay mode with a branching fraction of (1.11±0.06±0.05)×10^-4. The first error on the branching fractions is statistical and the second systematic. Note that no particle identification algorithm is applied to the charged tracks to distinguish pions from kaons. In addition, a 90% confidence level upper limit on the branching fraction of the τ^-→η^′(958)π^-ν_τ decay is measured to be 7.2×10^-6. This last decay proceeds through a second-class current and is expected to be forbidden in the limit of isospin symmetry.

This paper reports measurements of processes: e⁺e^-→γK_S⁰K^±π^∓, e⁺e^-→γK⁺K^-π⁰, e⁺e^-→γϕη, and e⁺e^-→γϕπ⁰. The initial-state radiated photon allows to cover the hadronic final state in the energy range from thresholds up to ≈4.6  GeV. The overall size of the data sample analyzed is 232  fb^-1, collected by the BABAR detector running at the PEP-II e⁺e^- storage ring. From the Dalitz plot analysis of the K_S⁰K^±π^∓ final state, moduli, and relative phase of the isoscalar and the isovector components of the e⁺e^-→KK^*(892) cross section are determined. Parameters of ϕ and ρ recurrences are also measured, using a global fitting procedure which exploits the interconnection among amplitudes, moduli, and phases of the e⁺e^-→K_S⁰K^±π^∓, K⁺K^-π⁰, ϕη final states. The cross section for the OZI-forbidden process e⁺e^-→ϕπ⁰, and the J/ψ branching fractions to KK^*(892) and K⁺K^-η are also measured.

The electronic structure and thermoelectric properties of ReSi_1.75 and its doped systems are studied by the first-principles calculation using the full-potential linearized augmented plane-wave method in the local density approximation with self-interaction correction and the semiclassical Boltzmann theory. ReSi_1.75 shows narrow gap semiconductor behavior with an indirect gap of 0.12 eV and a direct gap of 0.36 eV. The Fermi levels of Al- and Mo-doped systems move into the valence band. The Al-doped compound remains a semiconductor while the gap of Mo-doped compounds becomes small and nearly disappears. The relations between the electronic structure and thermoelectric properties of doped ReSi_1.75 are analyzed and explained in detail. Our calculations strongly suggest that an excellent thermoelectric performance can be obtained for p-doped ReSi_1.75 along [100] and for n-doped ReSi_1.75 along [001] compared with the undoped compound.

An efficient and straightforward approach is proposed to produce extreme ultraviolet frequency combs by using high-order harmonic generation from diatomic molecules driven by surface-enhanced high-repetition-rate femtosecond pulses. For a nonpolar molecule, the maximum (minimum) energy conversion efficiency is observed as the molecule is oriented at an angle of 27° (108°) relative to the surface. For a polar molecule, the frequency components of the generated ultraviolet frequency combs are doubled as both odd- and even-order harmonics are generated, which shows different orientation dependence and shifts from the nonpolar molecule. The generated ultraviolet frequency combs are mainly emitted with angles smaller than 30° close to the surface, while there is a noticeable emission probability around the directions perpendicular to the surface when the molecules are oriented with large angles.

The intrinsic transport properties of zigzag graphene nanoribbons (ZGNRs) are investigated using first-principles calculations. It is found that although all ZGNRs have similar metallic band structure, they show distinctly different transport behaviors under bias voltages, depending on whether they are mirror symmetric with respect to the midplane between two edges. Asymmetric ZGNRs behave as conventional conductors with linear current-voltage dependence, while symmetric ZGNRs exhibit unexpected very small currents with the presence of a conductance gap around the Fermi level. This difference is revealed to arise from different coupling between the conducting subbands around the Fermi level, which is dependent on the symmetry of the systems.

We present STAR results on the elliptic flow v₂ of charged hadrons, strange and multistrange particles from sqrt[s_NN]=200 GeV Au+Au collisions at the BNL Relativistic Heavy Ion Collider (RHIC). The detailed study of the centrality dependence of v₂ over a broad transverse momentum range is presented. Comparisons of different analysis methods are made in order to estimate systematic uncertainties. To discuss the nonflow effect, we have performed the first analysis of v₂ with the Lee-Yang zero method for K_S⁰ and Λ. In the relatively low p_T region, p_T≤2 GeV/c, a scaling with m_T-m is observed for identified hadrons in each centrality bin studied. However, we do not observe v₂(p_T) scaled by the participant eccentricity to be independent of centrality. At higher p_T,2≤p_T≤6 GeV/c,v₂ scales with quark number for all hadrons studied. For the multistrange hadron Ω, which does not suffer appreciable hadronic interactions, the values of v₂ are consistent with both m_T-m scaling at low p_T and number-of-quark scaling at intermediate p_T. As a function of collision centrality, an increase of p_T-integrated v₂ scaled by the participant eccentricity has been observed, indicating a stronger collective flow in more central Au+Au collisions.

We report on our search for decays proceeding via a tree-level b→c quark transition in which a gluon radiates into an ss̅ pair. We present observations of the decays B^-→D_s^(*)+K^-π^- and B̅ ⁰→D_s⁺K_S⁰π^- and evidence for B^-→D_s⁺K^-K^- and set upper limits on the branching fractions for B̅ ⁰→D_s^*+K_S⁰π^- and B^-→D_s^*+K^-K^- using 383×10⁶ Υ(4S)→BB̅ events collected by the BABAR detector at SLAC. We present evidence that the invariant mass distributions of D_s^(*)+K^- pairs from B^-→D_s^(*)+K^-π^- decays are inconsistent with the phase-space model, suggesting the presence of charm resonances lying below the D_s^(*)+K^- threshold.

We present partial branching fractions for inclusive charmless semileptonic B decays B̅ →X_uℓν̅ , and the determination of the Cabibbo-Kobayashi-Maskawa matrix element |V_ub|. The analysis is based on a sample of 383×10⁶ Υ(4S) decays into BB̅ pairs collected with the BABAR detector at the SLAC PEP-II e⁺e^- storage rings. We select events using the invariant mass M_X of the hadronic system, the invariant mass squared, q², of the lepton and neutrino pair, the kinematic variable P₊, or one of their combinations. We then determine partial branching fractions in limited regions of phase space: ΔB=(1.18±0.09_stat±0.07_syst±0.01_theor)×10^-3 (M_X<1.55  GeV/c²), ΔB=(0.95±0.10_stat±0.08_syst±0.01_theor)×10^-3 (P₊<0.66  GeV/c), and ΔB=(0.81±0.08_stat±0.07_syst±0.02_theor)×10^-3 (M_X<1.7  GeV/c², q²>8  GeV²/c⁴). Corresponding values of |V_ub| are extracted using several theoretical calculations.

YBa₂Cu₃O_7−x films with nanotube pores (YBCO/NTPs) aligned along normal of the film have been fabricated with thickness ranging from 0.1  to  1.2  μm. When compared to YBCO films without NTPs, YBCO/NTP samples show much reduced thickness dependence of the critical current density J_c, in addition to the overall higher J_c values, in magnetic fields below the matching field of the NTPs. These results suggest that the through-thickness NTPs may provide an enhanced pinning force per vortex length (F_p) which remains a constant at various thicknesses.

We present for the first time a measurement of the weak phase 2β+γ obtained from a time-dependent Dalitz plot analysis of B⁰→D^∓K⁰π^± decays. Using a sample of approximately 347×10⁶ BB̅ pairs collected by the BABAR detector at the PEP-II asymmetric-energy storage rings and assuming the ratio r of the b→u and b→c decay amplitudes to be 0.3, we obtain 2β+γ=(83±53±20)° and the equivalent solution at +180°. The magnitudes and phases for the resonances associated with the b→c transitions are also extracted from the fit.

We investigate the effect of edge defects (vacancies) and impurities (substitutional dopants) on the robustness of spin polarization in graphene nanoribbons (GNRs) with zigzag edges by using density-functional-theory calculations. The stability of the spin state and its magnetic moments is found to continuously decrease with increasing the concentration of the defects or impurities. The system generally becomes nonmagnetic at the concentration of one edge defect (impurity) per ∼10  Å. The spin suppression is shown to be caused by the reduction and removal of edge states at the Fermi energy. Our analysis implies an important criterion on the GNR samples for spintronics applications.

We report on the observed differences in production rates of strange and multistrange baryons in Au+Au collisions at sqrt[s_NN]=200 GeV compared to p+p interactions at the same energy. The strange baryon yields in Au+Au collisions, when scaled down by the number of participating nucleons, are enhanced relative to those measured in p+p reactions. The enhancement observed increases with the strangeness content of the baryon, and it increases for all strange baryons with collision centrality. The enhancement is qualitatively similar to that observed at the lower collision energy sqrt[s_NN]=17.3 GeV. The previous observations are for the bulk production, while at intermediate p_T,1<p_T<4 GeV/c, the strange baryons even exceed binary scaling from p+p yields.

The longitudinal space-charge (LSC) force can be a major cause of the microbunching instability in the linac for an x-ray free-electron laser. In this paper, the LSC-induced beam modulation is studied using an integral equation approach that takes into account the transverse (radial) variation of the LSC field for both the coasting-beam limit and a bunched beam. Variation of the beam energy and the transverse beam size is also incorporated. We discuss the validity of this approach and compare it with other analytical analyses as well as numerical simulations.

We report results of a search for CPT and Lorentz violation in B⁰-B̅ ⁰ oscillations using inclusive dilepton events from 232×10⁶ Υ(4S)→BB̅ decays recorded by the BABAR detector at the PEP-II B Factory at SLAC. We find 2.8σ significance, compatible with no signal, for variations in the complex CPT violation parameter z at the Earth’s sidereal frequency and extract values for the quantities Δa_μ in the general Lorentz-violating standard-model extension. The spectral powers for variations in z over the frequency range 0.26  yr^-1 to 2.1  solar day^-1 are also compatible with no signal.

Photoproduction reactions occur when the electromagnetic field of a relativistic heavy ion interacts with another heavy ion. The STAR Collaboration presents a measurement of ρ⁰ and direct π⁺π^- photoproduction in ultraperipheral relativistic heavy ion collisions at sqrt[s_NN]=200 GeV. We observe both exclusive photoproduction and photoproduction accompanied by mutual Coulomb excitation. We find a coherent cross section of σ(AuAu→Au^*Au^*ρ⁰)=530±19(stat.)±57(syst.) mb, in accord with theoretical calculations based on a Glauber approach, but considerably below the predictions of a color dipole model. The ρ⁰ transverse momentum spectrum (p_T²) is fit by a double exponential curve including both coherent and incoherent coupling to the target nucleus; we find σ_inc/σ_coh=0.29±0.03(stat.)±0.08(syst.). The ratio of direct π⁺π^- to ρ⁰ production is comparable to that observed in γp collisions at HERA and appears to be independent of photon energy. Finally, the measured ρ⁰ spin helicity matrix elements agree within errors with the expected s-channel helicity conservation.