Search Results (50 total)

A sampling algorithm is presented that generates spin-glass configurations of the two-dimensional Edwards-Anderson Ising spin glass at finite temperature with probabilities proportional to their Boltzmann weights. Such an algorithm overcomes the slow dynamics of direct simulation and can be used to study long-range correlation functions and coarse-grained dynamics. The algorithm uses a correspondence between spin configurations on a regular lattice and dimer (edge) coverings of a related graph: Wilson’s algorithm [D. B. Wilson, Proceedings of the Eighth Symposium on Discrete Algorithms (SIAM, Philadelphia, 1997), p 258] for sampling dimer coverings on a planar lattice is adapted to generate samplings for the dimer problem corresponding to both planar and toroidal spin-glass samples. This algorithm is recursive: it computes probabilities for spins along a “separator” that divides the sample in half. Given the spins on the separator, sample configurations for the two separated halves are generated by further division and assignment. The algorithm is simplified by using Pfaffian elimination rather than Gaussian elimination for sampling dimer configurations. For n spins and given floating point precision, the algorithm has an asymptotic run-time of O(n^3/2); it is found that the required precision scales as inverse temperature and grows only slowly with system size. Sample applications and benchmarking results are presented for samples of size up to n=128², with fixed and periodic boundary conditions.

Using e⁺e^- collision data taken with the CLEO-c detector at the Cornell Electron Storage Ring, we have investigated the direct photon spectrum in the decay ψ(2S)→γgg. We determine the ratio of the inclusive direct photon decay rate to that of the dominant three-gluon decay rate ψ(2S)→ggg (R_γ≡Γ(γgg)/Γ(ggg)) to be R_γ(z_γ>0.4)=0.070±0.002±0.019±0.011, with z_γ defined as the scaled photon energy relative to the beam energy. The errors shown are statistical, systematic, and that due to the uncertainty in the input branching fractions used to extract the ratio, respectively.

Using e⁺e^-→D_s^-D_s^*+ and D_s^*-D_s⁺ interactions at 4170 MeV collected with the CLEO-c detector, we investigate the semileptonic decays D_s⁺→f₀(980)e⁺ν and D_s⁺→ϕe⁺ν. By examining the decay rates as functions of the four-momentum transfer squared q², we measure the ratio [dB / dq²(D_s⁺→f₀(980)e⁺ν)B(f₀→π⁺π^-)]/[dB / dq²(D_s⁺→ϕe⁺ν)B(ϕ→K⁺K^-)] at q² of zero to be (42±11)%. This ratio has been predicted to equal the rate ratio [B(B_s→J/ψf₀)B(f₀→π⁺π^-)]/[B(B_s→J/ψϕ)B(ϕ→K⁺K^-)], thus indicating that the CP eigenstate J/ψf₀ could be useful for measuring CP violation via B_s mixing. Assuming a simple pole model for the form factor |f₊(q²)| in the f₀e⁺ν decay, we find a pole mass of (1.7_-0.7^+4.5±0.2)  GeV. We also determine the f₀ mass and width as (977_-9⁺¹¹±1) and (91_-22⁺³⁰±3)  MeV, respectively. In addition, we present updated results for B(D_s⁺→f₀(980)e⁺ν)B(f₀→π⁺π^-)=(0.20±0.03±0.01)% and B(D_s⁺→ϕe⁺ν)=(2.36±0.23±0.13)%. Assuming that the f₀ wave function is a combination of strange and nonstrange quark-antiquark components, we use our measurement for B(D_s⁺→f₀(980)e⁺ν) to extract a value of the mixing angle that we find consistent with ∣s̅ s⟩ dominance, adding to the mystery as to why the f₀ decays predominantly to two pions rather than two kaons.

Using a sample of 2.59×10⁷ ψ(2S) decays collected by the CLEO-c detector, we present results of a search for the decay chain ψ(2S)→π⁰h_c, h_c→n(π⁺π^-)π⁰, n=1, 2, 3. We observe no significant signals for n=1 and n=3 and set upper limits for the corresponding decay rates. First evidence for the decay h_c→π⁺π^-π⁺π^-π⁰ is presented, and a product branching fraction of B(ψ(2S)→h_cπ⁰)×B(h_c→2(π⁺π^-)π⁰)=1.88_-0.45-0.30^+0.48+0.47×10^-5 is measured. This result implies that h_c→hadrons and h_c→γη_c have comparable rates, in agreement with expectations.

Using data collected near the D_s^*±D_s^∓ peak production energy E_cm=4170  MeV by the CLEO-c detector, we search for D_s⁺ exclusive hadronic decays involving ω. We find B(D_s⁺→π⁺ω)=(0.21±0.09±0.01)%, B(D_s⁺→π⁺π⁰ω)=(2.78±0.65±0.25)%, B(D_s⁺→π⁺π⁺π^-ω)=(1.58±0.45±0.09)%, B(D_s⁺→π⁺ηω)=(0.85±0.54±0.06)%, B(D_s⁺→K⁺ω)<0.24%, B(D_s⁺→K⁺π⁰ω)<0.82%, B(D_s⁺→K⁺π⁺π^-ω)<0.54%, and B(D_s⁺→K⁺ηω)<0.79%. The upper limits are at 90% confidence level.

Using the entire CLEO-c ψ(3770)→DD̅ event sample, corresponding to an integrated luminosity of 818  pb^-1 and approximately 5.4×10⁶ DD̅ events, we present a study of the decays D⁰→π^-e⁺ν_e, D⁰→K^-e⁺ν_e, D⁺→π⁰e⁺ν_e, and D⁺→K̅ ⁰e⁺ν_e. Via a tagged analysis technique, in which one D is fully reconstructed in a hadronic mode, partial rates for semileptonic decays by the other D are measured in several q² bins. We fit these rates using several form factor parametrizations and report the results, including form factor shape parameters and the branching fractions B(D⁰→π^-e⁺ν_e)=(0.288±0.008±0.003)%, B(D⁰→K^-e⁺ν_e)=(3.50±0.03±0.04)%, B(D⁺→π⁰e⁺ν_e)=(0.405±0.016±0.009)%, and B(D⁺→K̅ ⁰e⁺ν_e)=(8.83±0.10±0.20)%, where the first uncertainties are statistical and the second are systematic. Taking input from lattice quantum chromodynamics, we also find |V_cd|=0.234±0.007±0.002±0.025 and |V_cs|=0.985±0.009±0.006±0.103, where the third uncertainties are from lattice quantum chromodynamics.

We study the inclusive decays of D_s⁺ mesons, using data collected near the D_s^*±D_s^∓ peak production energy E_cm=4170  MeV by the CLEO-c detector. We report the inclusive yields of D_s⁺ decays to K⁺X, K^-X, K_S⁰X, π⁺X, π^-X, π⁰X, ηX, η^′X, ϕX, ωX, and f₀(980)X, and also decays into pairs of kaons, D_s⁺→KK̅ X. Using these measurements, we obtain an overview of D_s⁺ decays.

Using data acquired with the CLEO-c detector at the CESR e⁺e^- collider, we measure branching fractions for J/ψ, ψ(2S), and ψ(3770) decays to γπ⁰, γη, and γη^′. Defining R_n≡B[ψ(nS)→γη]/B[ψ(nS)→γη^′], we obtain R₁=(21.1±0.9)% and, unexpectedly, an order of magnitude smaller limit, R₂<1.8% at 90% C.L. We also use J/ψ→γη^′ events to determine branching fractions of improved precision for the five most copious η^′ decay modes.

We compute, for the first time using lattice QCD methods, charmonium radiative transition rates involving states of high spin and exotics. Utilizing a large basis of interpolating fields we are able to project out various excited-state contributions to three-point correlators computed on quenched anisotropic lattices. In the first lattice QCD calculation of the exotic 1^-+ η_c1 radiative decay, we find a large partial width Γ(η_c1→J/ψγ)∼100  keV. We find clear signals for electric dipole and magnetic quadrupole transition form factors in χ_c2→J/ψγ, calculated for the first time in this framework, and study transitions involving excited ψ and χ_c1,2 states. We calculate hindered magnetic dipole transition widths without the sensitivity to assumptions made in model studies and find statistically significant signals, including a nonexotic vector hybrid candidate Y_hyb?→η_cγ. As well as comparison to experimental data, we discuss in some detail the phenomenology suggested by our results and the extent to which it mirrors that of quark-potential models, and make suggestions for the interpretation of our results involving exotic quantum numbered states.

We perform a Dalitz plot analysis of the decay D_s⁺→K⁺K^-π⁺ with the CLEO-c data set of 586  pb^-1 of e⁺e^- collisions accumulated at sqrt[s]=4.17  GeV. This corresponds to about 0.57×10⁶ D_s^±D_s^*∓ pairs from which we select 14 400 candidates with a background of roughly 15%. In contrast to previous measurements we find good agreement with our data only by including an additional f₀(1370)π⁺ contribution. We measure the magnitude, phase, and fit fraction of K^*(892)⁰K⁺, ϕ(1020)π⁺, K₀^*(1430)K⁺, f₀(980)π⁺, f₀(1710)π⁺, and f₀(1370)π⁺ contributions and limit the possible contributions of other KK and Kπ resonances that could appear in this decay.

We investigate how coupling of valence qq̅ to meson pairs can modify the properties of conventional qq̅ and hybrid mesons. In a symmetry limit the mixing between hybrids and conventional qq̅ with the same J^PC is shown to vanish. Flavor mixing between heavy and light qq̅ due to meson loops is shown to be dual to the results of gluon mediated pQCD and qualitatively different from mixing involving light flavors alone. The validity of the OZI rule for conventional qq̅ and hybrid mesons is discussed.

At the radioactive ion beam facility REX-ISOLDE, neutron-rich zinc isotopes were investigated using low-energy Coulomb excitation. These experiments have resulted in B(E2,2₁⁺→0₁⁺) values in ^74-80Zn, B(E2,4₁⁺→2₁⁺) values in ^74,76Zn and the determination of the energy of the first excited 2₁⁺ states in ^78,80Zn. The zinc isotopes were produced by high-energy proton- (A=74,76,80) and neutron- (A=78) induced fission of ²³⁸U, combined with selective laser ionization and mass separation. The isobaric beam was postaccelerated by the REX linear accelerator and Coulomb excitation was induced on a thin secondary target, which was surrounded by the MINIBALL germanium detector array. In this work, it is shown how the selective laser ionization can be used to deal with the considerable isobaric beam contamination and how a reliable normalization of the experiment can be achieved. The results for zinc isotopes and the N=50 isotones are compared to collective model predictions and state-of-the-art large-scale shell-model calculations, including a recent empirical residual interaction constructed to describe the present experimental data up to 2004 in this region of the nuclear chart.

Radiative hydrodynamics simulations of ignition experiments show that energy transfer between crossing laser beams allows tuning of the implosion symmetry. A new full-scale, three-dimensional quantitative model has been developed for crossed-beam energy transfer, allowing calculations of the propagation and coupling of multiple laser beams and their associated plasma waves in ignition hohlraums. This model has been implemented in a radiative-hydrodynamics code, demonstrating control of the implosion symmetry by a wavelength separation between cones of laser beams.

We show that the literature on pion exchange between charm and bottom mesons is inconsistent. We derive the formalism explicitly, expose differences between papers in the literature, and clarify the implications. We show that the X(3872) can be a bound state but that results are very sensitive to a poorly constrained parameter. We confirm that bound states in the BB̅ sector are possible. The circumstances whereby exotic combinations can bind with cc or bb quantum numbers are explored.

Slow dynamics in disordered materials prohibits direct simulation of their rich nonequilibrium behavior at large scales. “Patchwork dynamics” is introduced to mimic relaxation over a very broad range of time scales by directly equilibrating or optimizing on successive length scales. This dynamics is used to study coarsening and to replicate memory effects for spin glasses and random ferromagnets. It is also used to find, with high confidence, correct ground states in large toroidal samples.

We investigate crystalline order on a two-dimensional paraboloid of revolution by assembling a single layer of millimeter-sized soap bubbles on the surface of a rotating liquid, thus extending the classic work of Bragg and Nye on planar soap bubble rafts. Topological constraints require crystalline configurations to contain a certain minimum number of topological defects such as disclinations or grain boundary scars whose structure is analyzed as a function of the aspect ratio of the paraboloid. We find the defect structure to agree with theoretical predictions and propose a mechanism for scar nucleation in the presence of large Gaussian curvature.

We generalize results of lattice QCD to determine the spin-dependent symmetries and factorization properties of meson production in Okubo-Zweig-Iizuka allowed processes. This explains some conjectures previously made in the literature about axial meson decays and gives predictions for exclusive decays of vector charmonia, including ways of establishing the structure of Y(4260) and Y(4325) from their S-wave decays. Factorization gives a selection rule which forbids e⁺e^-→D^*D₂ near threshold with the tensor meson in helicity 2. The relations among amplitudes for double charmonia production e⁺e^-→ψ+χ_0,1,2 are expected to differ from the analogous relations among light flavor production such as e⁺e^-→ωf_0,1,2.

The root-mean-square (rms) nuclear charge radius of ⁸He, the most neutron-rich of all particle-stable nuclei, has been determined for the first time to be 1.93(3) fm. In addition, the rms charge radius of ⁶He was measured to be 2.068(11) fm, in excellent agreement with a previous result. The significant reduction in charge radius from ⁶He to ⁸He is an indication of the change in the correlations of the excess neutrons and is consistent with the ⁸He neutron halo structure. The experiment was based on laser spectroscopy of individual helium atoms cooled and confined in a magneto-optical trap. Charge radii were extracted from the measured isotope shifts with the help of precision atomic theory calculations.

As spin glass materials have extremely slow dynamics, devious numerical methods are needed to study low-temperature states. A simple and fast optimization version of the classical Kasteleyn treatment of the Ising model is described and applied to two-dimensional Ising spin glasses. The algorithm combines the Pfaffian and matching approaches to directly strip droplet excitations from an excited state. Extended ground states in Ising spin glasses on a torus, which are optimized over all boundary conditions, are used to compute precise values for ground state energy densities.

Neutron-rich, radioactive Zn isotopes were investigated at the Radioactive Ion Beam facility REX-ISOLDE (CERN) using low-energy Coulomb excitation. The energy of the 2₁⁺ state in ⁷⁸Zn could be firmly established and for the first time the 2⁺→0₁⁺ transition in ⁸⁰Zn was observed at 1492(1) keV. B(E2,2₁⁺→0₁⁺) values were extracted for ^74,76,78,80Zn and compared to large scale shell model calculations. With only two protons outside the Z=28 proton core, ⁸⁰Zn is the lightest N=50 isotone for which spectroscopic information has been obtained to date. Two sets of advanced shell model calculations reproduce the observed B(E2) systematics. The results for N=50 isotones indicate a good N=50 shell closure and a strong Z=28 proton core polarization. The new results serve as benchmarks to establish theoretical models, predicting the nuclear properties of the doubly magic nucleus ⁷⁸Ni.

The decay of the ground-state two-proton emitter ⁴⁵Fe was studied with a time-projection chamber and the emission of two protons was unambiguously identified. The total decay energy and the half-life measured in this work agree with the results from previous experiments. The present result constitutes the first direct observation of the individual protons in the two-proton decay of a long-lived ground-state emitter. In parallel, we identified for the first time directly two-proton emission from ⁴³Cr, a known β-delayed two-proton emitter. The technique developed in the present work opens the way to a detailed study of the mechanism of ground state as well as β-delayed two-proton radioactivity.

A model for ac-driven systems, based on the Tang-Wiesenfeld-Bak-Coppersmith-Littlewood automaton for an elastic medium, exhibits mode-locked steps with frequencies that are irrational multiples of the drive frequency, when the pinning is spatially quasiperiodic. Detailed numerical evidence is presented for the large-system-size convergence of such a mode-locked step. The irrational mode locking is stable to small thermal noise and weak disorder. Continuous-time models with irrational mode locking and possible experimental realizations are discussed.

The β-decay properties of ₂₈⁷²Ni₄₄ and ₂₉⁷²Cu₄₃ have been studied at the LISOL facility of Louvain-La-Neuve and at the CERN-ISOLDE facility, respectively. These neutron-rich nuclei have been produced in the proton-induced fission of ²³⁸U. Their decay schemes are presented and the lifetime T_1/2=6.63(3) s of ⁷²Cu was measured. No β-decaying isomeric state was found in ⁷²Cu, in line with a suggested spin (2) for its ground state. Spin and parity assignments of the observed excited states in the odd-odd nucleus ⁷²Cu are proposed and discussed in terms of coupling between the valence proton and neutrons. Comparison is made with a schematic shell-model picture of ⁷²Cu and with large-scale shell-model calculations performed in the (2p_3/21f_5/22p_1/21g_9/2) shell space outside the doubly magic ₂₈⁵⁶Ni₂₈ core.

A diagnostic is developed for the nonintrusive study of the azimuthal drift current in the coaxial E×B discharge of a Hall plasma accelerator. The technique of fast current interruption is used to generate a signal on several loop antenna that circle the outer wall of the discharge channel. The signal on the antenna is recorded, and used to determine the spatial distribution of the azimuthal drift at the moment of current interruption. The results of the experiment are compared to estimates derived via prior intrusive measurements, and the intrusive estimates are found to predict the spatial characteristics of the azimuthal drift, but underestimate its total magnitude. The self-induced magnetic field is then calculated and added to the applied magnetic field. The peak total magnetic field is seen to shift 2–5  mm towards the anode due to self-induction, and suffer a reduction in magnitude of 10%–15%. The peak in the total magnetic field is then found to more closely coincide with the peak of the measured electric field than the peak of the vacuum magnetic field. It is concluded that the self-induced magnetic field could be important to anomalous electron mobility in the Hall-effect thruster, and simulation efforts should try to include its impact.