Search Results (1,111 total)

We present a LIGO search for short-duration gravitational waves (GWs) associated with soft gamma ray repeater (SGR) bursts. This is the first search sensitive to neutron star f modes, usually considered the most efficient GW emitting modes. We find no evidence of GWs associated with any SGR burst in a sample consisting of the 27 Dec. 2004 giant flare from SGR 1806-20 and 190 lesser events from SGR 1806-20 and SGR 1900+14. The unprecedented sensitivity of the detectors allows us to set the most stringent limits on transient GW amplitudes published to date. We find upper limit estimates on the model-dependent isotropic GW emission energies (at a nominal distance of 10 kpc) between 3×10⁴⁵ and 9×10⁵² erg depending on waveform type, detector antenna factors and noise characteristics at the time of the burst. These upper limits are within the theoretically predicted range of some SGR models.

Based on 58×10⁶ J/ψ events collected with the BESII detector at the Beijing Electron-Positron Collider, the baryon pair processes J/ψ→Σ⁺Σ̅ ^- and J/ψ→Ξ⁰Ξ̅ ⁰ are observed for the first time. The branching fractions are measured to be B(J/ψ→Σ⁺Σ̅ ^-)=(1.50±0.10±0.22)×10^-3 and B(J/ψ→Ξ⁰Ξ̅ ⁰)=(1.20±0.12±0.21)×10^-3, where the first errors are statistical and the second ones are systematic.

We report the first observation of the strong effect of quantum rotational wave packets in atmospheric air on the long-range filamentary propagation of intense femtosecond laser pulses. In a pump-probe experiment, we find that the probe filament can be sucked into the pump filament’s molecular quantum wake and trapped or be destroyed by it.

We present measurements of the forward-backward charge asymmetry in top pair production using 1.9  fb^-1 of pp̅ collisions at sqrt[s]=1.96  TeV recorded with the Collider Detector at Fermilab II. Correcting for acceptance and measurement dilutions we obtain parton-level asymmetries of A_FB^pp̅ =0.17±0.08 in the pp̅ frame and A_FB^tt̅ =0.24±0.14 in the tt̅ frame. The values are consistent with the standard model expectation and disfavor exotic production mechanisms with significant negative values.

Neurons are subject to various kinds of noise. In addition to synaptic noise, the stochastic opening and closing of ion channels represents an intrinsic source of noise that affects the signal-processing properties of the neuron. We study the response of a stochastic Hodgkin-Huxley neuron to transient input subthreshold pulses. It is found that the average response time decreases but variance increases as the amplitude of channel noise increases. In the case of single-pulse detection, we show that channel noise enables one neuron to detect the subthreshold signals and an optimal membrane area (or channel noise intensity) exists for a single neuron to achieve optimal performance. However, the detection ability of a single neuron is limited by large errors. Here, we test a simple neuronal network that can enhance the pulse-detecting abilities of neurons and find that dozens of neurons can perfectly detect subthreshold pulses. The phenomenon of intrinsic stochastic resonance is also found at both the level of single neurons and the level of networks. At the network level, the detection ability of networks can be optimized for the number of neurons comprising the network.

We report a search for the flavor-changing neutral-current decay of the top quark t→Zq (q=u, c) in pp̅ collisions at sqrt[s]=1.96  TeV using a data sample corresponding to an integrated luminosity of 1.9  fb^-1 collected by the CDF II detector. This decay is strongly suppressed in the standard model and an observation of a signal at the Fermilab Tevatron would be an indication of physics beyond the standard model. Using Z+≥4 jet final state candidate events, with and without an identified bottom quark jet, we obtain an upper limit of B(t→Zq)<3.7% at 95% C.L.

We present the results of searches for large extra dimensions in samples of events with large missing transverse energy E̸_T and either a photon or a jet produced in pp̅ collisions at sqrt[s]=1.96  TeV collected with the Collider Detector at Fermilab II. For γ+E̸_T and jet+E̸_T candidate samples corresponding to 2.0 and 1.1  fb^-1 of integrated luminosity, respectively, we observe good agreement with standard model expectations and obtain a combined lower limit on the fundamental parameter of the large extra dimensions model M_D as a function of the number of extra dimensions in the model.

We report on the realization of a rewritable and local source inside a Si-based photonic crystal microcavity by infiltrating a solution of colloidal PbS quantum dots inside a single pore of the structure. We show that the resulting spontaneous emission from the source is both spatially and spectrally redistributed due to the mode structure of the photonic crystal cavity. The coupling of the quantum dot emission to the cavity mode is analyzed by mapping the luminescence signal of the infiltrated solution with a scanning near-field optical microscope at room temperature. Spectral characterization and the mode profile are in good agreement with a three-dimensional numerical calculation of the system.

We have studied the effect of memory on the evolution of the prisoner’s dilemma game in square lattice networks. Based on extensive simulations, we found that the density of cooperators was enhanced by an increasing memory effect for most parameters. However, we also observed that the density of cooperators decreased with an increased memory effect in the case of a large memory and moderate temptation. It is interesting to note that memory makes cooperators immune from temptation. The strength of protection reaches its maximal value only for a moderate memory effect.

We present a measurement of the shapes of b-jets using 300  pb^-1 of data obtained with the upgraded Collider Detector at Fermilab (CDF II) in pp̅ collisions at center-of-mass energy sqrt[s]=1.96  TeV. This measurement covers a wide transverse momentum range, from 52 to 300  GeV/c. Samples of heavy-flavor enhanced jets together with inclusive jets are used to extract the average shapes of b-jets. The b-jets are expected to be broader than inclusive jets. Moreover, b-jets containing a single b-quark are expected to be narrower than those containing a bb̅ pair from gluon splitting. The measured b-jet shapes are found to be significantly broader than expected from the pythia and herwig Monte Carlo simulations. This effect may arise from an underestimation of the fraction of b-jets originating from gluon splitting in these simulations. The jet shape distributions provided in this paper could be compared to any full Monte Carlo simulation and could be used to further constrain the various parameters.

We study sequential scattering in superradiance from a Bose-Einstein condensate pumped by a two-frequency laser beam. We find that the distribution of atomic side modes presents highly different patterns for various frequency difference between the two pump components. These observations reveal that the frequency overlap between the end-fire modes related to different side modes plays an essential role in the dynamics of sequential superradiant scattering. The numerical results from a semiclassical model qualitatively agree with our observations.

We report inelastic neutron scattering studies of magnetic excitations in antiferromagnetically ordered SrFe₂As₂ (T_N=200–220  K), the parent compound of the FeAs-based superconductors. At low temperatures (T=7  K), the magnetic spectrum S(Q,ℏω) consists of a Bragg peak at the elastic position (ℏω=0  meV), a spin gap (Δ≤6.5  meV), and sharp spin-wave excitations at higher energies. Based on the observed dispersion relation, we estimate the effective magnetic exchange coupling using a Heisenberg model. On warming across T_N, the low-temperature spin gap rapidly closes, with weak critical scattering and spin-spin correlations in the paramagnetic state. The antiferromagnetic order in SrFe₂As₂ is therefore consistent with a first order phase transition, similar to the structural lattice distortion.

Under normal incidence of circularly polarized light at room temperature, a charge current with swirly distribution has been observed in the two-dimensional electron gas in Al_0.25Ga_0.75N/GaN heterostructures. We believe that this anomalous charge current is produced by a radial spin current via the reciprocal spin Hall effect. It suggests a new way to research the reciprocal spin Hall effect and spin current on the macroscopic scale and at room temperature.

We report the results of a recent search for the lowest value of thermal noise that can be achieved in LIGO by changing the shape of mirrors, while fixing the mirror radius and maintaining a low diffractional loss. The result of this minimization is a beam with thermal noise a factor of 2.32 (in power) lower than previously considered Mesa Beams and a factor of 5.45 (in power) lower than the Gaussian beams employed in the current baseline design. Mirrors that confine these beams have been found to be roughly conical in shape, with an average slope approximately equal to the mirror radius divided by arm length, and with mild corrections varying at the Fresnel scale. Such a mirror system, if built, would impact the sensitivity of LIGO, increasing the event rate of observing gravitational waves in the frequency range of maximum sensitivity roughly by a factor of 3 compared to an Advanced LIGO using Mesa beams (assuming all other noises remain unchanged). We discuss the resulting beam and mirror properties and study requirements on mirror tilt, displacement, and figure error, in order for this beam to be used in LIGO detectors.

We present a measurement of the inclusive jet cross section in pp̅ collisions at sqrt[s]=1.96  TeV based on data collected by the CDF II detector with an integrated luminosity of 1.13  fb^-1. The measurement was made using the cone-based midpoint jet clustering algorithm in the rapidity region of |y|<2.1. The results are consistent with next-to-leading-order perturbative QCD predictions based on recent parton distribution functions (PDFs), and are expected to provide increased precision in PDFs at high parton momentum fraction x. The results are also compared to the recent inclusive jet cross section measurement using the k_T jet clustering algorithm, and we find that the ratio of the cross sections measured with the two algorithms is in agreement with theoretical expectations over a large range of jet transverse momentum and rapidity.

It has recently been shown that a stationary, asymptotically flat vacuum black hole in five space-time dimensions with two commuting axial symmetries must have an event horizon with either a spherical, ring or lens-space topology. In this paper, we study the third possibility, a so-called black lens with L(n,1) horizon topology. Using the inverse scattering method, we construct a black-lens solution with the simplest possible rod structure, and possessing a single asymptotic angular momentum. Its properties are then analyzed; in particular, it is shown that there must either be a conical singularity or a naked curvature singularity present in the space-time.

We search for pair production of doubly charged Higgs particles (H^±±) followed by decays into electron-tau (eτ) and muon-tau (μτ) pairs using data (350  pb^-1) collected from p̅ p collisions at sqrt[s]=1.96  TeV by the CDF II experiment. We search separately for cases where three or four final-state leptons are detected, and combine results for exclusive decays to left-handed eτ (μτ) pairs. We set an H^±± lower mass limit of 114(112)  GeV/c² at the 95% confidence level.

Currently planned second-generation gravitational-wave laser interferometers such as Advanced LIGO exploit the extensively investigated signal-recycling technique. Candidate Advanced LIGO configurations are usually designed to have two resonances within the detection band, around which the sensitivity is enhanced: a stable optical resonance and an unstable optomechanical resonance—which is upshifted from the pendulum frequency due to the so-called optical-spring effect. As an alternative to a feedback control system, we propose an all-optical stabilization scheme, in which a second optical spring is employed, and the test mass is trapped by a stable ponderomotive potential well induced by two carrier light fields whose detunings have opposite signs. The double optical spring also brings additional flexibility in reshaping the noise spectral density and optimizing toward specific gravitational-wave sources. The presented scheme can be extended easily to a multi-optical-spring system that allows further optimization.

We study the longitudinal polarization of the Σ̅ ^-, Σ̅ ⁺, Ξ̅ ⁰, and Ξ̅ ⁺ antihyperons in polarized high-energy pp collisions at large transverse momenta, extending a recent study of the Λ̅ antihyperon. We make predictions by using different parameterizations of the polarized parton densities and models for the polarized fragmentation functions. Similar to the Λ̅ polarization, the Ξ̅ ⁰ and Ξ̅ ⁺ polarizations are found to be sensitive to the polarized antistrange sea Δs̅ (x) in the nucleon. The Σ̅ ^- and Σ̅ ⁺ polarizations show sensitivity to the light sea quark polarizations Δu̅ (x) and Δd̅ (x), and their asymmetry.

We observe an obvious anomalous line shape of the e⁺e^-→ hadrons total cross sections in the energy region between 3.700 and 3.872 GeV. It is inconsistent with the explanation for only one simple ψ(3770) resonance with a statistical significance of 7σ. The anomalous line shape may be explained by two possible enhancements of the inclusive hadron production near the center-of-mass energies of 3.764 and 3.779 GeV, indicating that either there is likely a new structure in addition to the ψ(3770) resonance around 3.773 GeV, or there are some physics effects reflecting the DD̅ production dynamics.

Direct measurement of the remanent polarization of high quality (001)-oriented epitaxial BiFeO₃ thin films shows a strong strain dependence, even larger than conventional (001)-oriented PbTiO₃ films. Thermodynamic analysis reveals that a strain-induced polarization rotation mechanism is responsible for the large change in the out-of-plane polarization of (001) BiFeO₃ with biaxial strain while the spontaneous polarization itself remains almost constant.

We present the results of the first hadron collider search for heavy, long-lived neutralinos that decay via χ˜₁⁰→γG˜ in gauge-mediated supersymmetry breaking models. Using an integrated luminosity of 570±34  pb^-1 of pp̅ collisions at sqrt[s]=1.96  TeV, we select γ+jet+missing transverse energy candidate events based on the arrival time of a high-energy photon at the electromagnetic calorimeter as measured with a timing system that was recently installed on the CDF II detector. We find 2 events, consistent with the background estimate of 1.3±0.7 events. While our search strategy does not rely on model-specific dynamics, we set cross section limits and place the world-best 95% C.L. lower limit on the χ˜₁⁰ mass of 101  GeV/c² at τ_χ˜1⁰=5  ns.

Polarized and unpolarized neutron-diffraction measurements have been carried out to investigate the iron magnetic order in undoped NdFeAsO. Antiferromagnetic order is observed below 141(6) K, which is in close proximity to the structural distortion observed in this material. The magnetic structure consists of chains of parallel spins that are arranged antiparallel between chains, which is the same in-plane spin arrangement as observed in all the other iron oxypnictide materials. Nearest-neighbor spins along the c axis are antiparallel like LaFeAsO. The ordered moment is 0.25(7)  μ_B, which is the smallest moment found so far in these systems.

We present a search for standard model Higgs boson production in association with a W boson in proton-antiproton collisions (pp̅ →W^±H→ℓνbb̅ ) at a center of mass energy of 1.96 TeV. The search employs data collected with the CDF II detector which correspond to an integrated luminosity of approximately 1  fb^-1. We select events consistent with a signature of a single lepton (e^±/μ^±), missing transverse energy, and two jets. Jets corresponding to bottom quarks are identified with a secondary vertex tagging method and a neural network filter technique. The observed number of events and the dijet mass distributions are consistent with the standard model background expectations, and we set 95% confidence level upper limits on the production cross section times branching ratio ranging from 3.9 to 1.3 pb for Higgs boson masses from 110 to 150  GeV/c², respectively.