Search Results (116 total)

Vacancies in carbon nanotubes usually aggregate into larger vacancies. Using first-principles and tight-binding calculations, we investigate the alignment of missing atoms and the movement of pentagon-heptagon defects that are formed by reconstructions in large vacancy clusters V_n (n≤36), where n is the number of missing atoms. In nanotubes with small diameters, missing atoms have a tendency to form a serial network rather than a large hole due to the existence of large curvatures. It is generally found that the parallel alignment of missing atoms along the tube axis is energetically more favorable than the spiral alignment. Thus, the removal of atoms leads to the longitudinal movement of a pentagon-heptagon defect on the tube wall, which is in good agreement with the kink motion observed during superplastic deformation of single-wall nanotubes. The preference of the longitudinal motion of the pentagon-heptagon defect is more prominent in armchair tubes compared with other chiral tubes.

Photoproduction of beauty quarks in events with two jets and an electron associated with one of the jets has been studied with the ZEUS detector at HERA using an integrated luminosity of 120  pb^-1. The fractions of events containing b quarks, and also of events containing c quarks, were extracted from a likelihood fit using variables sensitive to electron identification as well as to semileptonic decays. Total and differential cross sections for beauty and charm production were measured and compared with next-to-leading-order QCD calculations and Monte Carlo models.

Inclusive K_S⁰K_S⁰ production in ep collisions at the DESY ep collider HERA was studied with the ZEUS detector using an integrated luminosity of 0.5  fb^-1. Enhancements in the mass spectrum were observed and are attributed to the production of f₂(1270)/a₂⁰(1320), f₂^′(1525) and f₀(1710). Masses and widths were obtained using a fit which takes into account theoretical predictions based on SU(3) symmetry arguments, and are consistent with the Particle Data Group values. The f₀(1710) state, which has a mass consistent with a glueball candidate, was observed with a statistical significance of 5 standard deviations. However, if this state is the same as that seen in γγ→K_S⁰K_S⁰, it is unlikely to be a pure glueball state.

Jet cross sections were measured in charged-current deep inelastic e^±p scattering at high boson virtualities Q² with the ZEUS detector at HERA II using an integrated luminosity of 0.36  fb^-1. Differential cross sections are presented for inclusive-jet production as functions of Q², Bjorken x and the jet transverse energy and pseudorapidity. The dijet invariant mass cross section is also presented. Observation of three- and four-jet events in charged-current e^±p processes is reported for the first time. The predictions of next-to-leading-order (NLO) QCD calculations are compared to the measurements. The measured inclusive-jet cross sections are well described in shape and normalization by the NLO predictions. The data have the potential to constrain the u and d valence-quark distributions in the proton if included as input to global fits.

Pre-testing and post-testing is a commonly used method in Physics Education Research to assess student learning gains. It is well recognized in the community that timings and incentives in delivering conceptual tests can impact test results. However, it is difficult to control these variables across different studies. As a common practice, a pre-test is often administered either at or near the beginning of a course, while a post-test can be given either at or near the end of a course. Also, in conducting such tests there often is no norm as to whether incentives should be offered to students. Because these variations can significantly affect test results, it is important to study and document their impact. We analyzed five years of data that were collected at The Ohio State University from over 2100 students, who took both the pre-test and post-test of the Conceptual Survey of Electricity and Magnetism under various timings and incentives. We observed that the actual time frame for giving a test has a marked effect on the test results and that incentive granting also has a significant influence on test outcomes. These results suggest that one should carefully monitor and document the conditions under which tests are administered.

The cross section for high-E_T dijet production in photoproduction has been measured with the ZEUS detector at HERA using an integrated luminosity of 81.8  pb^-1. The events were required to have a virtuality of the incoming photon, Q², of less than 1  GeV² and a photon-proton center-of-mass energy in the range 142<W_γp<293  GeV. Events were selected if at least two jets satisfied the transverse-energy requirements of E_T^jet1>20  GeV and E_T^jet2>15  GeV and pseudorapidity (with respect to the proton beam direction) requirements of -1<η^jet1,2<3, with at least one of the jets satisfying -1<η^jet<2.5. The measurements show sensitivity to the parton distributions in the photon and proton and to effects beyond next-to-leading order in QCD. Hence these data can be used to constrain further the parton densities in the proton and photon.

Using density functional theory (DFT) computations, we have demonstrated a substantial skeletal relaxation when the structure of 2,5-[bis-(4-anthracene-9-yl-phenyl]-1,1-dimethyl-3,4-diphenyl-silole (BAS) is optimized in the gas-phase comparing with the molecular structure determined from monocrystal x-ray diffraction. The origin of such a relaxation is explained by a strong environmental strains induced by the presence of anthracene entities. Moreover, the estimation of the frontier orbital levels showed that this structural relaxation affects mainly the LUMO that is lowered of 190 meV in the gas phase. To check if these theoretical findings would be confirmed for thin films of BAS, we turned to ultraviolet photoemission spectroscopy and/or inverse photoemission spectroscopy and electro-optical measurements. Interestingly, the study of the current density or voltage and luminance or voltage characteristics of an ITO∕PEDOT∕BAS∕Au device clearly demonstrated a very unusual temperature-dependent behavior. Using a thermally assisted tunnel transfer model, we found that this behavior likely originated from the variation of the electronic affinity of the silole derivative with the temperature. The thermal agitation relaxes the molecular strains in thin films as it is shown when passing from the crystalline to the gas phase. The relaxation of the intramolecular thus induces an increase of the electronic affinity and, as a consequence, the more efficient electron injection in organic light-emitting diodes.

The material properties of biomembranes can be measured by forming a tether, a thin bilayer tube that extends from the membrane surface. Recent experiments have demonstrated that the force required to maintain a tether is sensitive to the transmembrane potential. As a first approach towards understanding this phenomenon, a thermodynamic analysis of the influence of electrical fields on tether formation from an aspirated lipid vesicle is developed. The analysis considers contributions from Maxwell stresses as well as two forms of electromechanical coupling: coupling between the electric field and curvature strain (flexoelectric coupling) and between the electric field and areal strain (piezoelectric coupling). Predictions of equilibrium tether conformations are obtained numerically. For expected values of the dimensionless coupling parameters, flexoelectric coupling alters the force required to form a tether of a given length, while piezoelectric coupling and Maxwell forces do not greatly change the force versus tether length behavior. The results of this analysis indicate that tether experiments have the potential to characterize electromechanical coupling in both synthetic and cellular membranes.

Locomotion of bacteria by actin polymerization and in vitro motion of spherical beads coated with a protein catalyzing polymerization are examples of active motility. Starting from a simple model of forces locally normal to the surface of a bead, we construct a phenomenological equation for its motion. The singularities at a continuous transition between moving and stationary beads are shown to be related to the symmetries of its shape. Universal features of the phase behavior are calculated analytically and confirmed by simulations. Fluctuations in velocity are shown to be generically non-Maxwellian and correlated to the shape of the bead.

The results based on 1992–95 data (Run 1) from the CDF and D0 experiments on the measurements of the W boson mass and width are presented, along with the combined results. We report a Tevatron collider average M_W=80.456±0.059  GeV. We also report the Tevatron collider average of the directly measured W boson width Γ_W=2.115±0.105  GeV. We describe a new joint analysis of the direct W mass and width measurements. Assuming the validity of the standard model, we combine the directly measured W boson width with the width extracted from the ratio of W and Z boson leptonic partial cross sections. This combined result for the Tevatron is Γ_W=2.135±0.050  GeV. Finally, we use the measurements of the direct total W width and the leptonic branching ratio to extract the leptonic partial width Γ(W→eν)=224±13  MeV.

Our experiments and molecular dynamics simulations on a projectile penetrating a two-dimensional granular medium reveal that the mean deceleration of the projectile is constant and proportional to the impact velocity. Thus, the time taken for a projectile to decelerate to a stop is independent of its impact velocity. The simulations show that the probability distribution function of forces on grains is time independent during a projectile’s deceleration in the medium. At all times the force distribution function decreases exponentially for large forces.

We report an experimental and theoretical study of the properties of metallodielectric gratings with subwavelength slots in the thin metal limit. These structures were fabricated using a bench top nanopatterning method that produces metallic periodic structures with subwavelength features without the need for lithographic masks by directing the deposition of metal onto prepatterned surfaces. In the size and thickness regime explored in this study two excitations are possible: surface plasmons (SP’s) and Rayleigh anomalies. The transmissive and reflective properties of these structures are reported, and very good agreement between theory and experiment is achieved. The dispersion of both the Rayleigh anomalies and SPs is measured. In addition, the near-field properties of each of these excitations are calculated. Plasmon-plasmon interactions, which produce gaps in the dispersion relation for the surface waves, are observed.

We present the results of a search in pp̅ collisions at sqrt[s]=1.8 TeV for anomalous production of events containing a photon with large transverse energy and a lepton (e or μ) with large transverse energy, using 86 pb^-1 of data collected with the Collider Detector at Fermilab during the 1994–1995 collider run at the Fermilab Tevatron. The presence of large missing transverse energy (E_T), additional photons, or additional leptons in these events is also analyzed. The results are consistent with standard model expectations, with the possible exception of photon-lepton events with large E_T, for which the observed total is 16 events and the expected mean total is 7.6±0.7 events.

We present a search for the flavor-changing neutral current decay B_s→μ⁺μ^-φ in pp̅ collisions at sqrt[s]=1.8 TeV, using 91 pb^-1 of data collected at the Collider Detector at Fermilab (CDF). We find two candidate events for this decay, which are consistent with the background estimate of one event, and set an upper limit on the branching fraction of B(B_s→μ⁺μ^-φ)<6.7×10^-5 at a 95% confidence level. This is the first limit on the branching fraction of this decay.

We use the measurement of the cosmic microwave background taken during the MAXIMA-1 flight to estimate the bispectrum of cosmological perturbations. We propose an estimator for the bispectrum that is appropriate in the flat sky approximation, apply it to the MAXIMA-1 data, and evaluate errors using bootstrap methods. We compare the estimated value with what would be expected if the sky signal were Gaussian and find that it is indeed consistent, with a χ² per degree of freedom of approximately unity. This measurement places constraints on models of inflation.

The growth and development of “charged particle jets” produced in proton-antiproton collisions at 1.8 TeV  are studied over a transverse momentum range from 0.5 GeV/c to 50 GeV/c. A variety of leading (highest transverse momentum) charged jet observables are compared with the QCD Monte Carlo models HERWIG, ISAJET, and PYTHIA. The models describe fairly well the multiplicity distribution of charged particles within the leading charged jet, the size of the leading charged jet, the radial distribution of charged particles and transverse momentum around the leading charged jet direction, and the momentum distribution of charged particles within the leading charged jet. The direction of the leading “charged particle jet” in each event is used to define three regions of η-φ space. The “toward” region contains the leading “charged particle jet,” while the “away” region, on the average, contains the away-side jet. The “transverse” region is perpendicular to the plane of the hard 2-to-2 scattering and is very sensitive to the “underlying event” component of the QCD Monte Carlo models. HERWIG, ISAJET, and PYTHIA with their default parameters do not describe correctly all the properties of the “transverse” region.

We report a measurement of the diffractive structure function F_jj^D of the antiproton obtained from a study of dijet events produced in association with a leading antiproton in p̅ p collisions at sqrt[s]  =  630 GeV at the Fermilab Tevatron. The ratio of F_jj^D at sqrt[s]  =  630 GeV to F_jj^D obtained from a similar measurement at sqrt[s]  =  1800 GeV is compared with expectations from QCD factorization and other theoretical predictions. We also report a measurement of the ξ ( x-Pomeron) and β ( x of parton in Pomeron) dependence of F_jj^D at sqrt[s]  =  1800 GeV. In the region 0.035<ξ<0.095, |t|<1 GeV², and β<0.5, F_jj^D(β,ξ) is found to be of the form β^-1.0±0.1ξ^-0.9±0.1, which obeys β-ξ factorization.

We present a detailed examination of the heavy flavor content of the W+jet data sample collected with the Collider Detector at Fermilab during the 1992–1995 collider run at the Fermilab Tevatron. Jets containing heavy flavor quarks are selected via the identification of secondary vertices or semileptonic decays of b and c quarks. There is generally good agreement between the rates of secondary vertices and soft leptons in the data and in the standard model simulation including single and pair production of top quarks. An exception is the number of events in which a single jet has both a soft lepton and a secondary vertex tag. In W+2,3 jet data, we find 13 such events where we expected 4.4±0.6 events. The kinematic properties of this small sample of events are statistically difficult to reconcile with the simulation of standard model processes.

We have searched for evidence of physics beyond the standard model in events that include an energetic photon and an energetic b-quark jet, produced in 85 pb^-1 of p̅ p collisions at 1.8 TeV at the Tevatron Collider at Fermilab. This signature, containing at least one gauge boson and a third-generation quark, could arise in the production and decay of a pair of new particles, such as those predicted by supersymmetry, leading to a production rate exceeding standard model predictions. We also search these events for anomalous production of missing transverse energy, additional jets and leptons (e, μ and τ), and additional b quarks. We find no evidence for any anomalous production of γb or γb+X events. We present limits on two supersymmetric models: a model where the photon is produced in the decay χ̃₂⁰→γχ̃₁⁰, and a model where the photon is produced in the neutralino decay into the gravitino LSP, χ̃₁⁰→γG̃. We also present our limits in a model-independent form and test methods of applying model-independent limits.

We present the first general search for new heavy particles, X, which decay via X→WZ⁰→eν+jj as a function of M_X and Γ(X) in pp̅ collisions at sqrt[s]  =  1.8 TeV. No evidence is found for production of X in 110 pb^-1 of data collected by the Collider Detector at Fermilab. General cross section limits are set at the 95% C.L. as a function of mass and width of the new particle. The results are further interpreted as mass limits on the production of new heavy charged vector bosons which decay via W^′→WZ⁰ in an extended gauge model as a function of the width, Γ(W^′), and mixing factor between the W^′ and the standard model W bosons.

We report a study of the decays B⁰→J/ψK^(*)0π⁺π^-, which involve the creation of a uu̅ or dd̅ quark pair in addition to a b̅ →c̅ (cs̅ ) decay. The data sample consists of 110 pb^-1 of pp̅ collisions at sqrt[s]  =  1.8 TeV collected by the CDF detector at the Fermilab Tevatron collider during 1992–1995. We measure the branching fractions to be B(B⁰→J/ψK^*0π⁺π^-)  =  (6.6±1.9±1.1)×10^-4 and B(B⁰→J/ψK⁰π⁺π^-)  =  (10.3±3.3±1.5)×10^-4. Evidence is seen for contributions from ψ(2S)K^(*)0, J/ψK⁰ρ⁰, J/ψK^*+π^-, and J/ψK₁(1270).