Vert Bar Research Articles

Calculation of Feynman diagrams in superstring perturbation theory.

A method of perturbative calculation of multiloop amplitudes in superstring theories is proposed. In this method the multiloop superstring amplitudes are calculated from equations that are none other than Ward identities. These equations are derived from the requirement that the discussed amplitudes are independent of the choice of gauge of both the [ital vierbein] and the gravitino field. The amplitudes in question are determined in a unique way by these equations together with the factorization condition on the multiloop amplitudes when two handles move away from each other. The considered amplitudes are calculated in terms of vacuum correlators of superfields defined on the complex (1[vert bar]1) supermanifolds. These supermanifolds are described by superconformal versions of Schottky groups. The superconformal Schottky groups appropriate for this aim are built for all spinor structures. Being based only on gauge invariance together with the factorization requirement on the multiloop amplitudes when the handles move away from each other (unitarity), the proposed method can be used widely in critical (super)string theories. Moreover, after an appropriate modification is made, this method can be employed for the noncritical (super)string, too. In this paper the closed, oriented Ramond-Neveu-Schwarz superstring is considered, only boson emission amplitudes being discussed. The more » problem of the calculation of the multiloop boson emission amplitudes is concentrated mainly on those spinor structures where superfields are branched on the complex [ital z] plane where Riemann surfaces are mapped. In this case the vacuum superfield correlators cannot be derived by a simple extension of the boson string results. The method of calculation of the above correlators is proposed. The multiloop amplitudes associated with all the even spinor structures are calculated in explicit form. A previous discussion of the divergence problem is given. « less

Evaluation of the elastic pp single-flip helicity amplitude at low momentum transfer and high energies.

We report on the evaluation of the single-flip helicity amplitude by analyzing proton-proton elastic polarization data in the four-momentum-transfer[minus][ital t] region from 1.5[times]10[sup [minus]3] to 0.6 (GeV/[ital c])[sup 2]. Recent results around 200 GeV/[ital c] in the low [vert bar][ital t][vert bar] range, where the interference of the electromagnetic and hadronic amplitudes is significant, allow a more reliable evaluation of the single-flip hadronic helicity amplitude [phi][sub 5][sup [ital h]]. Within the experimental errors, our analysis indicates a small and negative real part ([minus]1 to [minus]4 %) and a positive imaginary part (8 to 30 %) for the reduced hadronic single-flip helicity amplitude ([ital m]/ [radical][minus][ital t] )[phi][sub 5][sup [ital h]] with respect to the helicity-averaged hadronic nonflip amplitude. To study the stability of these results, we have also considered data at 45--300 GeV/[ital c] in the 0.057--0.6 (GeV/[ital c])[sup 2] momentum transfer range in conjunction with the aforementioned data.

Simple model amplitude of the nucleon: A hint for important higher-twist effects?

The convergence properties of a simple model for the nucleon quark distribution amplitude into Appell polynomials are studied and the model is used to calculate nucleon form factors in perturbative QCD. Treating the gluon propagator and the strong coupling [alpha][sub [ital S]] in various ways, we demonstrate that the freedom left in the unknown [ital [rvec k]][sub [perpendicular]] dependence of the full wave function is large enough to account for a wide range of form factor results. This is interpreted as a hint that higher-twist effects may give significant contributions to hadronic form factors in PQCD. Additionally, we find a ratio of [vert bar][ital F][sub 1][sup [ital n]]/[ital F][sub 1][sup [ital p]][vert bar][approx]1/2 at large momentum transfers.

Determination of ||Vub|| from inclusive semileptonic decay spectra.

We discuss the possibility of a model-independent method to determine [vert bar][ital V][sub [ital u][ital b]][vert bar] from appropriately constructed moments of the energy spectrum of the charged lepton in inclusive semileptonic [ital B] decays. The method includes perturbative QCD corrections as well as nonperturbative ones.

Experimental investigations of absorption and dispersion profiles of a strongly driven transition: ssV-shaped three-level system with a strong probe.

This paper reports on experimental investigations of absorption, dispersion, and amplitude profiles of the Autler-Townes doublet in a [ital ssV]-shaped three-level system where the probe field intensities varied from weak to strong. The experiments were carried out on the ground-state hyperfine transitions of the nitrogen-vacancy color center in diamond using the Raman heterodyne technique, a sensitive optically detected magnetic resonance technique. A strong pump field is on resonance with the [ital I][sub [ital z]]=[vert bar]0[r angle][leftrightarrow][vert bar]1[r angle] transition at 4.7 MHz and the resultant Autler-Townes profiles are probed with a separate field scanning through the [ital I][sub [ital z]]=[vert bar]0[r angle][leftrightarrow][vert bar][minus]1[r angle] transition at 5.4 MHz. As the probe power increases the absorptive profile of the Autler-Townes doublet becomes saturated and power broadened and the two components overlap constructively. In contrast, the inner parts of the two dispersive doublet components interfere destructively due to a [pi] phase difference. The amplitude profile has an anomalous line shape at high probe intensity due to the different saturation behaviors of the absorptive and dispersive components. The experimental profiles are in good agreement with theoretical profiles calculated by solving the equation of motion of the density matrix under the steady-state limit.

Circular Rydberg States of the Hydrogen Atom in a Magnetic Field.

Dimensional perturbation theory is used to study circular Rydberg states ([vert bar][ital m][vert bar]=[ital n][minus]1[much gt]1) and other large [vert bar][ital m][vert bar] states of the hydrogen atom in a uniform magnetic field. Because of a degeneracy between states of increased angular momentum and states of increased Cartesian dimensionality, the accuracy of the zeroth-order [ital D][r arrow][infinity] limit and a dimensional perturbation expansion improves significantly for states with larger [vert bar][ital m][vert bar]. In contrast to other approaches, this method is applicable to the entire range of magnetic field strengths. Energies and expectation values are presented as functions of the field strength.

1H(d,2p)n reaction at 2 GeV deuteron energy.

The [sup 1]H([ital d],2[ital p])[ital n] deuteron breakup reaction was measured at 2 GeV deuteron energy in a kinematically complete experiment. Fivefold differential cross sections are given in a wide range of kinematical variables and analyzed in terms of impulse approximation and [ital NN] rescattering. The deuteron momentum density was determined and deviations were found depending on the value of the four-momentum transfer [vert bar][ital t][vert bar] in the scattering process. At low [vert bar][ital t][vert bar] the momentum densities are in good agreement with the impulse approximation whereas large discrepancies were found above [ital q][approx]200 MeV/[ital c] when the four-momentum transfer was large. Various possible origins of the anomalous behavior at high [ital q] values are discussed.

A stacking fault with a series of cross fringes in inconel 718 Ni-base superalloy

The stacking fault is the simplest planar fault in materials. However, as a special case of displacement fringes where [vert bar]Rn[vert bar] = a/3 for a fcc crystal, in the case of involving in coherent and partially coherent precipitate, the stacking faults become very complicated because the contrast of some precipitate is very similar to that of the stacking fault, for example, it is difficult to identify the displacement fringes contrast produced by the partially coherent [theta][prime] phase in Al-Cu alloy and stacking fault. A very complicated stacking fault, related to the segregation of niobium on the plane of an Inconel 718 matrix, with a series of cross fringes, will be presented in this work.

Numerical transfer-matrix study of a model with competing metastable states.

The Blume-Capel model, a three-state lattice-gas model capable of displaying competing metastable states, is investigated in the limit of weak, long-range interactions. The methods used are scalar field theory, a numerical transfer-matrix method, and dynamical Monte Carlo simulations. The equilibrium phase diagram and the spinodal surfaces are obtained by mean-field calculations. The model's Ginzburg-Landau-Wilson Hamiltonian is used to expand the free-energy cost of nucleation near the spinodal surfaces to obtain an analytic continuation of the free-energy density across the first-order phase transition. A recently developed transfer-matrix formalism is applied to the model to obtain complex-valued constrained'' free-energy densities [ital f][sub [alpha]]. For particular eigenvectors of the transfer matrix, the [ital f][sub [alpha]] exhibit finite-range scaling behavior in agreement with the analytically continued metastable free-energy density. This transfer-matrix approach gives a free-energy cost of nucleation that supports the proportionality relation for the decay rate of the metastable phase [Gamma][proportional to][vert bar]Im[ital f][sub [alpha]][vert bar], even in cases where two metastable states compete. The picture that emerges from this study is verified by Monte Carlo simulation.

Hole Localization or Delocalization? An Optical, Raman, and Redox Study of Lanthanide Porphyrin-Phthalocyanine Sandwich-Type Heterocomplexes

A systematic electrochemical, redox, optical absorption, and resonance Raman spectroscopic study of 22 gadolinium and cerium double- and triple-decker tetraphenylporphyrin-phthalocyanine (TPP-Pc) complexes is reported. The work is aimed at elucidating the extent of the localization or delocalization among the chromophore ligands in the mixed complexes, where there exists a charge imbalance between the metal centers and the dianion Pc/TPP ligands, as well as in different oxidation states for these types of complexes. For mixed double- and triple-decker complexes, [vert bar]Pc(M)TPP[vert bar] and [vert bar]TPP(M)TPP[vert bar] (where M = Gd[sup III], Ce[sup IV]), in which the metal cannot be oxidized, it appears that the ligand preferentially oxidized is the Pc moiety. This perfentially localized hole in the phthalocyanine results in new near-infrared transitions which show maxima around 1200-1300 nm for the double-decker complexes and shifts to 1800-2300 nm for the triple-decker derivatives. In this case, the removal of an electron from a complex redox orbital which is rich in Pc character is easily identified by the accompanying changes in the optical absorption characteristic of a Pc[sup [sm bullet][minus]] species and is also seen in FT Raman spectra (excited at 1064 nm) by a significant (>15 cm[sup [minus]1]) upshift of a redox-sensitivemore » Raman band at ca. 1500 cm[sup [minus]1]. 33 refs., 13 figs., 2 tabs.« less

WT identities of gauge theories from broken OSp(3,1 │2) symmetry in a superspace formulation

We show that Ward-Takahashi identities of gauge theories can be derived in a compact and elegant form [partial derivative][ital [bar W]]/[partial derivative][theta]=0 from partial OSp(3,1[vert bar]2) invariance of the action in a superspace formulation of gauge theories.

Renormalization in a superspace formulation of gauge theories.

We show that the renormalization transformations take a very simple form in a superspace formulation of gauge theories with a broken OSp (3,1[vert bar]2) symmetry. Interrelations between renormalizations of all sources of fields and of composite operators is shown to arise from the fact that they belong together in source supermultiplets which transform under renormalization as a whole, in a simple fashion. The symmetry-breaking piece [ital S][sub 1] of the action (which generates sources for fields [ital and] composite operators, gauge-fixing term, and ghost term) is entirely left form invariant by renormalization transformations. The freedom to perform an asymmetric renormalization of ghost and antighost fields is shown to correspond to that of an Sp (2) rotation in superspace.

206Pb states homologous to the 1.484 MeV, 11/2(-) state of 205Tl.

This paper is an extension of a previous study of the [sup 206]Pb nucleus in the framework of the homology concept. It is concerned with the multiplet of states homologous to the 1.484 MeV, 11/2[sup [minus]] level of [sup 205]Tl, due to [vert bar]9/2[sup [minus]][direct product] 11 / 2 [sup [minus]][r angle] coupling. The results obtained and the new assignments attributed to [sup 206]Pb confirm the importance of the homology concept in the spectroscopy of high excitation energy levels.

Explicit rephasing invariant parametrization of the Cabibbo-Kobayashi-Maskawa matrix.

An explicit rephasing invariant parametrization of the Cabibbo-Kobayashi-Maskawa (CKM) matrix is presented. Its four parameters [vert bar][ital V][sub 12][vert bar], [vert bar][ital V][sub 23][vert bar], [vert bar][ital V][sub 13][vert bar], and [phi][sub 13] are directly measurable. The phenomenological approximation of the parametrization is also obtained, which shows more clearly the correlations between the elements of the CKM matrix. The present bounds on the phase angle [phi][sub 13] are 31[degree][le][phi][sub 13][le]148[degree] coming from the [ital CP]-violating quantity [epsilon] in the kaon system.

Living Polymerization of Butadiene at Both Chain Ends via a Bimetallic Nickel Initiator. Preparation of Hydroxytelechelic Poly(butadiene) and Symmetric Poly(isocyanide-b-butadiene-b-isocyanide) Elastomeric Triblock Copolymers

A bifunctional nickel initiator, bis([mu]-trifluoroacetato) ([eta][sup 3]:[eta][sup 3]-2,2[prime]-biallyl)dinickel(II), IV, was synthesized to promote the living polymerization of butadiene to high cis-1,4-content polymer with both chain ends active. Other potential initiators, [1,4-phenylenebis(([eta][sup 3]-1-allyl)nickel trifluoroacetate)][sub n], II, and [1,4-cyclohexanediylbis(([eta][sup 3]-1-allyl)-nickel trifluoroacetate)][sub n], III, were found to be completely ineffective due to intermolecular carboxylate bridges which gave the complexes polymeric structures. The intramolecular bridges in IV and its molecularity were confirmed by X-ray crystallography. IV cocrystallizes in space group P[bar 1] with 0.5 equiv of 2.3-bis((trifluoroacetoxy)methyl)-1,3-butadiene in the unit cell with constants of a = 9.5600(8) [angstrom], b = 9.6554(9) [angstrom], c = 11.7615(10) [angstrom]; [alpha] = 73.711(7)[degree], [beta] = 73.537(7)[degree], [gamma]= 71.938(7)[degree]; V = 967.50(15) [angstrom][sup 3]; and Z = 2. Refinement of the model, which contained a disordered C[sub 6]H[sub 8] unit, led to convergence with R[sub F] = 5.8%, R[sub WF] = 6.9%, and GOF = 2.21 for 276 variables refined against those 2852 data with [vert bar]F[sub o][vert bar]> 3.0[sigma]/([vert bar]F[sub o][vert bar]). The living polybutadiene samples were successfully used to prepare triblock copolymers of the structure poly(1-phenylethyl isocyanide-b-butadiene-b-1-phenylethyl isocyanide). 24 refs., 5 figs., 1 tab.

Unusual properties of midband states in systems with off-diagonal disorder.

It is known that off-diagonal disorder results in anomalous localization at the band center, whereas diagonal disorder does not. We show that the important distinction is not between diagonal and off-diagonal disorder, but between bipartite and nonbipartite lattices. We prove that bipartite lattices in any dimension (and some generalizations that are not bipartite) have zero energy (i.e., band-center) eigenfunctions that vanish on one sublattice. We show that ln[vert bar][psi][sub [ital j]][vert bar] has random-walk behavior for one-dimensional systems with first-, or first- and third-neighbor random hopping, leading to exp([minus][lambda] [radical][ital r]) localization of the zero-energy eigenfunction. Addition of diagonal disorder leads to a [ital biased] random walk. First- and second-neighbor random hopping with no diagonal disorder leads to ordinary exponential [exp([minus][lambda][ital r])] localization. Numerical simulations show anomalous localization in dimensions 1 and 2, with additional periodic structure in some cases.

Measurement of Drell-Yan electron and muon pair differential cross sections in p-barp collisions at sqrt s =1.8 TeV.

We measure the Drell-Yan differential cross section [ital d][sup 2][sigma]/[ital dM] d[ital y][vert bar][sub [vert bar][ital y][vert bar][lt]1] over the mass range 11[lt][ital M][lt]150 GeV/[ital c][sup 2] using dielectron and dimuon data from [ital [bar p]p] collisions at a center-of-mass energy of [radical][ital s] =1.8 TeV. Our results show the 1/[ital M][sup 3] dependence that is expected from the naive Drell-Yan model. In comparison to the predictions of recent QCD calculations we find our data favor those parton distribution functions with the largest quark contributions in the [ital x] interval 0.006 to 0.03.

Constraints on ||Vtd|| from radiative decays of the B mesons.

The possibility of using a future measurement of the ratio [ital B]([ital B][r arrow][rho][gamma])/[ital B]([ital B][r arrow][ital K][sup *][gamma]) to improve the constraints on the CKM parameter [vert bar][ital V][sub [ital t][ital d]][vert bar] is investigated. Special attention is paid to the two main sources of theoretical uncertainties: the SU(3) flavor symmetry-breaking effects and the contribution from final-state interactions.

Quantum statistical foundation to the fermi liquid model and Ginzburg-Landau wave function

An energy eigenvalue equation for a quasi-particle is derived, starting with the Heisenberg equation of motion for an annihilation operator. An elementary derivation of the Fermi liquid model having a sharply defined Fermi surface in the k-space is given, starting with a realistic model of a metal including the Coulomb interaction among and between electrons and lattice-ions. The Ginzburg-Landau wave function [Psi][sub [sigma]](r), where [sigma] represents the superconducting pairon (Cooper-pair) state, is shown to be connected with the one-pairon density operator n by [Psi][sub [sigma]](r) = [l angle]r[vert bar]n[sup 1/2][vert bar][sigma][r angle]. A close analogy between supercurrent and laser is indicated. 31 refs.

Perturbative QCD effects observed in 490 GeV deep-inelastic muon scattering.

Results on forward charged hadrons in 490 GeV deep-inelastic muon scattering are presented. The transverse momenta, azimuthal asymmetry, and energy flow of events with four or more forward charged hadrons are studied. The range of the invariant hadronic mass squared 300[lt][ital W][sup 2][lt]900 GeV[sup 2]/[ital c][sup 4] extends higher than previous deep-inelastic muon scattering experiments. Data are compared to the predictions of the Lund Monte Carlo model with perturbative QCD simulated by matrix elements, parton showers, and color dipole radiation. All of the QCD-based models are consistent with the data while a model without QCD processes is not. Correlations with the multiplicity-independent event variable [Pi][congruent][summation][vert bar][ital p][sub [ital T]][vert bar] are studied. The relationship between the azimuthal asymmetry and transverse momentum of forward hadrons is also presented. The data are most consistent with intrinsic parton transverse momentum squared [ital k][sub [ital T]][sup 2] of 0.25 GeV[sup 2]/[ital c][sup 2].