1,595 research outputs found

    Assembly of Effective Halide Receptors from Components. Comparing Hydrogen, Halogen, Tetrel Bonds

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    Receptors for halide anions are constructed based on the imidazolium unit, and then replacing the H-bonding C-H group firstby halogen-bonding C-I and then by tetrel-bonding C-SnH3 and C-SiF3.Attaching a phenyl ring to any of these species has little effect on its ability to bind a halide, but incorporation of a second imidazolium to the benzene connector, forming a bidentate dicationic receptor, greatly enhances the binding. Addition of electron-withdrawing F atoms to each imidazolium adds a further increment. F- consistently binds more strongly to the various receptor models than does Cl-. Whereas replacement of the H atom on the imidazolium groups with the halogen-bonding I has an inconsistent perturbing effect, tetrel-bonding SnH3 significantly enhances the binding with either halide, and SiF3 even more so. Placement of the various complexes into aqueous solution reduces binding energies, but the trends that occur in the gas phase are largely reproduced in water. The tetrel-bonding receptors are the most selective for F- over Cl-, with an equilibrium ratio on the order of 1014 for SnH3 and1028 for SiF3. When combined with their strong halide binding, SiF3-ImF3-Bz-ImF3-SiF3+2 bipodal receptors represent an optimal choice in terms of both binding strength and selectivity

    Stellarator equilibria and the problem of position control

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    Path Integrals, Density Matrices, and Information Flow with Closed Timelike Curves

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    Two formulations of quantum mechanics, inequivalent in the presence of closed timelike curves, are studied in the context of a soluable system. It illustrates how quantum field nonlinearities lead to a breakdown of unitarity, causality, and superposition using a path integral. Deutsch's density matrix approach is causal but typically destroys coherence. For each of these formulations I demonstrate that there are yet further alternatives in prescribing the handling of information flow (inequivalent to previous analyses) that have implications for any system in which unitarity or coherence are not preserved.Comment: 25 pages, phyzzx, CALT-68-188

    Higher Order 1/m1/m Corrections at Zero Recoil

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    The general structure of the 1/m1/m corrections at zero recoil is studied. The relevant matrix elements are forward matrix elements of local higher dimensional operators and their time ordered products with higher order terms from the Lagrangian. These matrix elements may be classified in a simple way and the analysis at the non recoil point for the form factor of heavy quark currents simplifies drastically. The second order recoil corrections to the form factor hA1h_{A1} of the axial vector current, relevant for the Vcb|V_{cb}| determination from BDB \to D^* decays, are estimated to be 5%<hA11<0-5\% < h_{A1} - 1 < 0.Comment: LaTeX, 25 pages, one figure, appended after \end{document} as uu-encoded and compressed eps file, uses epsf, CERN-TH.7162/9

    Isospin Multiplet Structure in Ultra--Heavy Fermion Bound States

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    The coupled Bethe--Salpeter bound state equations for a QQˉQ\bar Q system, where Q=(U,D)Q=(U,D) is a degenerate, fourth generation, super--heavy quark doublet, are solved in several ladder approximation models. The exchanges of gluon, Higgs and Goldstone modes in the standard model are calculated in the ultra--heavy quark limit where weak γ,W±\gamma, W^\pm and Z0Z^0 contributions are negligible. A natural I=0I=0 and I=1I=1 multiplet pattern is found, with large splittings occuring between the different weak iso--spin states when MQM_Q, the quark masses, are larger than values in the range 0.4TeV<MQ<0.8TeV0.4 TeV<M_Q<0.8 TeV, depending on which model is used. Consideration of ultra--heavy quark lifetime constraints and UDU-D mass splitting constraints are reviewed to establish the plausibility of lifetime and mass degeneracy requirements assumed for this paper.Comment: 20 pages, 7 figures (hard copy available upon request), report# KU-HEP-93-2

    Unitarity of Quantum Theory and Closed Time-Like Curves

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    Interacting quantum fields on spacetimes containing regions of closed timelike curves (CTCs) are subject to a non-unitary evolution XX. Recently, a prescription has been proposed, which restores unitarity of the evolution by modifying the inner product on the final Hilbert space. We give a rigorous description of this proposal and note an operational problem which arises when one considers the composition of two or more non-unitary evolutions. We propose an alternative method by which unitarity of the evolution may be regained, by extending XX to a unitary evolution on a larger (possibly indefinite) inner product space. The proposal removes the ambiguity noted by Jacobson in assigning expectation values to observables localised in regions spacelike separated from the CTC region. We comment on the physical significance of the possible indefiniteness of the inner product introduced in our proposal.Comment: 13 pages, LaTeX. Final revised paper to be published in Phys Rev D. Some changes are made to expand our discussion of Anderson's Proposal for restoring unitarit

    QCD Factorization for BππB\to\pi\pi Decays: Strong Phases and CP Violation in the Heavy Quark Limit

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    We show that, in the heavy quark limit, the hadronic matrix elements that enter BB meson decays into two light mesons can be computed from first principles, including `non-factorizable' strong interaction corrections, and expressed in terms of form factors and meson light-cone distribution amplitudes. The conventional factorization result follows in the limit when both power corrections in 1/mb1/m_b and radiative corrections in αs\alpha_s are neglected. We compute the order-αs\alpha_s corrections to the decays Bdπ+πB_d\to\pi^+\pi^-, Bdπ0π0B_d\to\pi^0\pi^0 and B+π+π0B^+\to\pi^+\pi^0 in the heavy quark limit and briefly discuss the phenomenological implications for the branching ratios, strong phases and CP violation.Comment: 6 pages, 1 figur

    In search of multipolar order on the Penrose tiling

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    Based on Monte Carlo calculations, multipolar ordering on the Penrose tiling, relevant for two-dimensional molecular adsorbates on quasicrystalline surfaces and for nanomagnetic arrays, has been analyzed. These initial investigations are restricted to multipolar rotors of rank one through four - described by spherical harmonics Ylm with l=1...4 and restricted to m=0 - positioned on the vertices of the rhombic Penrose tiling. At first sight, the ground states of odd-parity multipoles seem to exhibit long-range multipolar order, indicated by the appearance of a superstructure in the form of the decagonal Hexagon-Boat-Star tiling, in agreement with previous investigations of dipolar systems. Yet careful analysis establishes that long-range multipolar order is absent in all cases investigated here, and only short-range order exists. This result should be taken as a warning for any future analysis of order in either real or simulated arrangements of multipoles on quasiperiodic templates

    A Bjorken sum rule for semileptonic Ωb\Omega_b decays to ground and excited charmed baryon states

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    We derive a Bjorken sum rule for semileptonic Ωb\Omega_b decays to ground and low-lying negative-parity excited charmed baryon states, in the heavy quark limit. We discuss the restriction from this sum rule on form factors and compare it with some models.Comment: 10 pages, RevTex, no figure, Alberta Thy--26--9
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