30,875 research outputs found

    Baryon Masses in the 1/N Expansion

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    The masses of baryons and heavy quark baryons are studied analytically in an expansion in 1/N, SU(3) flavor symmetry breaking and heavy-quark symmetry breaking. The measured baryon masses are in striking agreement with the 1/N hierarchy.Comment: Lattice 2000 (Spectrum

    Naturalness of the Coleman-Glashow Mass Relation in the 1/N_c Expansion: an Update

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    A new measurement of the Xi^0 mass verifies the accuracy of the Coleman-Glashow relation at the level predicted by the 1/N_c expansion. Values for other baryon isospin mass splittings are updated, and continue to agree with the 1/N_c hierarchy.Comment: 6 pages, revte

    QCD Baryons in the 1/N_c Expansion

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    The 1/N_c expansion provides a theoretical method for analyzing the spin-flavor symmetry properties of baryons in QCD that is quantitative, systematic and predictive. An exact spin-flavor symmetry exists for large-N_c baryons, whereas for QCD baryons, the spin-flavor symmetry is approximate and is broken by corrections proportional to the symmetry-breaking parameter 1/N_c = 1/3. The 1/N_c expansion predicts a hierarchy of spin and flavor symmetry relations for QCD baryons that is observed in nature. It provides a quantitative understanding of why some SU(3) flavor symmetry relations in the baryon sector, such as the Gell-Mann--Okubo mass formula, are satisfied to a greater precision than expected from flavor symmetry-breaking suppression factors alone.Comment: 25 pages, Invited lectures VIII Mexican Workshop on Particles and Fields, Nov. 14-20, 2001, Zacatecas, Mexico, to be published AIP Proceeding

    Baryon Chiral Perturbation Theory in the 1/N Expansion

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    The chiral Lagrangian for baryons is formulated in an expansion in 1/N_c. The chiral Lagrangian implements the contracted spin-flavor symmetry of large-N_c baryons as well as nonet symmetry of the leading planar diagrams. Large-N_c consistency conditions ensure that chiral loop corrections are suppressed in 1/N_c through exact cancellation of chiral loop graphs to fixed orders in 1/N_c. Application of 1/N_c baryon chiral perturbation theory to the flavor-27 baryon mass splittings and the baryon axial vector currents are considered as examples.Comment: 10 pages, Invited talk, The Phenomenology of Large-N_c QCD, to be published Proceedings of the Institute of Nuclear Theor

    Bosonic Operator Methods for the Quark Model

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    Quark model matrix elements can be computed using bosonic operators and the holomorphic representation for the harmonic oscillator. The technique is illustrated for normal and exotic baryons for an arbitrary number of colors. The computations are much simpler than those using conventional quark model wavefunctions

    Spin-Flavor Structure of Large N Baryons

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    The spin-flavor structure of large N baryons is described in the 1/N expansion of QCD using quark operators. The complete set of quark operator identities is obtained, and used to derive an operator reduction rule which simplifies the 1/N expansion. The operator reduction rule is applied to the axial currents, masses, magnetic moments and hyperon non-leptonic decay amplitudes in the SU(3)SU(3) limit, to first order in SU(3)SU(3) breaking, and without assuming SU(3)SU(3) symmetry. The connection between the Skyrme and quark representations is discussed. An explicit formula is given for the quark model operators in terms of the Skyrme model operators to all orders in 1/N1/\N for the two flavor case.Comment: 36 pages, 2 eps figures, uses revte

    Observations of Interstellar Lyman-alpha Absorption

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    Interstellar Lyman alpha absorption spectra discrepancies emphasizing neutral hydroge

    Observations of the interstellar medium with IUE

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    Limitations of the International Ultraviolet Explorer (IUE) are discussed, as well as improved powers of research in these areas: (1) Si and C IV doublet absorptions in the high dispersion stellar spectra of distant stars; (2) observation of spectra outside the galactic plans; (3) high velocity gas

    Changes in interstellar atomic abundances from the galactic plane to the halo

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    A few, specially selected interstellar absorption lines were measured in the high resolution, far ultraviolet spectra of 200 O and B type stars observed by the International Ultraviolet Explorer (IUE). For lines of sight extending beyond about 500 pc from the galactic plane, the abundance of singly ionized iron atoms increases relative to singly ionized sulfur. However, the relative abundances of singly ionized sulfur, silicon and aluminum do not seem to change appreciably. An explanation for the apparent increase of iron is the partial sputtering of material off the surfaces of dust grains by interstellar shocks. Another possibility might be that the ejecta from type I supernovae enrich the low density medium in the halo with iron

    1/N Expansion for Exotic Baryons

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    The 1/N expansion for exotic baryons is developed, and applied to the masses, meson couplings and decay widths. Masses and widths of the 27 and 35 pentaquark states in the same tower as the Theta+ are related by spin-flavor symmetry. The 27 and 35 states can decay within the pentaquark tower, as well as to normal baryons, and so have larger decay widths than the lightest pentaquark Theta. The 1/N expansion also is applied to baryon exotics containing a single heavy antiquark. The decay widths of heavy pentaquarks via pion emission, and to normal baryons plus heavy D^(*),B^(*) mesons are studied, and relations following from large-N spin-flavor symmetry and from heavy quark symmetry are derived.Comment: Major additions: plots of widths and branching ratios, discussion of strong decays of heavy pentaquarks, including consequences of heavy quark symmetr
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