1,243 research outputs found

    Improvement of the Theta+ width estimation method on the Light Cone

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    Recently, Diakonov and Petrov have suggested a formalism in the Relativistic Mean Field Approximation allowing one to derive the 3-, 5-, 7-,... quark wavefunctions for the octet, decuplet and antidecuplet. They have used this formalism and many strong approximations in order to estimate the exotic Theta+ width. The latter has been estimated to ~4 MeV. Besides they obtained that the 5-quark component of the nucleon is about 50% of its 3-quark component meaning that relativistic effects are not small. We have improved the technique by taking into account some relativistic corrections and considering the previously neglected 5-quark exchange diagrams. We also have computed all nucleon axial charges. It turns out that exchange diagrams affect very little Diakonov's and Petrov's results while relativistic corrections reduce the Theta+ width to ~2 MeV and the 5- to 3-quark component of the nucleon ratio to 30%.Comment: 28 pages, 15 figures; typo corrected and few comments adde

    Mixed quark-gluon condensate from instantons

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    We calculate the vacuum expectation value of the dimension-5 "mixed" quark-gluon operator, Oσ=ψˉ(λa/2)σμνψFμνaO_\sigma = \bar\psi (\lambda^a /2) \sigma_{\mu\nu} \psi F_{\mu\nu}^a, in the instanton vacuum. Within the 1/Nc1/N_c--expansion the QCD operator is replaced by an effective many-fermion operator, which is averaged over the effective theory of massive quarks derived from instantons. We find m02Oσ/ψˉψ4ρˉ2=1.4GeV2m_0^2 \equiv \langle O_\sigma \rangle / \langle \bar\psi\psi \rangle \approx 4 \bar\rho^{-2} = 1.4 GeV^2, somewhat larger than the estimate from QCD sum rules for the nucleon.Comment: 10 p, LaTeX, 1 figure included using eps

    Justifying the exotic Theta+ pentaquark

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    The existence of a light S=+1 baryon resonance follows from Quantum Field Theory applied to baryons. This is illustrated in the Skyrme model (where Theta+ exists but is too strong) and in a new mean field approach where Theta+ arises as a consequence of three known resonances: Lambda(1405), N(1440) and N(1535).Comment: 3 p., contribution to the PANIC-08 proceeding

    Chiral symmetry and pentaquarks

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    Spontaneous chiral symmetry breaking, mesons and baryons are illustrated in the language of the Dirac theory. Various forces acting between quarks inside baryons are discussed. I explain why the naive quark models typically overestimate pentaquark masses by some 500 MeV and why in the fully relativistic approach to baryons pentaquarks turn out to be light. I discuss briefly why it can be easier to produce pentaquarks at low than at high energies.Comment: Combined talks at Electron-Nucleus Scattering (Elba, June 20-25, 2004) and Pentaquarks-04 (Osaka, July 20-23, 2004) workshops. 18 p., 11 fig
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