1,243 research outputs found

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

    Full text link
    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

    Get PDF
    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

    Full text link
    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

    Full text link
    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