1,243 research outputs found

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

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

We calculate the vacuum expectation value of the dimension-5 "mixed"
quark-gluon operator, $O_\sigma = \bar\psi (\lambda^a /2) \sigma_{\mu\nu} \psi
F_{\mu\nu}^a$, in the instanton vacuum. Within the $1/N_c$--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
$m_0^2 \equiv \langle O_\sigma \rangle / \langle \bar\psi\psi \rangle \approx 4
\bar\rho^{-2} = 1.4 GeV^2$, 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

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

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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