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Register a new userFive-quark components in baryons
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Authors
B.S.Zou
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Evidence has been accumulating for the existence of significant intrinsic non-perturbative five-quark components in various baryons. The inclusion of the five-quark components gives a natural explanation of the excess of $bar d$ over $bar u$, significant quark orbital angular momentum in the proton, the problematic mass and decay pattern of the lowest $1/2^-$ baryon nonet, etc.. A breathing mode of $qqqleftrightarrow qqqqbar q$ is suggested for the lowest $1/2^-$ baryon octet. Evidence of a predicted member of the new scheme, $Sigma^*(1/2^-)$ around 1380 MeV, is introduced.
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Within an extended chiral constituent quark model, three- and five-quark structure of the $S_{01}$ resonance $Lambda(1405)$ is investigated. Helicity amplitudes for the electromagnetic decays ($Lambda(1405) to Lambda(1116)gamma$, $Sigma(1194)gamma$), and transition amplitudes for strong decays ($Lambda(1405)toSigma(1194)pi$, $ K^{-}p$) are drived, as well as the relevant decay widths. The experimental value for the strong decay width, $Gamma_{Lambda(1405)to (Sigma pi)^circ}=50pm 2$ MeV, is well reproduced with about 50% of five-quark admixture in the $Lambda(1405)$. Important effects due to the configuration mixings among $Lambda^{2}_{1}P_{A}$, $Lambda^{2}_{8}P_{M}$ and $Lambda^{4}_{8}P_{M}$ are found. In addition, transitions between the three- and five-quark components in the baryons turn out to be significant in both radiative and strong decays of the $Lambda(1405)$ resonance.
In this contribution, we present a study of ground- and excited-state $Omega_c$ and $Omega_b$ baryons consisting of two strange quarks and a heavy charm or bottom quark. An analysis in the quark model shows that the recently observed excited $Omega_c$ and $Omega_b$ states can be interpreted in terms of $lambda$-mode excitations.
The baryon-baryon interactions for the complete baryon octet (B_8) are investigated in a unified framework of the resonating-group method, in which the spin-flavor SU_6 quark-model wave functions are employed. Model parameters are determined to reproduce properties of the nucleon-nucleon system and the low-energy cross section data for the hyperon-nucleon interaction. We then proceed to explore B_8 B_8 interactions in the strangeness S=-2, -3 and -4 sectors. The S-wave phase-shift behavior and total cross sections are systematically understood by 1) the spin-flavor SU_6 symmetry, 2) the special role of the pion exchange, and 3) the flavor symmetry breaking.
We use a symmetry-preserving truncation of meson and baryon bound-state equations in quantum field theory in order to develop a unified description of systems constituted from light- and heavy-quarks. In particular, we compute the spectrum and leptonic decay constants of ground-state pseudoscalar- and vector-mesons: $q^prime bar q$, $Q^prime bar Q$, with $q^prime,q=u,d,s$ and $Q^prime,Q = c,b$; and the masses of $J^P=3/2^+$ baryons and their first positive-parity excitations, including those containing one or more heavy quarks. This Poincare-covariant analysis predicts that such baryons have a complicated angular momentum structure. For instance, the ground states are all primarily $S$-wave in character, but each possesses $P$-, $D$- and $F$-wave components, with the $P$-wave fraction being large in the $qqq$ states; and the first positive-parity excitation in each channel has a large $D$-wave component, which grows with increasing current-quark mass, but also exhibits features consistent with a radial excitation. The configuration space extent of all such baryons decreases as the mass of the valence-quark constituents increases.
The task of mapping and explaining the spectrum of baryons and the structure of these states in terms of quarks and gluons is a longstanding challenge in hadron physics, which is likely to persist for another decade or more. We review the progress made in this topic using a functional method based on Dyson-Schwinger equations. This framework provides a non-perturbative, Poincare-covariant continuum formulation of Quantum Chromodynamics which is able to extract novel insight on baryon properties since the physics at the hadron level is directly related with the underlying quark-gluon substructure, via convolution of Green functions.
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