Unquenched Quark Model For Baryons Research Articles

Meson-loop contributions in the quark model

We present a review of some recent results obtained in the unquenched quark model for baryons. The effects of sea quarks are taken into account in an explicit form via a QCD-inspired creation mechanism of the quark-antiquark pairs. In this approach, the contribution of the quark-antiquark pairs can be studied for any inital baryon and for any flavor of the qq̄ pairs (uū, dd̄ and ss̄). It is shown that, whereas the inclusion of qq̄ pairs (or meson loops) does not affect the baryon magnetic moments, it immediately leads to an excess of d̄ over ū in the proton and introduces a sizeable contribution of orbital angular momentum to the spin of the proton. The contribution of ss̄ pairs to the magnetic moment and the radius of the proton is found to be small, in agreement with the latest experimental results and recent lattice calculations.

Recent Results for the Unquenched Quark Model

We discuss a new generation of unquenched quark models for baryons is presented in which the effects of quark-antiquark pairs are taken into account in an explicit form by means of a 3 P0 quark-antiquark creation mechanism. This provides the possibility to address many open problems in baryons structure and spectroscopy. The applications to magnetic moments and to the spin and flavor content of baryons will be reviewed here. 1 Open Problems in Baryons Physics There is compelling evidence for the existence of exotic degrees of freedom, i.e. other than valence quarks, in baryons, in particular for the inclusion of the effects of quark-antiquark pairs (virtual intermediate meson- baryon states). The importance of these quark-antiquark configurations (or higher Fock components in baryon wave functions) is evident from measurements of the ¯ d/ ¯ u asymmetry in the nucleon sea (1), parity-violating electron scattering experiments (2,3), the proton spin crisis (4-6), as well as from analysis of helicity ampli- tudes (7,8) and strong couplings of baryon resonances (9-11). The coupling with the continuum and higher Fock configurations can also be the key solution of the missing resonances problem, which are resonances predicted by all quark models based on three constituent quarks, such as the (non)relativistic quark model, bag models and chiral soliton models, but which have never been observed (12). This problem is common to all quark models even below 2 GeV, as well as for most quark-diquark models (see (13) and references therein for a review on quark-diquark models), even though in this case the number of missing states is much smaller, since they are based on a reduced number of effective degrees of freedom. The aim of this article is to present a short overview of constituent quark models (CQM) and the need for unquenching of the quark model. The recently introduced unquenched quark model for baryons (14,15) will be reviewed and briefly and some recent results for the magnetic moments, and the spin and flavor contents of octet baryons will be discussed.

Unquenching the Quark Model

The formalism for a new generation of unquenched quark models for baryons in which the effects of quark–antiquark pairs are taken into account in an explicit form via a microscopic, QCD-inspired, quark–antiquark creation mechanism, is presented. As an application the quark–antiquark contribution to the spin of the proton and the Σ+ is discussed. Finally the strangeness radius of the Σ+ in the closure limit is calculated.

Unquenched quark model for baryons: Magnetic moments, spins, and orbital angular momenta

We present an unquenched quark model for baryons in which the effects of the quark-antiquark pairs (up, down and strange) are taken into account in an explicit form via a microscopic, QCD-inspired, quark-antiquark creation mechanism. In the present approach, the contribution of the quark-antiquark pairs can be studied for any inital baryon and for any flavor of the pairs. It is shown that, while the inclusion of quark-antiquark pairs does not affect the baryon magnetic moments, it leads to a sizeable contribution of the orbital angular momentum to the spin of the proton and the Lambda hyperon.

Quark-antiquark effects in baryons

The formalism for a new generation of unquenched quark models for baryons is presented. The effects of quark-antiquark pairs are taken into account in an explicit form via a microscopic, QCD-inspired, quark-antiquark creation mechanism. As an application, the quark-antiquark contribution to the spin of the proton is discussed.