Recent Results for the Unquenched Quark Model

Abstract

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.