Flavor asymmetry of sea quarks in the unquenched quark model

Abstract

The flavor asymmetry of the nucleon sea is studied in the framework of the unquenched quark model in which the effects of quark-antiquark pairs (uū, dd, and ss) are taken into account via a microscopic, QCD-inspired, quark-antiquark creation mechanism. The inclusion of the qq pairs leads to an excess ofd overū, in agreement with the experimental data for the proton. In addition, the results for the flavor asymmetry of all ground-state octet and decuplet baryons are presented. The isospin symmetry leads to simple relations among the flavor asymmetries of octet and decuplet baryons. The flavor asymmetry of the + hyperon is predicted to be very similar to that of the proton and much larger than that for the 0 hyperon. A comparison with other approaches shows large differences in the predictions for the flavor asymmetries of the hyperons.Introduction. From an experimental point of view, the flavor asymmetry betweenū andd quarks in the proton is now well established [1-8]; for review, see also Refs. [9,10]. It is well known [11,12] that perturbative QCD is not able to account for this asymmetry. In fact, for quark-antiquark pairs created perturbatively, the sea quarks generated by leading twist evolution, i.e., from gluon splitting, are flavor symmetric with the same amount of uū as dd and ss. Thus the flavor asymmetry of the nucleon sea is assumed to have a nonperturbative origin, and since currently it is still a big challenge to perform calculations from the first principles of QCD in the nonperturbative region, one has to try to understand the situation with effective models of hadrons, in terms of constituent quark degrees of freedom and/or meson-baryon degrees of freedom.The flavor content of the nucleon sea provides an important test for models of the nucleon structure. The quark-parton model predicts a flavor symmetric sea that leads to the Gottfried sum rule S G = 1/3 [13], whereas any deviation from this value is an indication of thed/ū asymmetry of the nucleon sea, thus providing a clean evidence of the existence of nonperturbative higher Fock components (such as qqq-qq configurations) in the proton wave function. The first clear evidence of a flavor asymmetric sea and a related violation of the Gottfried sum rule came in 1991 from the New Muon Collaboration (NMC) [1], which was later confirmed in Drell-Yan experiments [4][5][6][7] that probe the ratioū/d, as well as in semi-inclusive deep-inelastic scattering (SIDIS) experiments [8]. All these experiments show evidence that there are morē d quarks in the proton than there areū quarks [10].Many phenomenological models have been applied to the flavor asymmetry of the nucleon sea, e.g., meson-cloud models [14-16] (see Ref.[17] for other references), chiral quark

Higher bottomonia in the unquenched quark model

“…The wave functions of the mesons A, B and C are written as harmonic oscillator wave functions, depending on a single oscillator parameter = 0.5 GeV [4][5][6][17][18][19]. The self energies (E a )'s of Eq.…”

“…In the UQM approach [4][5][6][17][18][19], which is a generalization of the unitarized quark model by Törnqvist and Zenczykowski [1], the effects of qq pairs are introduced explicitly into the QM through a 3 P 0 pair-creation mechanism. The dispersive equation for the self energy (E a ) in the UQM is given by…”

“…These continuum coupling (or pair-creation) effects, neglected in the naive quark model (QM) [9][10][11][12][13][14] (see also Refs. [15,16]), are also important in the study of other observables, such as the importance of the orbital angular momentum in the spin of the proton [17], the flavor asymmetry of the proton [18] and the strange content of the electromagnetic form factors of the nucleon [19]. Important informations on mesons are also provided by their strong, electromagnetic and weak decay modes.…”

“…In this contribution, we show an application of the unquenched quark model (UQM) [4][5][6][17][18][19]] to the calculation of the bottomonium spectrum with self energy corrections [6]. We also discuss our results for the open bottom strong decay widths of higher bottomonia [6], computed within a modified version of the 3 P 0 pair-creation model [4][5][6].…”

Dynamical study of Ωc0 in the chiral quark model

“…Lots of theoretical calculations have been performed to investigate these two exotic states [7][8][9][10][11][12][13][14][15][16][17], even though the Θ þ ð1540Þ pentaquark was reported by several experimental groups [18][19][20] in 2003 and has been denied by JLab with more higher precision results [21] [LEPS Collaboration still insisted on the existence of pentaquark Θ þ ð1540Þ [22]]. Besides, it is shown that there should be notable five-quark components in the baryon resonances [23][24][25][26]. In addition, the valencesea quark mixing (Fock space expansion) model (q 3 þ q 3 qq) of the nucleon ground state had been used to explain the mysterious proton spin structure well [27].…”