Spin and flavor content of octet baryons

Abstract

Abstract. We discuss the spin and flavor content of the proton in an unquenched quark model. It is shown that the inclusion of hadron loops leads automatically to an excess ofd over u and introduces a sizeable contribution of orbital angular momentum to the spin of the proton. Whereas the flavor asymmetry and the orbital angular momentum are dominated by pion loops, the contribution of the sea quark spins to the proton spin arises almost entirely from (excited) vector meson loops. IntroductionIn the constituent quark model (CQM) hadrons are described in terms of system of constituent (or valence) quarks and antiquarks, qqq for baryons and qq for mesons. Despite the success of the quark model there is strong evidence for the existence of exotic degrees of freedom (other than valence quarks) in hadrons, in particular for the need to include the effects of quark-antiquark pairs (hadron loops). The importance of these quark-antiquark configurations (or higher Fock components in baryon wave functions) is evident from measurements of thed/ū asymmetry in the nucleon sea [1], parity-violating electron scattering experiments [2,3], the proton spin crisis [4], as well as from analysis of helicity amplitudes [5] and strong couplings of baryon resonances [6,7].The aim of this contribution is to study the spin and flavor content of the proton in an unquenched quark model in which the effects of quark-antiquark pair creation (uū, dd and ss) are taken into account in an explicit form via a 3 P 0 coupling mechanism [8,9,10]. The numerical results for the flavor asymmetry [9] and the proton spin [8] are analyzed by means of a simple exactly solvable meson-cloud model. Finally, we show some predictions for other octet baryons as well.

Orbital angular momentum of the proton and intrinsic five-quark Fock states

“…The only adjusted parameter, V in Eq. (19), for the Goldstone boson exchange model, was determined by fitting [32,33] the experimental data for the sea flavor asymmetry I a =d −ū = 0.118 ± 0.012 of the proton [1]. This ensemble allowed us postdicting, on the one hand, the strangeness magnetic moment µ s and the strangeness magnetic moment G s M of the proton [37], and on other hand, shedding a light [38] on the measured [39] quark-antiquark ratios r ℓ = uū/dd, r s = ss/dd, and the strangeness content of the proton κ s = 2ss/(uū + dd).…”

“…Moreover, while the CQM also predicts a vanishing value for the OAM, the Q Q components lead to L q = 0. Actually, the contribution of the OAM to the spin of proton was found to be comparable to that of the sea quark ( ≈ 30% each) [19], and much larger than that of the gluons [20].…”