Spin-Dependent Forces Between Quarks in Hadrons

Abstract

Most of what I say in this talk is within the framework of the constituent quark model. in which explicit gluon degrees of freedom are integrated out, leaving only a potential between quarks. In this framework, the question, “What per cent of the spin of a hadron is carried by the quarks?” has the simple answer, “100 per cent.” The sea of gluons and quark-antiquark pairs surrounding a current quark is considered an integral part of it, and both the quark and the sea together make the constituent quark. When a constituent quark is tweaked gently, it drags its sea around with it, and therefore has more inertia than a current quark. The size of a constituent quark. including its sea, is substantially greater than the size of a current quark, and a constituent quark may not be much smaller than the hadron in which it is bound. In the constituent quark model there is still a puzzle about the nature of the spin-dependent force between two quarks (or between a quark and an antiquark). Specifically, does the spin-dependent interaction arise from the chromomagnetic interaction of one-gluon exchange,1 from Goldstone-boson exchange in connection with the breaking of chiral symmetry,2 or from the effects of instantons3? It is the purpose of my talk to show that the one-gluon-exchange mechanism provides a consistent picture of the spin-splittings in ground-state mesons and baryons. independently of whether those hadrons contain heavy or light quarks. Isgur4 has recently taken a somewhat similar point of view. However, I cannot rule out the possibility that meson exchange and instantons also contribute to the spin splittings. Shortly after the invention of QCD, De Rujula et al.1 discussed consequences of the one-gluon exchange interaction between two (constituent) quarks in a baryon and between a quark and antiquark in a meson. In their work, the spin-dependent splitting of ground-state hadrons arises from the chromomagnetic interaction. Since then, many authors have written papers in which the mass splittings of the vector and pseudoscalar mesons, and the splittings of the spin-3/2 and spin-1/2 baryons arise from chromomagnetic forces. These forces have three salient features: 1) they are two-body forces, 2) they are short range, and 3) they are attractive in spin-0 states and repulsive in spin-1 states of two quarks in a baryon or a quarkantiquark in a meson.