Eur. Phys. J. A 12, 353-359 (2001)

Understanding the proton spin "puzzle" with a new "minimal" quark model

B.-Q. Ma¹, I. Schmidt² and J.-J. Yang<sup>2, 3

1</sup>  Department of Physics, Peking University, Beijing 100871 and Institute of Theoretical Physics, Academia Sinica, Beijing 100080, PRC
²  Departamento de Física, Universidad Técnica Federico Santa María, Casilla 110-V, Valparaíso, Chile
³  Department of Physics, Nanjing Normal University, Nanjing 210097, PRC

mabq@phy.pku.edu.cn
ischmidt@fis.utfsm.cl
jjyang@fis.utfsm.cl

(Received: 4 November 2000 / Revised version: 2 August 2001 Communicated by A. Schäfer)

Abstract
We investigate the spin structure of the nucleon in an extended Jaffe-Lipkin quark model. In addition to the conventional 3q structure, different admixtures in the nucleon wave function are also taken into account. The contributions to the nucleon spin from various components of the nucleon wave function are discussed. The effect due to the Melosh-Wigner rotation is also studied. It is shown that the Jaffe-Lipkin term is only important when antiquarks are negatively polarized. We arrive at a new "minimal" quark model, which is close to the naive quark model, in order to understand the proton spin "puzzle".

PACS
12.39.-x - Phenomenological quark models.
13.60.Hb - Total and inclusive cross sections (including deep-inelastic processes).
13.88.+e - Polarization in interactions and scattering.
14.20.Dh - Protons and neutrons.

© Società Italiana di Fisica, Springer-Verlag 2001