The spin structure of the proton

It is shown that the proton " spin crisis " or " spin puzzle " can be understood by the relativistic effect of quark transversal motions due to the Melosh-Wigner rotation. The quark helicity ∆q measured in polarized deep inelastic scattering is actually the quark spin in the infinite momentum frame or in the light-cone formalism, and it is different from the quark spin in the nucleon rest frame or in the quark model. The flavor asymmetry of the Melosh-Wigner effect for the valence u and d quarks and the intrinsic sea q ¯ q pairs are also the important ingredients in a SU(6) quark-spectator-diquark model framework to understand the " spin puzzle ". Such a picture of the spin structure can be tested by use of several simple relations to measure the quark spin distributions in the quark model. §1. The proton " spin crisis " The spin structure of the proton has received attention in the particle physics society for a decade, and there has been a vast number of theoretical and experimental investigations. Parton sum rules and similar relations played important roles in the establishment of the quark-parton picture for nucleons in deep inelastic scattering. Thus any violation of a parton sum rule is of essential importance to reveal possible new content concerning our understanding of the underlying quark-gluon structure of hadrons. From the SU(6) quark model one would expect that the spin of the proton is fully provided by the valence quark spins. Therefore the observation of the Ellis-Jaffe sum rule violation received extensive attention by its implication that the sum of the quark helicities is much smaller than the proton spin. The EMC result of a much smaller integrated spin-dependent structure function data than that expected from the Ellis-Jaffe sum rule triggered the proton " spin crisis " , i.e., the intriguing question of how the spin of the proton is distributed among its quark spin, gluon spin and orbital angular momentum 1). It is commonly taken for granted that the EMC result implies that there must be some contribution due to gluon polarization or orbital angular momentum to the proton spin. It will be reported here based on previous works 2)−6) , however, that the the proton spin problem raised by the Ellis-Jaffe sum rule violation does not in conflict with the SU(6) quark model in which the spin of the proton, when viewed …