Chiral symmetry on the light-cone and effective models

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This project followed the previous one "Models of hadrons on the light-cone" (project no.11640258) supported by the Grant-in-Aid for Scientific Research (Basic Research (C) (2)) for the period 1999-2002, and aimed at clarifying how to describe the spontaneous breakdown of chiral symmetry, which is related to the structure of vacuum in the conventional formulation of field theory, in the light-cone formulation, where the vacuum is known to be trivial, and how it is reflected in effective models.In the conventional formulation, the Nambu-Goldstone (NG) particle, which appears as a result of the symmetry brealing, is generated by a local soft modulation of the symmetry transformation generator, and therefore the connection to the underlying symmetry is transparent. This was not the case in the light-cone formulation. We have clarified the connection by considering the peculiar kinematics of the light-cone coordinate and introducing a non-local modulation of the symmetry operator. Actual construction of the modulated operator was made for the Nambu-Jona-Lasinio (NJL) model, which is considered to be an effective theory of QCD. This work has been published in Physical Review D.Concerning effective models, I have continued the collaboration and used the quark-diquark model, which was derived from a relativistic Faddeev approach to 3-quark system in the NJL model, to study the effect of nuclear medium on the structure functions of the nucleon. The calculation gave a reasonable description of the experimentally observed EMC effect. This work has been published in Nuclear Physics A.