Programa de Pós-graduação em Física

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Navegando Programa de Pós-graduação em Física por Por Orientador "Braghin, Fábio Luis"

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    Strong magnetic field corrections to the NJL coupling constant from vacuum polarization
    (Universidade Federal de Goiás, 2022-01-28) Moreira, Thiago Henrique; Braghin, Fábio Luis; http://lattes.cnpq.br/9375085677737347; Braghin, Fábio Luis; Shovkovy, Igor; Markó, Gergely
    In this work we calculate corrections to the NJL model coupling constant in the presence of a uniform magnetic field. Starting with the Nambu-Jona-Lasinio Lagrangian coupled with an external magnetic field, we separate the quark field into two components: one corresponding to the condensed quarks and the other corresponding to the interacting quasiparticle quarks. By integrating out the former, an effective action in terms of the interacting quarks is obtained. We then expand the quark determinant in terms of the quark currents and show that the first-order term provides a correction to the quark masses given by the gap equation. The values obtained for the constituent quark masses increase with the magnetic field, which signals the enhancement of chiral symmetry breaking by the external field and therefore showing that the system exhibits magnetic catalysis. The second-order term of the quark determinant expansion provides a correction to the NJL coupling constant, which decreases with increasing magnetic field for the scalar interactions and increases with increasing magnetic field for the pseudoscalar ones. We then consider a NJL model with the flavor- and B-dependent couplings obtained from vacuum polarization and compute quark and meson masses. While the scalar couplings seem to improve the conciliation with lattice results for the quark masses, the same cannot be said about the pseudoscalar couplings which alters the pseudoscalar meson masses in a different way than what is known in the literature due to its behavior with the applied field.
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    Interações efetivas entre quarks a baixas energias a partir da Cromodinâmica Quântica considerando o condensado de quark-antiquark
    (Universidade Federal de Goiás, 2014-02-17) Paulo Júnior, Ademar; Braghin, Fábio Luis; http://lattes.cnpq.br/9375085677737347; Krein, Gastão Inácio; http://lattes.cnpq.br/5704289678296630; Avelar, Ardiley Torres; http://lattes.cnpq.br/5732286631137637
    The Quantum Chromodynamics (QCD) is the current theory that aim to describe the strong interactions in the Standard Model. It is a non-abelian gauge theory where quarks and gluons are the fundamental particles. However, QCD exhibits difficulties when low energy regimes are considered and perturbative methods cannot be employed. Thus, effective models are extremely important and useful when low energy limits are considered. Starting with QCD we develop an effetive model taking into account the second order gluon condensate, ⟨Aa A a ⟩, and quark-antiquark, ⟨ qq⟩. The coupled GAP equations are derived for Ns = 3 and numerical solutions are found for that. Furthermore, a functional expansion from effective potencial to the sixth order is calculated and contributions from that terms inserted into the effective lagrangian.