Nonperturbative Physics in a Magnetic Field

Non-Perturbative Quantum Field Theory has played an important role in the study of phenomena where a fermion condensate can appear under certain physical conditions. The familiar phenomenon of electric superconductivity, the color superconductivity of very dense quark matter, and the chiral symmetry breaking of low energy effective chiral theories are all examples of that sort. Often one is interested in the behavior of these systems in the presence of an external magnetic field. In this talk I will outline the effects of an external magnetic field on theories with either fermion-fermion or fermion-antifermion condensates.Comment: Invited talk at XII Mexican Workshop on Particles & Fields. Mazatlan, Sinaloa, Mexico, Nov.200

Yukawa Interactions and Dynamical Generation of Mass in an External Magnetic Field

In this work we study the dynamical generation of a fermion mass induced by a constant and uniform external magnetic field in an Abelian gauge model with a Yukawa term. We show that the Yukawa coupling not only enhances the dynamical generation of the mass, but it substantially decreases the magnetic field required for the mass to be generated at temperatures comparable to the electroweak critical temperature. These results indicate that if large enough primordial magnetic fields were present during the early universe evolution, the field-induced generation of fermion masses, which in turn corresponds to the generation of fermion bound states, may play an important role in the electroweak phase transition.Comment: Corrected mistake in last reference. Work presented at SILAFAE'98, 8 pages. To be published in the proceedings of SILAFAE'98, April 8-11, San Juan, Puerto Ric

Absence of Landau-Peierls Instability in the Magnetic Dual Chiral Density Wave Phase of Dense QCD

We investigate the stability of the Magnetic Dual Chiral Density Wave (MDCDW) phase of cold and dense QCD against collective low-energy fluctuations of the order parameter. The appearance of additional structures in the system free-energy due to the explicit breaking of the rotational and isospin symmetries by the external magnetic field and the field-induced asymmetry of the lowest Landau level modes play a crucial role in the analysis. The new structures not only affect the condensate minimum equations, but also the spectrum of the thermal fluctuations, which lacks the transverse soft modes that typically affect single-modulated inhomogeneous phases in the absence of a magnetic field. Consequently, the long-range order of the MDCDW phase is preserved at finite temperature. The lack of Landau-Peierls instabilities in the MDCDW phase makes this inhomogeneous phase of dense quark matter particularly relevant for the physics of neutron stars.Comment: Typos corrected, new discussions adde

Neutrinos under Strong Magnetic Fields

In this talk we review the results on neutrino propagation under external magnetic fields. We concentrate on the effects of strong magnetic fields $M_{W}^{2}\gg B\gg m_{e}^{2}$ in neutral media. It is shown that the neutrino energy density get a magnetic contribution in the strong-field, one-loop approximation, which is linear in the Fermi coupling constant as in the charged medium. It is analyzed how this correction produces a significant oscillation resonance between electron-neutrinos and the other two active flavors, as well as with sterile neutrinos. The found resonant level-crossing condition is highly anisotropic. Possible cosmological applications are discussed. Effects due to primordial hypermagnetic fields on neutrinos propagating in the symmetric phase of the electroweak model are also presented. At sufficiently strong hypermagnetic fields, $B\geq T^{2}$, the neutrino energy is found to be similar to that of a massless charged particle with one-degree of freedom.Comment: Talk given at the Fourth Tropical Workshop on Particle Physics and Cosmology, Cairns, Australia, June 9-13, 200