The Background Field Method for N = 2 Super Yang-Mills Theories in Harmonic Superspace

The background field method for N=2 super Yang-Mills theories in harmonic superspace is developed. The ghost structure of the theory is investigated. It is shown that the ghosts include two fermionic real omega-hypermultiplets (Faddeev-Popov ghosts) and one bosonic real omega-hypermultiplet (Nielsen-Kallosh ghost), all in the adjoint representation of the gauge group. The one-loop effective action is analysed in detail and it is found that its structure is determined only by the ghost corrections in the pure super Yang-Mills theory. As applied to the case of N=4 super Yang-Mills theory, realized in terms of N=2 superfields, the latter result leads to the remarkable conclusion that the one-loop effective action of the theory does not contain quantum corrections depending on the N=2 gauge superfield only. We show that the leading low-energy contribution to the one-loop effective action in the N=2 SU(2) super Yang-Mills theory coincides with Seiberg's perturbative holomorphic effective action.Comment: 17 pages, Late

Two-loop low-energy effective action in Abelian supersymmetric Chern-Simons matter models

We compute two-loop low-energy effective actions in Abelian Chern-Simons matter models with N=2 and N=3 supersymmetry up to four-derivative order. Calculations are performed with a slowly-varying gauge superfield background. Though the gauge superfield propagator depends on the gauge fixing parameter, it is shown that the obtained results are independent of this parameter. In the massless case the considered models are superconformal. We demonstrate that the superconformal symmetry strongly restricts the form of two-loop quantum corrections to the effective actions such that the obtained terms have simpler structure than the analogous ones in the effective action of three-dimensional supersymmetric electrodynamics (SQED) with vanishing topological mass.Comment: 1+30 pages; v2: references added, misprints corrected; v3: minor changes, published versio

Ionization corrections in a multi-phase interstellar medium: Lessons from a z~2 sub-DLA

We present a high resolution (FWHM=2.7 km/s), high S/N echelle spectrum for the z = 2.26 QSO J2123-0050 and determine elemental abundances for the z = 2.06 sub-DLA in its line of sight. This high redshift sub-DLA has a complex kinematic structure and harbours detections of neutral (SI, CI), singly (e.g. CII, SII) and multiply ionized (e.g. CIV, SiIV) species as well as molecular H and HD. The plethora of detected transitions in various ionization stages is indicative of a complex multi-phase structure present in this high redshift galaxy. We demonstrate that the ionization corrections in this sub-DLA are significant (up to ~0.7 dex). For example, if no ionization correction is applied, a super-solar metallicity is derived ([S/H] = +0.36), whereas a single phase ionization correction reduces this to [S/H] = -0.19. The theoretical impact of a multi-phase medium is investigated through Cloudy modelling and it is found that the abundances of Si, S and Fe are always over-estimated (by up to 0.15 dex in our experiments) if a single-phase is assumed. Therefore, although Cloudy models improve estimates of metal column densities, the simplification of a single phase medium leaves a systematic error in the result, so that even ionization-corrected abundances may still be too high. Without ionization corrections the properties of this sub-DLA appear to require extreme scenarios of nucleosynthetic origins. After ionization corrections are applied the ISM of this galaxy appears to be similar to some of the sightlines through the Milky Way.Comment: Accepted for publication in MNRA

Generic chiral superfield model on nonanticommutative N=1/2 superspace

We consider the generic nonanticommutative model of chiral-antichiral superfields on ${\cal N}={1\over 2}$ superspace. The model is formulated in terms of an arbitrary K\"ahlerian potential, chiral and antichiral superpotentials and can include the nonanticommutative supersymmetric sigma-model as a partial case. We study a component structure of the model and derive the component Lagrangian in an explicit form with all auxiliary fields contributions. We show that the infinite series in the classical action for generic nonanticommutative model of chiral-antichiral superfields in D=4 dimensions can be resumed in a compact expression which can be written as a deformation of standard Zumino's lagrangian and chiral superpotential. Problem of eliminating the auxiliary fields in the generic model is discussed and the first perturbative correction to the effective scalar potential is obtained.Comment: 12 pages, LaTeX; text revised and extended, references adde

Covariant Harmonic Supergraphity for N = 2 Super Yang--Mills Theories

We review the background field method for general N = 2 super Yang-Mills theories formulated in the N = 2 harmonic superspace. The covariant harmonic supergraph technique is then applied to rigorously prove the N=2 non-renormalization theorem as well as to compute the holomorphic low-energy action for the N = 2 SU(2) pure super Yang-Mills theory and the leading non-holomorphic low-energy correction for N = 4 SU(2) super Yang-Mills theory.Comment: 17 pages, LAMUPHYS LaTeX, no figures; based on talks given by I. Buchbinder and S. Kuzenko at the International Seminar ``Supersymmetries and Quantum Symmetries'', July 1997, Dubna; to be published in the proceeding