Discrete Symmetries (C,P,T) in Noncommutative Field Theories

In this paper we study the invariance of the noncmmutative gauge theories under C, P and T transformations. For the noncommutative space (when only the spatial part of $\theta$ is non-zero) we show that NCQED is Parity invariant. In addition, we show that under charge conjugation the theory on noncommutative $R^4_{\theta}$ is transformed to the theory on $R^4_{-\theta}$, so NCQED is a CP violating theory. The theory remains invariant under time reversal if, together with proper changes in fields, we also change $\theta$ by $-\theta$. Hence altogether NCQED is CPT invariant. Moreover we show that the CPT invariance holds for general noncommutative space-time.Comment: Revtex File, 4 pages, no figures, minor changes from previous verion. To appear in Phys. Rev. Let

One Loop Renormalizability of Supersymmetric Yang-Mills Theories on Noncommutative Two-Torus

We argue that Yang-Mills theory on noncommutative torus, expressed in the Fourrier modes, is described by a gauge theory in a usual commutative space, the gauge group being a generalization of the area-preserving diffeomorphisms to the noncommutative case. In this way, performing the loop calculation in this gauge theory in the continuum limit we show that this theory is {\it one loop renormalizable}, and discuss the UV and IR limits. The moduli space of the vacua of the noncommutative super Yang-Mills theories in (2+1) dimensions is discussed.Comment: 16 pp, one figure, v2: One reference added, typos corrected. v3: minor exchange

Classification of Different Branes at Angles

In this paper, we consider two D-branes rotated with respect to each other, and argue that in this way one can find brane configurations preserving {1 \f 16} of SUSY. Also we classify different brane configurations preserving {1 \f 2}, {1 \f 4}, {3 \f 16},{1 \f 8}, {1 \f 16} of SUSY.Comment: Tex, 11 page, no figure

More on Mixed Boundary Conditions and D-branes Bound States

In this article, applying different types of boundary conditions; Dirichlet, Neumann, or Mixed, on open strings we realize various new brane bound states in string theory. Calculating their interactions with other D-branes, we find their charge densities and their tension. A novel feature of $(p-2,p)$ brane bound state is its "non-commutative" nature which is manifestly seen both in the open strings mode expansions and in their scattering off a $D_p$-brane. Moreover we study three or more object bound states in string theory language. Finally we give a M-theoretic picture of these bound states.Comment: Latex file, pages, No Figure

Noncommutative Open String Theories and Their Dualities

The recently found non-critical open string theories is reviewed. These open strings, noncommutative open string theories (NCOS), arise as consistent quantum theories describing the low energy theory of D-branes in a background electric B-field in the critical limit. Focusing on the D3-brane case, we construct the most general (3+1) NCOS, which is described by four parameters. We study S and T -dualities of these theories and argue the existence of a U-duality group.Comment: 10 pages, no figures, The invited talk, presented in the conference "Brane New World and Noncommutative Geometry", Torino, Villa Gualino,(Italy) October, 200