The Conformal Window from the Worldline Formalism

We use the worldline formalism to derive a universal relation for the lower boundary of the conformal window in non-supersymmetric QCD-like theories. The derivation relies on the convergence of the expansion of the fermionic determinant in terms of Wilson loops. The expansion shares a similarity with the lattice strong coupling expansion and the genus expansion in string theory. Our result relates the lower boundary of the conformal window in theories with different representations and different gauge groups. Finally, we use SQCD to estimate the boundary of the conformal window in QCD-like theories and compare it with other approaches.Comment: 14 pages. 4 eps figures. v2: refs. added. To appear in Nuclear Physics

Comments on Mesonic Correlators in the Worldline Formalism

We elaborate on how to incorporate mesonic correlators into the worldline formalism. We consider possible applications to QCD-like theories in various dimensions. We focus on large-N_c two dimensional QCD (the 't Hooft model) and relate it to a single harmonic oscillator. We also discuss the dependence of the Peskin S-parameter on the number of massless flavors and their representation and compare our expression to the corresponding expression obtained at weak coupling. Finally, we use the worldline formalism to discuss how the Veneziano limit of QCD is realized in holography in the limit of small N_f/N_c.Comment: 14 pages, LaTex. 6 eps figures. v2: minor changes, typos fixed. To appear in Nuclear Physics

Non-Supersymmetric Brane Configurations, Seiberg Duality and Dynamical Symmetry Breaking

We consider type IIA brane configurations, similar to those that realize SO(2N) supersymmetric QCD, that include orientifold planes and anti-branes. Such brane configurations lead to Sp(2N) field theories that become supersymmetric in the large-N limit and break supersymmetry upon the inclusion of 1/N corrections. We argue that this class of field theories admit Seiberg duality and interpret the potential between branes and orientifolds as field theory phenomena. In particular we find in the magnetic theory a meson potential that leads to dynamical symmetry breaking and a meson condensate similar to the anticipated quark condensate in QCD.Comment: 22 pages. LaTex. 5 eps figures. v2: minor changes, reference and a comment about the GMOR relation added. To appear in Phys.Rev.

Universality of k-string Tensions from Holography and the Lattice

We consider large Wilson loops with quarks in higher representations in SU(N) Yang-Mills theories. We consider representations with common N-ality and check whether the expectation value of the Wilson loop depends on the specific representation or only on the N-ality. In the framework of AdS/CFT we show that = dim R exp -sigma_k A, namely that the string tension depends only on the N-ality k but the pre-exponent factor is representation dependent. The lattice strong coupling expansion yields an identical result at infinite N, but shows a representation dependence of the string tension at finite N, a result which we interpret as an artifact. In order to confirm the representation independence of the string tension we re-analyse results of lattice simulations involving operators with common N-ality in pure SU(N) Yang-Mills theory. We find that the picture of the representation-independence of the string tension is confirmed by the spectrum of excited states in the stringy sector, while the lowest-lying states seem to depend on the representation. We argue that this unexpected result is due to the insufficient distance of the static sources for the asymptotic behaviour to be visible and give an estimate of the distance above which a truly representation-independent spectrum should be observed.Comment: Refs. added, discussion in sect. 3 improved, other minor changes; to appear in JHE

Level-Rank Duality in Chern-Simons Theory from a Non-Supersymmetric Brane Configuration

We derive level-rank duality in pure Chern-Simons gauge theories from a non-supersymmetric Seiberg duality by using a non-supersymmetric brane configuration in type IIB string theory. The brane configuration consists of fivebranes, N D3 antibranes and an O3 plane. By swapping the fivebranes we derive a 3d non-supersymmetric Seiberg duality. After level shifts from loop effects, this identifies the IR of Sp(2N)_{2k-2N+2} and Sp(2k-2N+2)_{-2N} pure Chern-Simons theories, which is a level-rank pair. We also derive level-rank duality in a Chern-Simons theory based on a unitary group.Comment: 10 pages, 3 figure

S-Dual of Maxwell Chern-Simons Theory

We discuss the dynamics of three dimensional Maxwell theory coupled to a level $k$ Chern-Simons term. Motivated by S-duality in string theory we argue that the theory admits an S-dual description. The S-dual theory contains a non-gauge 1-form field, previously proposed by Deser and Jackiw and a level $k$ $U(1)$ Chern-Simons term, $Z_{\rm MCS}=Z_{\rm DJ}Z_{\rm CS}$. The couplings to external electric and magnetic currents and their string theory realisations are also discussed.Comment: 11 pages,LaTex. 3 figures. v2: Few comments added, in particular how to incorporate dynamical matter. Version to appear in PR

Quantum Broadening of k-Strings from the AdS/CFT Correspondence

We study the quantum broadening (width) of k-strings. We generalise an old result by Luscher, Munster and Weisz to the case of a k-string, by using the gauge/gravity correspondence. When the fundamental QCD-string is replaced by a bound state of k strings, the bound state is better described by a wrapped D-brane. We calculate the width of the k-string (the wrapped D-brane) in several AdS/CFT backgrounds by using a D-brane probe and find universally that omega_k ^2 = 1 / (2\pi \sigma_k) log R/r .Comment: 13 pages, LaTex. v2: minor changes, references added. To appear in Nucl.Phys.

Equivalence of meson scattering amplitudes in strong coupling lattice and flat space string theory

We consider meson scattering in the framework of the lattice strong coupling expansion. In particular we derive an expression for the 4-point function of meson operators in the planar limit of scalar Chromodynamics. Interestingly, in the naive continuum limit the expression coincides with an independently known result, that of the worldline formalism. Moreover, it was argued by Makeenko and Olesen that (assuming confinement) the resulting scattering amplitude in momentum space is the celebrated expression proposed by Veneziano several decades ago. This motivates us to also use holography in order to argue that the continuum expression for the scattering amplitude is related to the result obtained from flat space string theory. Our results hint that at strong coupling and large-Nc the naive continuum limit of the lattice formalism can be related to a flat space string theory