Spectrum of orientifold QCD in the strong coupling and hopping expansion approximation

We use the strong coupling and hopping parameter expansions to calculate the pion and rho meson masses for lattice Yang-Mills gauge theories with fermions in irreducible two-index representations, namely the adjoint, symmetric and antisymmetric. The results are found to be consistent with orientifold planar equivalence, and leading order 1/N corrections are calculated in the lattice phase. An estimate of the critical bare mass, for which the pion is massless, is obtained as a function of the bare coupling. A comparison to data from the two-flavour SU(2) theory with adjoint fermions gives evidence for a bulk phase transition at beta~2, separating a pure lattice phase from a phase smoothly connected to the continuum.Comment: 16 page

SUSY Relics in One-Flavor QCD from a New 1/N Expansion

We suggest a new large-N_c limit for multi flavor QCD. Since fundamental and two-index antisymmetric representations are equivalent in SU(3), we have the option to define SU(N_c) QCD keeping quarks in the latter. We can then define a new 1/N_c expansion (at fixed number of flavors N_f) that shares appealing properties with the topological (fixed N_f/N_c) expansion while being more suitable for theoretical analysis. In particular, for N_f=1, our large-N_c limit gives a theory that we recently proved to be equivalent, in the bosonic sector, to N=1 supersymmetric gluodynamics. Using known properties of the latter, we derive several qualitative and semi-quantitative predictions for N_f=1 massless QCD which can be easily tested in lattice simulations. Finally, we comment on possible applications for pure SU(3) Yang-Mills theory and real QCD.Comment: 4 pages, RevTex. v2: note added at the end of the paper, ref. added. To appear in Phys.Rev.Let

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

The Hagedorn Temperature and Open QCD-String Tachyons in Pure N=1 Super Yang-Mills

We consider large-N confining gauge theories with a Hagedorn density of states. In such theories the potential between a pair of colour-singlet sources may diverge at a critical distance r_c = 1/ T_H. We consider, in particular, pure N=1 super Yang-Mills theory and argue that when a domain-wall and an anti domain-wall are brought to a distance near r_c the interaction potential is better described by an ``open QCD-string channel''. We interpret the divergence of the potential in terms of a tachyonic mode and relate its mass to the Hagedorn temperature. Finally we relate our result to a theorem of Kutasov and Seiberg and argue that the presence of an open string tachyonic mode in the annulus amplitude implies an exponential density of states in the UV of the closed string channel.Comment: 9 pages, LaTex. v2: a paragraph about the couplings between the glueballs and the source is added. To appear in Phys.Lett.

Screening in Supersymmetric Gauge Theories in Two Dimensions

We show that the string tension in N=1 two-dimensional super Yang-Mills theory vanishes independently of the representation of the quark anti-quark external source. We argue that this result persists in SQCD_2 and in two-dimensional gauge theories with extended supersymmetry or in chiral invariant models with at least one massless dynamical fermion. We also compute the string tension for the massive Schwinger model, as a demonstration of the method of the calculation.Comment: 8 pages, Latex. Minor changes. Accepted to Phys.Lett.

Confining k-string tensions with D-Branes in Super Yang-Mills theories

We discuss confining k strings in four dimensional gauge theories using D5 branes in AdS5xS5, and D3 branes in Klebanov-Strassler and Maldacena-Nunez backgrounds. We present two results: The first that confining k string tensions in N=4 can be calculated using D5 branes in AdS5xS5 with a cut-off in the bulk AdS. Using an embedding of R2 times S4 in S5, we show that the D5 brane replicates a string of rank k in the antisymmetric representation. The second result shows that the S-Dual calculation to hep-th/0111078 reproduces the action in the Klebanov-Strassler and Maldacena-Nunez backgrounds exactly, while providing a more natural manifestation of the string charge k.Comment: LaTeX, 12 pages; v2 references adde

Predictions for orientifold field theories from type 0' string theory

Two predictions about finite-N non-supersymmetric "orientifold field theories" are made by using the dual type 0' string theory on C^3 / Z_2 x Z_2 orbifold singularity. First, the mass ratio between the lowest pseudoscalar and scalar color-singlets is estimated to be equal to the ratio between the axial anomaly and the scale anomaly at strong coupling, M_- / M_+ ~ C_- / C_+. Second, the ratio between the domain wall tension and the value of the quark condensate is computed.Comment: latex, 14 page

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

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

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.