Orbifold resolutions and fermion localization

We study the Dirac equation of chiral fermions on a regularized version of the two-dimensional T^2/Z_2 orbifold, where the conical singularities are replaced by suitable spherical caps with constant curvature. This study shows how localized and bulk fermions arise in the orbifold as the resolved space approaches the orbifold limit. Our analysis also shows that not all possible fermion configurations on T^2/Z_2 admit such a simple resolution. We focus our study to a fermion coupled to a U(1) gauge field. It is explicitly shown how a resolution of the orbifold puts severe constraints on the allowed chiralities and U(1) charges of the massless four dimensional fermions, localized or not, that can be present in the orbifold. The limit in which T^2/Z_2 (and its corresponding resolved space) collapses to S^1/Z_2 is also studied in detail.Comment: 37 pages, 5 figures; v2: minor improvements and references adde

The Electroweak Phase Transition in Models with Gauge-Higgs Unification

The dynamics of five dimensional Wilson line phases at finite temperature is studied in the one-loop approximation. We show that at temperatures of order $T\sim 1/L$, where L is the length of the compact space, the gauge symmetry is always restored and the electroweak phase transition appears to be of first order. We focus on a specific model where the Wilson line phase is identified with the Higgs field (gauge-Higgs unification). The transition is of first order even for values of the Higgs mass above the current experimental limit. If large localized gauge kinetic terms are present, the transition might be strong enough to give baryogenesis at the electroweak transition.Comment: 4 pages, 2 figures, contribution to the Proceedings of the Int. Europhysics Conf. on High Energy Physics (HEP2005), July 21-27 2005, Lisboa, Portuga

Moduli Stabilization in Meta-Stable Heterotic Supergravity Vacua

We revisit the issue of moduli stabilization in a class of N=1 four dimensional supergravity theories which are low energy descriptions of standard perturbative heterotic string vacua compactified on Calabi-Yau spaces. In particular, we show how it is possible to stabilize the universal dilaton and Kahler moduli in a de Sitter/Minkowski vacuum with low energy supersymmetry breaking by means of non-perturbative gauge dynamics, including recent results by Intriligator, Seiberg and Shih. The non-SUSY vacua are meta-stable but sufficiently long-lived.Comment: 1+31 pages, LaTeX, uses JHEP3 class, v2: references corrected and added, version published in JHE

On string models with Scherk-Schwarz supersymmetry breaking

We construct a general class of chiral four-dimensional string models with Scherk--Schwarz supersymmetry breaking, involving freely acting orbifolds. The basic ingredient is to combine an ordinary supersymmetry-preserving Z_N projection with a supersymmetry-breaking projection Z_M' acting freely on a subspace of the internal manifold. A crucial condition is that any generator of the full orbifold group Z_N x Z_M' must either preserve some supersymmetry or act freely in order to become irrelevant in some large volume limit. Tachyons are found to be absent or limited to a given region of the tree-level moduli space. We find several new models with orthogonal supersymmetries preserved at distinct fixed-points. Particular attention is devoted to an interesting Z_3 x Z_3' heterotic example.Comment: v2: Minor corrections and a reference added; v3: Title and abstract slightly changed, figure 2 corrected and a reference adde

Gauge and gravitational anomalies in D=4 N=1 orientifolds

The cancellation of U(1)-gauge and U(1)-gravitational anomalies in certain D=4 N=1 Type IIB orientifolds is analyzed in detail, from a string theory point of view. We verify the proposal that these anomalies are cancelled by a Green-Schwarz mechanism involving only twisted Ramond-Ramond fields. By factorizing one-loop partition functions, we also get the anomalous couplings of D-branes, O-planes and orbifold fixed-points to these twisted fields. Twisted sectors with fixed-planes participate to the inflow mechanism in a peculiar way.Comment: 21 pages, JHEP LaTex, no figures; minor corrections and reference adde

Anomaly inflow and RR anomalous couplings

We review the anomaly inflow mechanism on D-branes and O-planes. In particular, we compute the one-loop world-volume anomalies and derive the RR anomalous couplings required for their cancellation.Comment: 9 pages, LaTex, Talk given by C.A. Scrucca at TMR meeting "Quantum aspects of gauge theories, supersymmetry and unification", ENS, Paris, 1-7 September 199

D=11 SUGRA as the Low Energy Effective Action of Matrix Theory: Three Form Scattering

We employ the LSZ reduction formula for Matrix Theory introduced in our earlier work to compute the t-pole S-matrix for three form-three form scattering. The result agrees completely with tree level D=11 SUGRA. Taken together with previous results on graviton-graviton scattering this shows that Matrix Theory indeed reproduces the bosonic sector of the D=11 SUGRA action including the Chern-Simons term. Furthermore we provide a detailed account of our framework along with the technology to compute any Matrix Theory one-loop t-pole scattering amplitude at vanishing p^- exchange.Comment: 24 pages, LaTex, no figures, Ref. added, typos correcte

On the Cut-off Estimate in Lifshitz Five Dimensional Field Theories

We analyze if and to what extent the high energy behaviour of five-dimensional (5D) gauge theories can be improved by adding certain higher dimensional operators of "Lifshitz" type, without breaking the ordinary four-dimensional Lorentz symmetries. We show that the UV behaviour of the transverse gauge field polarizations can be improved by the Lifshitz operators, while the longitudinal polarizations get strongly coupled at energies lower than the ones in ordinary 5D theories, spoiling the usefulness of the construction in non-abelian gauge theories. We conclude that the improved behaviour as effective theories of the ordinary 5D models is not only related to locality and 5D gauge symmetries, but is a special property of the standard theories defined by the lowest dimensional operators.Comment: 18 pages, one appendix; v2: minor improvements, to appear in Phys. Rev. D; v3: one typo fixed, incorrect argument at the end of section 5 removed, acknowledgments added, conclusions unchanged, supersedes published version v

Cancellation of IR Divergences in 3d Abelian Gauge Theories

Three dimensional abelian gauge theories classically in a Coulomb phase are affected by IR divergences even when the matter fields are all massive. Using generalizations of Ward-Takahashi identities, we show that correlation functions of gauge-invariant operators are IR finite to all orders in perturbation theory. Gauge invariance is sufficient but not necessary for IR finiteness. In particular we show that specific gauge-variant correlators, including the two-point function of matter fields, are also IR finite to all orders in perturbation theory. Possible applications of these results are briefly discussed

A Simple UV-Completion of QED in 5D

We construct a Lifshitz-like version of five-dimensional (5D) QED which is UV - completed and reduces at low energies to ordinary 5D QED. The UV quantum behaviour of this theory is very smooth. In particular, the gauge coupling constant is finite at all energy scales and at all orders in perturbation theory. We study the IR properties of this theory, when compactified on a circle, and compare the one-loop energy dependence of the coupling in the Lifshitz theory with that coming from the standard 5D QED effective field theory. The range of validity of the 5D effective field theory is found to agree with the more conservative version of Naive Dimensional Analysis.Comment: 24 pages, 7 figures; v2: Minor improvements, matches journal versio