177,311 research outputs found
Bounds in 4D conformal field theories with global symmetry
No full textWe explore the constraining power of OPE associativity in 4D conformal field theory with a continuous global symmetry group. We give a general analysis of crossing symmetry constraints in the 4-point function (φφφ †φ†, where φ is a primary scalar operator in a given representation R. These constraints take the form of 'vectorial sum rules' for conformal blocks of operators whose representations appear in R ⊗ R and R ⊗ R̄. The coefficients in these sum rules are related to the Fierz transformation matrices for the R ⊗ R ⊗ R̄ ⊗ R̄ invariant tensors. We show that the number of equations is always equal to the number of symmetry channels to be constrained. We also analyze in detail two cases-the fundamental of SO(N) and the fundamental of SU(N). We derive the vectorial sum rules explicitly, and use them to study the dimension of the lowest singlet scalar in the φ × φ† OPE. We prove the existence of an upper bound on the dimension of this scalar. The bound depends on the conformal dimension of φ and approaches 2 in the limit dim(φ) → 1. For several small groups, we compute the behavior of the bound at dim(φ) > 1. We discuss implications of our bound for the conformal technicolor scenario of electroweak symmetry breaking. © 2011 IOP Publishing Ltd
Graviton loops and brane observables
No full textWe discuss how to consistently perform effective Lagrangian computations in quantum gravity with branes in compact extra dimensions. A reparametrization invariant and infrared finite result is obtained in a non trivial way. It is crucial to properly account for brane fluctuations and to correctly identify physical observables. Our results correct some confusing claims in the literature. We discuss the implications of graviton loops on electroweak precision observables and on the muon g-2 in models with large extra dimensions. We model the leading effects, not controlled by effective field theory, by introducing a hard momentum cut-off.We discuss how to consistently perform effective Lagrangian computations in quantum gravity with branes in compact extra dimensions. A reparametrization invariant and infrared finite result is obtained in a non trivial way. It is crucial to properly account for brane fluctuations and to correctly identify physical observables. Our results correct some confusing claims in the literature. We discuss the implications of graviton loops on electroweak precision observables and on the muon g-2 in models with large extra dimensions. We model the leading effects, not controlled by effective field theory, by introducing a hard momentum cut-off
Galileon as a local modification of gravity
No full textIn the Dvali-Gabadadze-Porrati (DGP) model, the "self-accelerating" solution is plagued by a ghost instability, which makes the solution untenable. This fact, as well as all interesting departures from general relativity (GR), are fully captured by a four-dimensional effective Lagrangian, valid at distances smaller than the present Hubble scale. The 4D effective theory involves a relativistic scalar pi, universally coupled to matter and with peculiar derivative self-interactions. In this paper, we study the connection between self-acceleration and the presence of ghosts for a quite generic class of theories that modify gravity in the infrared. These theories are defined as those that at distances shorter than cosmological, reduce to a certain generalization of the DGP 4D effective theory. We argue that for infrared modifications of GR locally due to a universally coupled scalar, our generalization is the only one that allows for a robust implementation of the Vainshtein effect-the decoupling of the scalar from matter in gravitationally bound systems-necessary to recover agreement with solar-system tests. Our generalization involves an internal Galilean invariance, under which pi's gradient shifts by a constant. This symmetry constrains the structure of the pi Lagrangian so much so that in 4D there exist only five terms that can yield sizable nonlinearities without introducing ghosts. We show that for such theories in fact there are "self-accelerating" de Sitter solutions with no ghostlike instabilities. In the presence of compact sources, these solutions can support spherically symmetric, Vainshtein-like nonlinear perturbations that are also stable against small fluctuations. We investigate a possible infrared completion of these theories at scales of order of the Hubble horizon, and larger. There are however some features of our theories that may constitute a problem at the theoretical or phenomenological level: the presence of superluminal excitations; the extreme subluminality of other excitations, which makes the quasistatic approximation for certain solar-system observables unreliable due to Cherenkov emission; the very low strong-interaction scale for pi pi scatterings.LPT
Energy's and amplitudes' positivity
No full textIn QFT, the null energy condition (NEC) for a classical field configuration is usually associated with that configuration's stability against small perturbations, and with the sub-luminality of these. Here, we exhibit an effective field theory that allows for stable NEC-violating solutions with exactly luminal excitations only. The model is the recently introduced 'galileon', or more precisely its conformally invariant version. We show that the theory's low-energy S-matrix obeys standard positivity as implied by dispersion relations. However we also show that if the relevant NEC-violating solution is inside the effective theory, then other (generic) solutions allow for superluminal signal propagation. While the usual association between sub-luminality and positivity is not obeyed by our example, that between NEC and sub-luminality is, albeit in a less direct way than usual.LPT
Phenomenology of deflected anomaly-mediation
No full textWe explore the phenomenology of a class of models with anomaly-mediated supersymmetry breaking. These models retain the successful flavor properties of the minimal scenario while avoiding the tachyons. The mass spectrum is predicted in terms of a few parameters. However various qualitatively different spectra are possible, often strongly different from the ones usually employed to explore capabilities of new accelerators. One stable feature is the limited spread of the spectrum, so that squarks and gluinos could be conceivably produced at TEVII. The Lightest superpartner of standard particles is often a charged slepton or a neutral higgsino. It behaves as a stable particle in collider experiments but it decays at or before nucleosynthesis. We identify the experimental signatures at hadron colliders that can help distinguish this scenario from the usual ones. (C) 2000 Elsevier Science B,V. All rights reserved
Unificaxion
No full textDark matter, gauge coupling unification, and the strong CP problem find a common and simple solution (in the absence of naturalness) within axion models. We show that such solution, even without specifying the details of the model implementation, makes testable predictions for the experimentally measurable axion parameters: the axion mass and its coupling to photons.Dark matter, gauge coupling unification, and the strong CP problem find a common and simple solution (in the absence of naturalness) within axion models. We show that such a solution, even without specifying the details of the model implementation, makes testable predictions for the experimentally measurable axion parameters: the axion mass and its coupling to photons.Dark matter, gauge coupling unification, and the strong CP problem find a common and simple solution (in the absence of naturalness) within axion models. We show that such solution, even without specifying the details of the model implementation, makes testable predictions for the experimentally measurable axion parameters: the axion mass and its coupling to photons
Probing the Scattering of Equivalent Electroweak Bosons
No full textWe analyze the kinematic conditions under which the scattering of equivalent massive spin-1 vector bosons factorizes out of the complete process. In practice, we derive the conditions for the validity of the effective W approximation, proposed long ago but never established on a firm basis. We also present a parametric estimate of the corrections to the approximation and explicitly check its validity in two examples
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