1,721,096 research outputs found
Anomalies, Unitarity and Quantum Irreversibility
No full textThe trace anomaly in external gravity is the sum of three terms at criticality: the square of the Weyl tensor, the Euler density and Box R, with coefficients, properly normalized, called c, a and a', the latter being ambiguously defined by an additive constant. Unitarity and positivity properties of the induced actions allow us to show that the total RG flows of a and a' are equal and therefore the a'-ambiguity can be consistently removed through the identification a'=a. The picture that emerges clarifies several long-standing issues. The interplay between unitarity and renormalization implies that the flux of the renormalization group is irreversible. A monotonically decreasing a-function interpolating between the appropriate values is naturally provided by a'. The total a-flow is expressed non-perturbatively as the invariant (i.e. scheme-independent) area of the graph of the beta function between the fixed points. We test this prediction to the fourth loop order in perturbation theory, in QCD with Nf ~< 11/2 Nc and in supersymmetric QCD. There is agreement also in the absence of an interacting fixed point (QED and phi^4-theory). Our arguments do not seem to prove that a is strictly positive, but put a lower bound to its value.The trace anomaly in external gravity is the sum of three terms at criticality: the square of the Weyl tensor, the Euler density and Box R, with coefficients, properly normalized, called c, a and a', the latter being ambiguously defined by an additive constant. Considerations about unitarity and positivity properties of the induced actions allow us to show that the total RG flows of a and a' are equal and therefore the a'-ambiguity can be consistently removed through the identification a'=a. The picture that emerges clarifies several long-standing issues. The interplay between unitarity and renormalization implies that the flux of the renormalization group is irreversible. A monotonically decreasing a-function interpolating between the appropriate values is naturally provided by a'. The total a-flow is expressed non-perturbatively as the invariant (i.e. scheme-independent) area of the graph of the beta function between the fixed points. We test this prediction to the fourth loop order in perturbation theory, in QCD with Nf ~< 11/2 Nc and in supersymmetric QCD. There is agreement also in the absence of an interacting fixed point (QED and phi^4-theory). Arguments for the positivity of a are also discussed.The trace anomaly in external gravity is the sum of three terms at criticality: the square of the Weyl tensor, the Euler density and Box R, with coefficients, properly normalized, called c, a and a', the latter being ambiguously defined by an additive constant. Considerations about unitarity and positivity properties of the induced actions allow us to show that the total RG flows of a and a' are equal and therefore the a'-ambiguity can be consistently removed through the identification a'=a. The picture that emerges clarifies several long-standing issues. The interplay between unitarity and renormalization implies that the flux of the renormalization group is irreversible. A monotonically decreasing a-function interpolating between the appropriate values is naturally provided by a'. The total a-flow is expressed non-perturbatively as the invariant (i.e. scheme-independent) area of the graph of the beta function between the fixed points. We test this prediction to the fourth loop order in perturbation theory, in QCD with Nf ~< 11/2 Nc and in supersymmetric QCD. There is agreement also in the absence of an interacting fixed point (QED and phi^4-theory). Arguments for the positivity of a are also discussed.The trace anomaly in external gravity is the sum of three terms at criticality: the square of the Weyl tensor, the Euler density and □ R , with coefficients, properly normalized, called c , a , and a ′, the latter being ambiguously defined by an additive constant. Considerations about unitarity and positivity properties of the induced actions allow us to show that the total RG flows of a and a ′ are equal and therefore the a ′-ambiguity can be consistently removed through the identification a ′= a . The picture that emerges clarifies several long-standing issues. The interplay between unitarity and renormalization implies that the flux of the renormalization group is irreversible. A monotonically decreasing a -function interpolating between the appropriate values is naturally provided by a ′. The total a -flow is expressed non-perturbatively as the invariant (i.e., scheme-independent) area of the graph of the beta function between the fixed points. We test this prediction to the fourth loop order in perturbation theory, in QCD with N f ≲11/2 N c and in supersymmetric QCD. There is agreement also in the absence of an interacting fixed point (QED and ϕ 4 -theory). Arguments for the positivity of a are also discussed
A new perspective on the philosophical implications of quantum field theory
No full textI discuss issues concerning the philosophical foundations and implications of quantum field theory, renormalization in particular. A new understanding of the correspondence principle, an unexpected role of perturbation theory and, most of all, a criterion to reduce the set of consistent theories from infinitely many to finitely many, are the key concepts of a theoretical set-up that appears to overcome in a natural way various consistency problems of quantum mechanics and offer several hints for further developments
Fakeons, unitarity, massive gravitons, and the cosmological constant
Full text linkWe give a simple proof of perturbative unitarity in gauge theories and quantum gravity using a special gauge that allows us to separate the physical poles of the free propagators, which are quantized by means of the Feynman prescription, from the poles that belong to the gauge-trivial sector, which are quantized by means of the fakeon prescription. The proof applies to renormalizable theories, including the ultraviolet complete theory of quantum gravity with fakeons formulated recently, as well as low-energy (nonrenormalizable) theories. We clarify a number of subtleties related to the study of scattering processes in the presence of a cosmological constant Λ. The scattering ampli- tudes, defined by expanding the metric around flat space, obey the optical theorem up to corrections due to Λ, which are negligible for all practical purposes. Problems of interpretation would arise if such corrections became important. In passing, we obtain local, unitary (and “almost” renormalizable) theories of massive gravitons and gauge fields, which violate gauge invariance and general covariance explicitly
REMOVAL OF DIVERGENCES WITH THE BATALIN-VILKOVISKY FORMALISM
No full textWe show how to remove the divergences in an arbitrary gauge-field theory (possibly nonrenormalizable, i.e. involving infinitely many parameters) in the context of the Batalin-Vilkovisky formalism. We show that this can be achieved by performing, order by order in the loop expansion, a redefinition of the parameters of the classical Lagrangian (possibly infinitely many) and a canonical transformation (in the sense of Batalin and Vilkovisky) of fields and BRS sources. Gauge-invariance is turned into a suitable quantum generalization of BRS invariance. We define quantum observables in this formal context and study their properties. We show the independence of the on-shell physical amplitudes from gauge fixing. We apply the result to renormalizable gauge-field theories that are gauge-fixed with a non-renormalizable gauge fixing and prove that their predictivity is retained. A corollary is that topological field theories are predictive
Adler-Bardeen theorem and cancellation of gauge anomalies to all orders in nonrenormalizable theories
No full textWe prove the Adler-Bardeen theorem in a large class of general gauge theories, including nonrenormalizable ones. We assume that the gauge symmetries are general covariance, local Lorentz symmetry and Abelian and non-Abelian Yang-Mills symmetries, and that the local functionals of vanishing ghost numbers satisfy a variant of the Kluberg-Stern–Zuber conjecture. We show that if the gauge anomalies are trivial at one loop, for every truncation of the theory there exists a subtraction scheme where they manifestly vanish to all orders, within the truncation. Outside the truncation the cancellation of gauge anomalies can be enforced by fine-tuning local counterterms. The framework of the proof is worked out by combining a recently formulated chiral dimensional regularization with a gauge invariant higher-derivative regularization. If the higher-derivative regularizing terms are placed well beyond the truncation, and the energy scale Λ associated with them is kept fixed, the theory is super-renormalizable and has the property that, once the gauge anomalies are canceled at one loop, they manifestly vanish from two loops onwards by simple power counting. When the Λ divergences are subtracted away and Λ is sent to infinity, the anomaly cancellation survives in a manifest form within the truncation and in a nonmanifest form outside. The standard model coupled to quantum gravity satisfies all the assumptions, so it is free of gauge anomalies to all orders
Consistent irrelevant deformations of interacting conformal field theories
No full textI show that under certain conditions it is possible to define consistent irrelevant deformations of interacting conformal field theories. The deformations are finite or have a unique running scale ("quasi-finite"). They are made of an infinite number of lagrangian terms and a finite number of independent parameters that renormalize coherently. The coefficients of the irrelevant terms are determined imposing that the beta functions of the dimensionless combinations of couplings vanish ("quasi-finiteness equations"). The expansion in powers of the energy is meaningful for energies much smaller than an effective Planck mass. Multiple deformations can be considered also. I study the general conditions to have non-trivial solutions. As an example, I construct the Pauli deformation of the IR fixed point of massless non-abelian Yang-Mills theory with N-c colors and N(f)less than or similar to11 N-c/2 flavors and compute the couplings of the term F-3 and the four-fermion vertices. Another interesting application is the construction of finite chiral irrelevant deformations of N=2 and N=4 superconformal field theories. The results of this paper suggest that power-counting non-renormalizable theories might play a role in the description of fundamental physics
Gauge theories and quantum gravity in a finite interval of time, on a compact space manifold
Full text linkWe study gauge theories and quantum gravity in a finite interval of time
, on a compact space manifold . The initial, final and boundary
conditions are formulated in gauge invariant and general covariant ways by
means of purely virtual extensions of the theories, which allow us to
"trivialize" the local symmetries and switch to invariant fields (the invariant
metric tensor, invariant quark and gluon fields, etc.). The evolution operator
is worked out diagrammatically for arbitrary
initial and final states, as well as boundary conditions on ,
and shown to be well defined and unitary for arbitrary . We illustrate the basic properties in
Yang-Mills theory on the cylinder.Comment: 52 pages; PR
ANOMALIES IN INSTANTON CALCULUS
No full textI develop a formalism for solving topological field theories explicitly, in the case when the explicit expression of the instantons is known. I solve topological Yang-Mills theory with the k = 1 instanton of Belavin et al. and topological gravity with the Eguchi-Hanson instanton. It turns out that naively empty theories are indeed nontrivial. Many unexpected interesting hidden quantities (punctures, contact terms, nonperturbative anomalies with or without gravity) are revealed. Topological Yang-Mills theory with G = SU(2) is not just Donaldson theory, but contains a certain link theory. Indeed, local and non-local observables have the property of marking cycles. Moreover, from topological gravity one learns that an object can be considered BRST exact only if it is so all over the moduli space M, boundary included. Being BRST exact in any interior point of M is not sufficient to make an amplitude vanish. Presumably, recursion relations and hierarchies can be found to solve topological field theories in four dimensions, in particular topological Yang-Mills theory with G = SU(2) on R(4) and topological gravity with the full set of asymptotically locally Euclidean manifolds
Higher-spin current multiplets in operator-product expansions
No full textVarious formulas for currents with arbitrary spin are worked out in general space-time dimension, in the free field limit and, at the bare level, in presence of interactions. As the n-dimensional generalization of the (conformal) vector field, the (n/2-1)-form is used. The two-point functions and the higher-spin central charges are evaluated at one loop. As an application, the higher-spin hierarchies generated by the stress-tensor operator-product expansion are computed in supersymmetric theories. The results exhibit an interesting universality.Various formulas for currents with arbitrary spin are worked out in general space-time dimension, in the free field limit and, at the bare level, in presence of interactions. As the n-dimensional generalization of the (conformal) vector field, the (n/2-1)-form is used. The two-point functions and the higher-spin central charges are evaluated at one loop. As an application, the higher-spin hierarchies generated by the stress-tensor operator-product expansion are computed in supersymmetric theories. The results exhibit an interesting universality
Renormalization of quantum gravity coupled with matter in three dimensions
No full textIn three spacetime dimensions, where no graviton propagates, pure gravity is known to be finite. It is natural to inquire whether finiteness survives the coupling with matter. Standard arguments ensure that there exists a subtraction scheme where no Lorentz-Chern-Simons term is generated by radiative corrections, but are not sufficiently powerful to ensure finiteness. Therefore, it is necessary to perform an explicit (two-loop) computation in a specific model. I consider quantum gravity coupled with Chem-Simons U(1) gauge theory and massless fermions and show that renormalization originates four-fermion divergent vertices at the second loop order. I conclude that quantum gravity coupled with matter, as it stands, is not finite in three spacetime dimensions. (C) 2004 Elsevier B.V. All rights reserved
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