Course

19S1 D. Anselmi
Theories of gravitation

Program

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Book

D. Anselmi
From Physics To Life

A journey to the infinitesimally small and back

In English and Italian

Available on Amazon:
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IT: book | ebook  (in IT)




Recent Papers




Quantum field theory

Program

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Talk given at the Department of Physics and Astronomy of Southampton University, UK, on Nov 16th, 2018



I introduce the concept of fake particle and study how it is used to formulate a consistent theory of quantum gravity. Fakeons arise from a new quantization prescription, alternative to the Feynman one, for the poles of higher-derivative theories, which avoids the problem of ghosts. The fake particles mediate interactions and simulate true particles in many situations. Nevertheless, they are not asymptotic states and cannot be detected directly. The Wick rotation and the S matrix are regionwise analytic and the amplitudes can be calculated in all regions starting from the Euclidean one by means of an unambiguous, but nonanalytic operation. By reconciling renormalizability and unitarity in higher-derivative theories, the models containing both true and fake particles are good candidates to explain quantum gravity. In pole position is the unique theory that is strictly renormalizable. One of the major physical predictions due to the fakeons is the violation of microcausality. I discuss the classical limit of the theory and the acausal corrections to the Einstein equations.

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Talk given at the conference

Progress and Visions in Quantum Theory in View of Gravity: Bridging foundations of physics and mathematics

Max Planck Institute for Mathematics in the Sciences, Leipzig

October 04, 2018



I claim that the best correspondence principle for quantum field theory and quantum gravity is made of unitarity, locality and proper renormalizability (which is a refinement of strict renormalizability), combined with fundamental local symmetries and the requirement of having a finite number of fields. Quantum gravity is identified in an essentially unique way. It emerges from a new quantization prescription, which introduces the notion of fake particle, or “fakeon”, and uses it to resolve the long-standing problem of the higher-derivative ghosts. I discuss the major physical prediction of the theory, which is the violation of causality at small distances. The correspondence principle identifies the gauge interactions uniquely in form, but does not predict the gauge group. On the other hand, the matter sector remains almost completely unrestricted.

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We discuss the fate of the correspondence principle beyond quantum mechanics, specifically in quantum field theory and quantum gravity, in connection with the intrinsic limitations of the human ability to observe the external world. We conclude that the best correspondence principle is made of unitarity, locality, proper renormalizability (a refinement of strict renormalizability), combined with fundamental local symmetries and the requirement of having a finite number of fields. Quantum gravity is identified in an essentially unique way. The gauge interactions are uniquely identified in form. Instead, the matter sector remains basically unrestricted. The major prediction is the violation of causality at small distances.

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OSF preprints | DOI: 10.31219/osf.io/d2nj7

Philpapers ANSTCP-2

PhilSci 15287 (v1: PhilSci 15048)

Preprints 2018, 2018110213 | DOI: 10.20944/preprints201811.0213.v1

hal-01900207

Course on renormalization, taught in Pisa in 2015. (More chapters will be added later.)

Last update: May 9th 2015, 230 pages

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Contents:

Preface

1. Functional integral

  • 1.1 Path integral
    • Schroedinger equation
    • Free particle
  • 1.2 Free field theory
  • 1.3 Perturbative expansion
    • Feynman rules
  • 1.4 Generating functionals, Schwinger-Dyson equations
  • 1.5 Advanced generating functionals
  • 1.6 Massive vector fields
  • 1.7 Fermions

2. Renormalization

  • 2.1 Dimensional regularization
    • 2.1.1 Limits and other operations in $D$ dimensions
    • 2.1.2 Functional integration measure
    • 2.1.3 Dimensional regularization for vectors and fermions
  • 2.2 Divergences and counterterms
  • 2.3 Renormalization to all orders
  • 2.4 Locality of counterterms
  • 2.5 Power counting
  • 2.6 Renormalizable theories
  • 2.7 Composite fields
  • 2.8 Maximum poles of diagrams
  • 2.9 Subtraction prescription
  • 2.10 Regularization prescription
  • 2.11 Comments about the dimensional regularization
  • 2.12 About the series resummation

3. Renormalization group

  • 3.1 The Callan-Symanzik equation
  • 3.2 Finiteness of the beta function and the anomalous dimensions
  • 3.3 Fixed points of the RG flow
  • 3.4 Scheme (in)dependence
  • 3.5 A deeper look into the renormalization group

4. Gauge symmetry

  • 4.1 Abelian gauge symmetry
  • 4.2 Gauge fixing
  • 4.3 Non-Abelian global symmetry
  • 4.4 Non-Abelian gauge symmetry

5. Canonical gauge formalism

  • 5.1 General idea behind the canonical gauge formalism
  • 5.2 Systematics of the canonical gauge formalism
  • 5.3 Canonical transformations
  • 5.4 Gauge fixing
  • 5.5 Generating functionals
  • 5.6 Ward identities

6. Quantum electrodynamics

  • 6.1 Ward identities
  • 6.2 Renormalizability of QED to all orders

7 Non-Abelian gauge field theories

  • 7.1 Renormalizability of non-Abelian gauge theories to all orders
    • Raw subtraction

A. Notation and useful formulas

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We define a modified dimensional-regularization technique that overcomes several difficulties of the ordinary technique, and is specially designed to work efficiently in chiral and parity violating quantum field theories, in arbitrary dimensions greater than 2. When the dimension of spacetime is continued to complex values, spinors, vectors and tensors keep the components they have in the physical dimension, therefore the $\gamma $ matrices are the standard ones. Propagators are regularized with the help of evanescent higher-derivative kinetic terms, which are of the Majorana type in the case of chiral fermions. If the new terms are organized in a clever way, weighted power counting provides an efficient control on the renormalization of the theory, and allows us to show that the resulting chiral dimensional regularization is consistent to all orders. The new technique considerably simplifies the proofs of properties that hold to all orders, and makes them suitable to be generalized to wider classes of models. Typical examples are the renormalizability of chiral gauge theories and the Adler-Bardeen theorem. The difficulty of explicit computations, on the other hand, may increase.

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Phys. Rev. D 89 (2014) 125024 | DOI: 10.1103/PhysRevD.89.125024

arXiv: 1405.3110 [hep-th]

We reconsider the Adler-Bardeen theorem for the cancellation of gauge anomalies to all orders, when they vanish at one loop. Using the Batalin-Vilkovisky formalism and combining the dimensional-regularization technique with the higher-derivative gauge invariant regularization, we prove the theorem in the most general perturbatively unitary renormalizable gauge theories coupled to matter in four dimensions, and identify the subtraction scheme where anomaly cancellation to all orders is manifest, namely no subtractions of finite local counterterms are required from two loops onwards. Our approach is based on an order-by-order analysis of renormalization, and, differently from most derivations existing in the literature, does not make use of arguments based on the properties of the renormalization group. As a consequence, the proof we give also applies to conformal field theories and finite theories.

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Eur. Phys. J. C 74 (2014) 3083 | DOI: 10.1140/epjc/s10052-014-3083-0

arXiv: 1402.6453 [hep-th]

We investigate the background field method with the Batalin-Vilkovisky formalism, to generalize known results, study parametric completeness and achieve a better understanding of several properties. In particular, we study renormalization and gauge dependence to all orders. Switching between the background field approach and the usual approach by means of canonical transformations, we prove parametric completeness without making use of cohomological theorems, namely show that if the starting classical action is sufficiently general all divergences can be subtracted by means of parameter redefinitions and canonical transformations. Our approach applies to renormalizable and non-renormalizable theories that are manifestly free of gauge anomalies and satisfy the following assumptions: the gauge algebra is irreducible and closes off shell, the gauge transformations are linear functions of the fields, and closure is field-independent. Yang-Mills theories and quantum gravity in arbitrary dimensions are included, as well as effective and higher-derivative versions of them, but several other theories, such as supergravity, are left out.

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Phys. Rev. D 89 (2014) 045004 | DOI: 10.1103/PhysRevD.89.045004

arXiv: 1311.2704 [hep-th]

We investigate the renormalization of gauge theories without assuming cohomological properties. We define a renormalization algorithm that preserves the Batalin-Vilkovisky master equation at each step and automatically extends the classical action till it contains sufficiently many independent parameters to reabsorb all divergences into parameter-redefinitions and canonical transformations. The construction is then generalized to the master functional and the field-covariant proper formalism for gauge theories. Our results hold in all manifestly anomaly-free gauge theories, power-counting renormalizable or not. The extension algorithm allows us to solve a quadratic problem, such as finding a sufficiently general solution of the master equation, even when it is not possible to reduce it to a linear (cohomological) problem.

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Eur. Phys. J. C 73 (2013) 2508 | DOI: 10.1140/epjc/s10052-013-2508-5

arXiv:1301.7577 [hep-th]

We develop a general field-covariant approach to quantum gauge theories. Extending the usual set of integrated fields and external sources to “proper” fields and sources, which include partners of the composite fields, we define the master functional $\Omega$, which collects one-particle irreducible diagrams and upgrades the usual $\Gamma$-functional in several respects. The functional $\Omega$ is determined from its classical limit applying the usual diagrammatic rules to the proper fields. Moreover, it behaves as a scalar under the most general perturbative field redefinitions, which can be expressed as linear transformations of the proper fields. We extend the Batalin-Vilkovisky formalism and the master equation. The master functional satisfies the extended master equation and behaves as a scalar under canonical transformations. The most general perturbative field redefinitions and changes of gauge-fixing can be encoded in proper canonical transformations, which are linear and do not mix integrated fields and external sources. Therefore, they can be applied as true changes of variables in the functional integral, instead of mere replacements of integrands. This property overcomes a major difficulty of the functional $\Gamma$. Finally, the new approach allows us to prove the renormalizability of gauge theories in a general field-covariant setting. We generalize known cohomological theorems to the master functional and show that when there are no gauge anomalies all divergences can be subtracted by means of parameter redefinitions and proper canonical transformations.

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Eur. Phys. J. C 73 (2013) 2363 | DOI: 10.1140/epjc/s10052-013-2363-4

arXiv:1205.3862 [hep-th]

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Book

14B1 D. Anselmi
Renormalization

Course on renormalization, taught in Pisa in 2015. (More chapters will be added later.)

Last update: May 9th 2015, 230 pages

Avaibable on Amazon:

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Contents:
Preface
1. Functional integral
2. Renormalization
3. Renormalization group
4. Gauge symmetry
5. Canonical formalism
6. Quantum electrodynamics
7. Non-Abelian gauge field theories
Notation and useful formulas
References


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