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Andrea Quadri

Publications and source records attributed to Andrea Quadri.

At least 37 records · Page 2Linked to original sources
arXiv2011

Background field dependence from the Slavnov-Taylor identity in (non-perturbative) Yang-Mills theory

We show that in Yang-Mills theory the Slavnov-Taylor (ST) identity, extended in the presence of a background gauge connection, allows to fix in a unique way the dependence of the vertex functional on the background, once the 1-PI amplitudes at zero background are known. The reconstruction of the background dependence is carried out by purely algebraic techniques and therefore can be applied in a non-perturbative scheme (e.g. on the lattice or in the Schwinger-Dyson approach), provided that the latter preserves the ST identity. The field-antifield redefinition, which replaces the classical background-quantum splitting when quantum corrections are taken into account, is considered on the example of an instanton background in SU(2) Yang-Mills theory.

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arXiv2011

Non-trivial Backgrounds in (non-perturbative) Yang-Mills Theory by the Slavnov-Taylor Identity

We show that in the background field method (BFM) quantization of Yang-Mills theory the dependence of the vertex functional on the background field is controlled by a canonical transformation w.r.t. the Batalin-Vilkovisky bracket, naturally associated with the BRST symmetry of the theory. Since it only relies on the Slavnov-Taylor identity of the model, this result holds both in perturbation theory and in the non-perturbative regime. It provides a general consistent framework for the systematic implementation of the BFM in non-perturbative approaches to QCD, like e.g. those based on the Schwinger-Dyson equations or the lattice, in the presence of topologically non-trivial background configurations. The analysis is carried out in an arbitrary (background) R_ξ-gauge.

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arXiv2011

Slavnov-Taylor constraints for non-trivial backgrounds

We devise an algebraic procedure for the evaluation of Green's functions in SU(N) Yang-Mills theory in the presence of a non-trivial background field. In the ghost-free sector the dependence of the vertex functional on the background is shown to be uniquely determined by the Slavnov-Taylor identities in terms of a certain 1-PI correlator of the covariant derivatives of the ghost and the anti-ghost fields. At non-vanishing background this amplitude is shown to encode the quantum deformations to the tree-level background-quantum splitting. The approach only relies on the functional identities of the model (Slavnov-Taylor identities, $b$-equation, anti-ghost equation) and thus it is valid beyond perturbation theory, and in particular in a lattice implementation of the background field method. As an example of the formalism we analyze the ghost two-point function and the Kugo-Ojima function in an instanton background in SU(2) Yang-Mills theory, quantized in the background Landau gauge.

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arXiv2010

The SU(2) X U(1) Electroweak Model based on the Nonlinearly Realized Gauge Group. II. Functional Equations and the Weak Power-Counting

In the present paper, that is the second part devoted to the construction of an electroweak model based on a nonlinear realization of the gauge group SU(2) X U(1), we study the tree-level vertex functional with all the sources necessary for the functional formulation of the relevant symmetries (Local Functional Equation, Slavnov-Taylor identity, Landau Gauge Equation) and for the symmetric removal of the divergences. The Weak Power Counting criterion is proven in the presence of the novel sources. The local invariant solutions of the functional equations are constructed in order to represent the counterterms for the one-loop subtractions. The bleaching technique is fully extended to the fermion sector. The neutral sector of the vector mesons is analyzed in detail in order to identify the physical fields for the photon and the Z boson. The identities necessary for the decoupling of the unphysical modes are fully analyzed. These latter results are crucially bound to the Landau gauge used throughout the paper.

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arXiv2010

One-loop Self-energies in the Electroweak Model with Nonlinearly Realized Gauge Group

We evaluate at one loop the self-energies in generic D dimensions for the W,Z mesons in the Electroweak model where the gauge group is nonlinearly realized. In this model the Higgs boson parameters are absent, while a second mass parameter appears together with a scale for the radiative corrections. We estimate these parameters in a simplified fit on leptons and gauge bosons data. We check physical unitarity and the absence of infrared divergences. Landau gauge is used.

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arXiv2010

The SU(2) X U(1) Electroweak Model based on the Nonlinearly Realized Gauge Group

The electroweak model is formulated on the nonlinearly realized gauge group SU(2) X U(1). This implies that in perturbation theory no Higgs field is present. The paper provides the effective action at the tree level, the Slavnov Taylor identity (necessary for the proof of unitarity), the local functional equation (used for the control of the amplitudes involving the Goldstone bosons) and the subtraction procedure (nonstandard, since the theory is not power-counting renormalizable). Particular attention is devoted to the number of independent parameters relevant for the vector mesons; in fact there is the possibility of introducing two mass parameters. With this choice the relation between the ratio of the intermediate vector meson masses and the Weinberg angle depends on an extra free parameter. We briefly outline a method for dealing with γ_5 in dimensional regularization. The model is formulated in the Landau gauge for sake of simplicity and conciseness: the QED Ward identity has a simple and intriguing form.

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arXiv2010

The Algebra of Physical Observables in Nonlinearly Realized Gauge Theories

We classify the physical observables in spontaneously broken nonlinearly realized gauge theories in the recently proposed loopwise expansion governed by the Weak Power-Counting (WPC) and the Local Functional Equation. The latter controls the non-trivial quantum deformation of the classical nonlinearly realized gauge symmetry, to all orders in the loop expansion. The Batalin-Vilkovisky (BV) formalism is used. We show that the dependence of the vertex functional on the Goldstone fields is obtained via a canonical transformation w.r.t. the BV bracket associated with the BRST symmetry of the model. We also compare the WPC with strict power-counting renormalizability in linearly realized gauge theories. In the case of the electroweak group we find that the tree-level Weinberg relation still holds if power-counting renormalizability is weakened to the WPC condition.

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arXiv2010

The Electroweak Model based on the Nonlinearly realized Gauge Group. Theoretical Foundations and Phenomenological Prospects

A consistent strategy for the subtraction of the divergences in the nonlinearly realized Electroweak Model in the loop expansion is presented. No Higgs field enters into the perturbative spectrum. The local functional equation (LFE), encoding the invariance of the SU(2) Haar measure under local left SU(2) transformations, the Slavnov-Taylor identity, required in order to fulfill physical unitarity, and the Landau gauge equation hold in the nonlinearly realized theory. The quantization is performed in the Landau gauge for the sake of simplicity and elegance. The constraints on the admissible interactions arising from the Weak Power-Counting (WPC) are discussed. The same symmetric pattern of the couplings as in the Standard Model is shown to arise, as a consequence of the defining functional identities and the WPC. However, two independent mass invariants in the vector meson sector are possible, i.e. no tree-level Weinberg relation holds between the Z and W mass. Majorana neutrino masses can be implemented in the nonlinearly realized Electroweak Model in a way compatible with the WPC and all the symmetries of the theory.

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arXiv2008

A Massive Yang-Mills Theory based on the Nonlinearly Realized Gauge Group

We propose a subtraction scheme for a massive Yang-Mills theory realized via a nonlinear representation of the gauge group (here SU(2)). It is based on the subtraction of the poles in D-4 of the amplitudes, in dimensional regularization, after a suitable normalization has been performed. Perturbation theory is in the number of loops and the procedure is stable under iterative subtraction of the poles. The unphysical Goldstone bosons, the Faddeev-Popov ghosts and the unphysical mode of the gauge field are expected to cancel out in the unitarity equation. The spontaneous symmetry breaking parameter is not a physical variable. We use the tools already tested in the nonlinear sigma model: hierarchy in the number of Goldstone boson legs and weak power-counting property (finite number of independent divergent amplitudes at each order). It is intriguing that the model is naturally based on the symmetry SU(2)_L local times SU(2)_R global. By construction the physical amplitudes depend on the mass and on the self-coupling constant of the gauge particle and moreover on the scale parameter of the radiative corrections. The Feynman rules are in the Landau gauge.

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arXiv2007

Further Comments on the Symmetric Subtraction of the Nonlinear Sigma Model

Recently a perturbative theory has been constructed, starting from the Feynman rules of the nonlinear sigma model at the tree level in the presence of an external vector source coupled to the flat connection and of a scalar source coupled to the nonlinear sigma model constraint (flat connection formalism). The construction is based on a local functional equation, which overcomes the problems due to the presence (already at one loop) of non chiral symmetric divergences. The subtraction procedure of the divergences in the loop expansion is performed by means of minimal subtraction of properly normalized amplitudes in dimensional regularization. In this paper we complete the study of this subtraction procedure by giving the formal proof that it is symmetric to all orders in the loopwise expansion. We provide further arguments on the issue that, within our subtraction strategy, only two parameters can be consistently used as physical constants.

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arXiv2007

The Hierarchy Principle and the Large Mass Limit of the Linear Sigma Model

In perturbation theory we study the matching in four dimensions between the linear sigma model in the large mass limit and the renormalized nonlinear sigma model in the recently proposed flat connection formalism. We consider both the chiral limit and the strong coupling limit of the linear sigma model. Our formalism extends to Green functions with an arbitrary number of pion legs,at one loop level,on the basis of the hierarchy as an efficient unifying principle that governs both limits. While the chiral limit is straightforward, the matching in the strong coupling limit requires careful use of the normalization conditions of the linear theory, in order to exploit the functional equation and the complete set of local solutions of its linearized form.

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arXiv2007

Path-integral over non-linearly realized groups and Hierarchy solutions

The technical problem of deriving the full Green functions of the elementary pion fields of the nonlinear sigma model in terms of ancestor amplitudes involving only the flat connection and the nonlinear sigma model constraint is a very complex task. In this paper we solve this problem by integrating, order by order in the perturbative loop expansion, the local functional equation derived from the invariance of the SU(2) Haar measure under local left multiplication. This yields the perturbative definition of the path-integral over the non-linearly realized SU(2) group.

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arXiv2006

A Weak Power-Counting Theorem for the Renormalization of the Non-Linear Sigma Model in Four Dimensions

The formulation of the non-linear sigma model in terms of flat connection allows the construction of a perturbative solution of a local functional equation encoding the underlying gauge symmetry. In this paper we discuss some properties of the solution at the one-loop level in D=4. We prove the validity of a weak power-counting theorem in the following form: although the number of divergent amplitudes is infinite only a finite number of divergent amplitudes have to be renormalized. The proof uses the linearized functional equation of which we provide the general solution in terms of local functionals. The counteterms are given in terms of linear combinations of these invariants and the coefficients are fixed by a finite number of divergent amplitudes. The latter contain only insertions of the composite operators $ϕ_0$ (the constraint of the non-linear sigma model) and $F_μ$ (the flat connection). These amplitudes are at the top of a hierarchy implicit in the functional equation. As an example we derive the counterterms for the four-point amplitudes.

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arXiv2006

The Abelian Embedding Formulation of the Stueckelberg Model and its Power-counting Renormalizable Extension

We elucidate the geometry of the polynomial formulation of the non-abelian Stueckelberg mechanism. We show that a natural off-shell nilpotent BRST differential exists allowing to implement the constraint on the sigma field by means of BRST techniques. This is achieved by extending the ghost sector by an additional U(1) factor (abelian embedding). An important consequence is that a further BRST-invariant but not gauge-invariant mass term can be written for the non-abelian gauge fields. As all versions of the Stueckelberg theory, also the abelian embedding formulation yields a non power-counting renormalizable theory in D=4. We then derive its natural power-counting renormalizable extension and show that the physical spectrum contains a physical massive scalar particle. Physical unitarity is also established. This model implements the spontaneous symmetry breaking in the abelian embedding formalism.

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arXiv2005

Renormalization of the Non-Linear Sigma Model in Four Dimensions. A two-loop example

The renormalization procedure of the non-linear SU(2) sigma model in D=4 proposed in hep-th/0504023 and hep-th/0506220 is here tested in a truly non-trivial case where the non-linearity of the functional equation is crucial. The simplest example, where the non-linear term contributes, is given by the two-loop amplitude involving the insertion of two ϕ_0 (the constraint of the non-linear sigma model) and two flat connections. In this case we verify the validity of the renormalization procedure: the recursive subtraction of the pole parts at D=4 yields amplitudes that satisfy the defining functional equation. As a by-product we give a formal proof that in D dimensions (without counterterms) the Feynman rules provide a perturbative symmetric solution.

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arXiv2005

Slavnov-Taylor parameterization of Yang-Mills theory with massive fermions in the presence of singlet axial-vector currents

We study the all-order restoration of the Slavnov-Taylor (ST) identities for Yang-Mills theory with massive fermions in the presence of singlet axial-vector currents. By making use of the ST parameterization of the symmetric quantum effective action a natural set of normalization conditions is derived allowing to reduce the algebraic complexity of higher orders ST identities up to a homogeneous linear problem. Explicit formulas for the action-like part of the symmetric vertex functional are given to all orders in the loop expansion.

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arXiv2005

Some Conjectures on the Limit of Infinite Higgs Mass

We consider a possible field theory candidate for the electroweak SU(2) x U(1) model where the limit of infinitely sharp Higgs potential is performed. We show that it is possible to formulate such a limit as a Stueckelberg massive non abelian gauge theory.

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