Gaugeon formalism in the framework of generalized BRST symmetry

We consider gaugeon formulations which discuss the quantum gauge freedom covariantly in the framework of generalized BRST transformation for the Yang-Mills theory as well as the BRST invariant Higgs model. We generalize the BRST symmetries of both the Yang-Mills theory and the Higgs model by making the transformation parameter finite and field-dependent. Remarkably, we observe that the gaugeon Lagrangian which describe the quantum gauge freedom appear automatically in the effective theories along with the natural shift in gauge parameters under specific finite field-dependent parameters.


INTRODUCTION
In usual canonical quantization of the gauge theories there exits no gauge freedom at the quantum level as quantum theory is defined only after gauge fixing. The gaugeon formulation provides a wider framework in which one can consider quantum gauge transformation among a family of linear covariant gauges [1][2][3][4][5][6]. In this formulation the so-called gaugeon fields are introduced as the quantum gauge freedom and such a formulation was originally proposed to restore the problem of gauge parameter renormalization. The shift of gauge parameter which arises through renormalization [7] is naturally derived in this formulation by connecting theories in two different gauges within the same family by a q-number gauge transformation [1]. The main drawback of this formulation is the unphysical gaugeon fields which do not contribute in the physical process. Thus, it is necessary to remove the gaugeon modes by imposing subsidiary conditions. Initially it was performed by putting Gupta-Bleuler type of restriction which has its own limitations. Gaugeon formulation is improved in certain cases by further extending the configuration space to incorporate the BRST quartet for gaugeon fields [8,9] where Gupta-Bleuler type subsidiary condition is replaced by Kugo-Ojima type restrictions [10,11]. The geugeon formalism with and without BRST symmetry have been studied in many different contexts in quantum field theory [8,9,[12][13][14][15][16][17][18] as well as in the perturbative gravity [19].
In the present work we would like to consider gaugeon formulation in the light of generalized BRST transformation [20]. The generalized BRST symmetry by making the infinitesimal parameter finite and field-dependent is known as finite field-dependent BRST (FFBRST) transformation and has been found many implications in gauge theories [20][21][22][23][24][25][26][27][28][29][30][31][32]. This provides us sufficient motivations to analyse the gaugeon formulation through generalized BRST symmetry. For this purpose, we consider two different models, (i) the Yang-Mills theory [33], the backbone of all frontier high energy physics and (ii) the Higgs model [34,35] which provides a general framework to explain the observed masses of the gauge bosons by means of charged and neutral Goldstone bosons that end up as the longitudinal components of the gauge bosons, to describe the quantum gauge symmetry in the framework of gaugeon formalism. gaugeon fields do not contribute in physical processes and therefore lead to unphysical gaugeon modes. To remove the unphysical modes, we put a Gupta-Bleuler type condition on gaugeon fields which finds certain limitations. This situation is further improved in the Higgs model by extending the action by introducing the Faddeev-Popov ghosts associated with gaugeon fields. Such an action remains invariant under both the extended BRST symmetry and the extended quantum gauge symmetry. Now, we generalized the full BRST symmetries of the theories by allowing the infinitesimal parameter to be finite and field-dependent with continuous interpolation of arbitrary parameter κ. Further, we calculate the Jacobian of path integral measure in each case for a specific finite field-dependent parameter and show that the Jacobian produces the exact gaugeon part to the effective action with renormalized gauge parameters. Therefore, we claim that FFBRST transformation with an appropriate transformation parameter produces the gaugeon effective action having accurate shift in gauge parameter to describe the quantum gauge freedom. Even though we establish these results with the help of two different but typical models, namely, Yang-Mills theory and Higgs model, these results hold good for any arbitrary models in gaugeon formulation.
The paper is organized as follows. In section II, we discuss the preliminaries about FFBRST transformation. Section III is devoted to study the gaugeon formalism of both the Yang-Mills as well as of Higgs model. Within this section, we also investigate the FFBRST transformation and emergence of gaugeon mode through Jacobian of path integral measure. The conclusions are drawn in the last section.

II. THE GENERALIZED BRST TRANSFORMATION: PRELIMINARIES
In this section, we recapitulate the generalized BRST transformation with finite field-dependent parameter which is also known as FFBRST transformation [20]. For this purpose, let us begin with the usual BRST transformation defined by where s b φ is Slavnov variation of generic field φ.
The properties of the usual BRST transformation do not depend on whether the transformation parameter η whether it is finite or infinitesimal and field-dependent or not, however, it must be anticommuting and space-time independent. These inspections give the freedom to generalize the BRST transformation by making the parameter, η finite and field-dependent without affecting its properties. To generalize the BRST transformation we start by making the infinitesimal parameter field-dependent with introduction of an arbitrary parameter κ (0 ≤ κ ≤ 1). We allow the fields, φ(x, κ), to depend on κ in such a way that φ(x, κ = 0) = φ(x) and φ(x, κ = 1) = φ ′ (x), the transformed field. Furthermore, the usual infinitesimal BRST transformation is defined generically as [20] dφ( where the Θ ′ [φ(x, κ)]dκ is the infinitesimal but field-dependent parameter. The FFBRST transformation with the finite field-dependent parameter then can be constructed by integrating such infinitesimal transformation from κ = 0 to κ = 1, to obtain where is the finite field-dependent parameter [20]. This FFBRST transformation is the symmetry of the effective action. However, being transformation parameter field-dependent the path integral measure is no more invariant under such transformation. The Jacobian of path integral measure changes non-trivially under FFBRST transformation. To estimate the Jacobian J(κ) of the path integral measure (Dφ) under FFBRST transformations for a particular choices of the finite field-dependent parameter, Θ[φ(x)], we first calculate the infinitesimal change in Jacobian using Taylor expansion as follows [20] 1 J where ± sign refers to whether φ is a bosonic or a fermionic field.
Further, the Jacobian J(κ) can be replaced (within the functional integral) by without changing the theory defined by the action S ef f numerically if and only if the following essential condition is satisfied [20] Dφ where S 1 [φ] refers to some local functional of fields.
Consequently, the functional S 1 within functional integral accumulate to effective action S ef f and therefore effective action modifies to S ef f + S 1 which becomes an extended effective action. Hence, FFBRST transformation with an appropriate parameter Θ extends the effective action of the theory. We utilise this fact to show that the gaugeon modes in the effective theory which describes the quantum gauge freedom are generated through FFBRST transformation. To produce the extra piece S 1 in the effective action having some extra fields through Jacobian calculation, we first insert a well-defined path integral measure corresponding to that extra fields in the functional integral by hand before performing FFBRST transformation and thereafter Jacobian factor compensates the divergence factor. However, if the extra piece S 1 contains only original fields then we don't need any extra path integral measure before the performing FFBRST transformation.

III. GAUGEON FORMALISM AND ITS EMERGENCE THROUGH GENERALIZED BRST SYMMETRY
In this section, we review the Yokoyama gaugeon formalism to discuss the quantum gauge freedom for the Yang-Mills theory as well as for the Higgs model.

A. BRST symmetric Yokoyama-Yang-Mills theory
To analyse the gaugeon formalism for Yang-Mills theory, let us start with the effective Lagrangian density for four dimensional Yang-Mills theory in Landau gauge where A a µ , B a , c a and c a ⋆ are gauge field, multiplier field, ghost field and anti-ghost field respectively. Here field-strength tensor (F a µν ) and covariant derivative (D ab µ ) are defined by with coupling constant g. The Lagrangian density (8) is invariant under following nilpotent BRST transformations: where η is an infinitesimal, anticommuting and global parameter. Now, by introducing the gaugeon field Y and its associated field Y ⋆ subject to the Bose-Einstein statistics, the Yokoyama Lagrangian density for the Yang-Mills theory is demonstrated as where α a is the group vector valued gauge-fixing parameter and ε(±) is the sign factor. Here ∇ µ refers the form covariant derivative defined as The Lagrangian density (11) remains invariant under following BRST transformation: Now, we define the following quantum gauge transformation under which the Lagrangian density (11) remains form invariant [2]: where τ is an infinitesimal transformation parameter having bosonic nature. The form invariance of Lagrangian density (11) under quantum gauge transformation (14) leads to the following shift in parameter: Further, to remove the unphysical modes of the theory and to define physical states we impose two subsidiary conditions [2] Q b |phys = 0, where Q b is the BRST charge calculated as The Kugo-Ojima type subsidiary condition [first of Eq. (16)] is subjected to remove the unphysical gauge field modes from the total Fock space. However, the second Gupta-Bleuler type condition guarantees that no gaugeon appears in the physical states. The second subsidiary condition is well-defined when the combination (Y ⋆ + α a B a ) satisfies the following free equation [2] ∂ µ ∂ µ (Y ⋆ + α a B a ) = 0.
The above free equation assures the decomposition of (Y ⋆ + α a B a ) in positive and negative frequency parts. The subsidiary conditions (16 guarantee the metric of our physical state-vector space to be positive semi-definite and consequently, we have a desirable physical subspace on which our unitary physical S-matrix exists. Now, we analyse the emergence of gaueon mode in the effective Yang-Mills theory by calculating the Jacobian of path integral measure under FFBRST transformation. First of all, we construct the FFBRST transformation by making the infinitesimal parameter η of Eq. (13) finite and field-dependent (in the same fashion as discussed in earlier section) as follows: where Θ[φ] is an arbitrary finite field-dependent parameter with ghost number −1. Now, we choose the following infinitesimal field-dependent parameter to construct the specific Θ[φ] using relation (4), whereα denotes the shifted gauge parameter as defined in (15). Now, exploiting the relation (5), the infinitesimal change in Jacobian for above Θ Now, the Jacobian J can be written properly in terms of local fields as e iS1 for the following assumption where ξ i (i = 1, 2, .., 6) are arbitrary κ-dependent constants satisfying following initial boundary conditions At the physical ground, the theory remains unaltered when the above S 1 and change in Jacobian given in (22) satisfy the crucial condition (7). To check this consistency, we first calculate the infinitesimal difference in S 1 with respect to parameter κ with the help of (2) as follows Now, the consistency condition (7) together with Eqs. (22) and (25) leads where the non-local (Θ ′ dependent) term vanishes leading to following constraint: However, the disappearance of local terms from the LHS of expression (26) leads to following exactly solvable linear differential equations The solutions of above equations satisfying the initial boundary conditions (24) are With these solutions, the expression (23) at κ = 1 receives the following form: Now, by adding S 1 [φ(x, 1), 1] to the effective action corresponding to (8), we get which is nothing but the gaugeon action for Yang-Mills theory with shifted gauge parameterα a = α a (1 + τ ). Hence, we end up the subsection with following remark that under specific generalized BRST transformation the gaugeon modes in the effective Yang-Mills action appears manifestly.

B. BRST symmetric Higgs model
To describe the gaugeon formulation of Higgs model in the framework of FFBRST transformation, we begin with the classical Lagrangian density of the Higgs model defined by where ϕ is the complex scalar field, µ 2 and λ are positive constants . The field-strength tensor and covariant derivative are defined, respectively, by Here the complex scalar field ϕ has the following vacuum expectation value: It is well known that the Lagrangian density (32) is gauge invariant. Therefore, to quantize it correctly we need to break the local gauge invariance by fixing a suitable gauge. There are many choices for gaugefixing condition. For example, the gauge-fixed Lagrangian density corresponding to R ξ gauge condition introduced by Fujikawa, Lee and Sanda [36] is given by where B is multiplier field and ξ is a numerical gauge-fixing parameter. here M = ev is the mass of A µ acquired through the spontaneous symmetry breaking and hermitian field χ is the Goldstone mode defined along with physical Higgs mode ψ as Now, the Faddeev-Popov ghost term corresponding to the above gauge-fixing term is constructed as where c ad c ⋆ are the ghost and anti-ghost fields respectively.
To analyse the gaugeon formalism for the R ξ gauge avoiding non-polynomial terms into the Lagrangian density, we use the following parametrization [15] ϕ instead of (36). Here fields ρ and π get resemblance with fields ψ and χ of Eq. (36). In terms of the parametrization, the Lagrangian density given in Eq. (32) is expressed as where √ λv is the mass of Higgs boson ρ. The above Lagrangian density is invariant under following classical gauge transformations: where Λ(x) is an arbitrary local parameter of transformation. Now, we recast the gauge-fixing and ghost terms given in (35) and (37) in accordance with parametrization (38) as follows: where α 1 and β 1 are gauge parameters. Now, the effective Lagrangian density for Higgs model, possesses the following nilpotent BRST transformation: where η is an anticommuting global parameter.
Further, to analyse the quantum gauge freedom of Higgs model we extend the effective Lagrangian density (42) to a most general gaugeon Lagrangian density by introducing the gaugeon field Y and its associated field Y ⋆ as well as the corresponding ghost fields K and K ⋆ as where α i (i = 2, 3) and β i (i = 2, 3, 4) are constant gauge parameters. Now, the gaugeon fields and respective ghost fields vary under the BRST transformation as follows: and form the BRST quartet. The gaugeon Lagrangian density (44) is invariant under the effect of combined BRST transformations (43) and (45). Consequently, the corresponding BRST charge Q b annihilates the physical subspace of V phys of total Hilbert space, i.e.
This single subsidiary condition of Kugo-Ojima type removes both the unphysical gauge modes as well as unphysical gaugeon modes.
The gaugeon Lagrangian density (44) also admits the following quantum gauge transformations: Under the above quantum gauge transformation L Y H remains form invariant leading to the following shift in gauge parameters: We observe that the quantum gauge transformations (47) commute with BRST transformations mentioned in (45). Consequently, it is confirmed that the Hilbert space spanned from physical states annihilated by BRST charge is also invariant under the quantum gauge transformations. Now, we analyse the emergence of gaueon mode in effective action for Higgs model by calculating the Jacobian of path integral measure under FFBRST transformation. To achieve this goal, we construct the FFBRST transformation by making the infinitesimal parameter η of (43) and (45) finite and fielddependent such that where Θ[φ] is finite field-dependent parameter is constructed from following infinitesimal field-dependent parameter Hereα 2 ,α 3 ,β 1 ,β 2 ,β 3 andβ 4 are the shifted gauge parameters having same definition as in (48). However, parameterα is defined in terms of τ explicitly aŝ We again calculate the infinitesimal change in Jacobian for path integral measure under FFBRST transformation in the same way as calculated in the last subsection Dropping the total derivative terms the above expression reduces to Further, by considering all the terms appearing in the above expression, we postulate the functional S 1 to have following form: where all κ dependent constant parameters (ξ i , i = 1, 2, ..., 10) are prescribed to satisfy the following initial boundary conditions Now, the infinitesimal change in S 1 is evaluated as where we have utilized the relation (2). The essential condition (7) which validates the functional S 1 together with Eqs. (53) and (56) yields Comparing the coefficients of the various terms present in the above expression from LHS to RHS, we get following differential equations together with The solutions of above equations satisfying the initial conditions (55) are ξ 1 = −κ, ξ 2 = −iκ, ξ 3 = κ, ξ 4 = iκ, ξ 5 = κ, ξ 6 = iκ, ξ 7 = κ, ξ 8 = iκ, ξ 9 = 1 2 κ, With these identifications of constant parameters ξ i , the exact form of S 1 is given by which vanishes at κ = 0. However, functional S 1 at κ = 1 (under FFBRST transformation) accumulate to the effective action (42) within functional integral as which is nothing but the BRST invariant effective action for gaugeon Higgs model with shifted gauge parameters. Therefore, we conclude that under generalized BRST transformations with appropriate finite field-dependent parameter the gaugeon modes to describe quantum gauge freedom in Higgs model appear naturally in the effective action.

IV. CONCLUSIONS
We have first evoked the gaugeon formalism for both the Yang-Mills theory [12] and Higgs model [15]. Following the Refs. [12,15], we have extended the configuration space by introducing the gaugeon field and its associated field in the effective actions of these models. Further, the quantum gauge transformation has been derived for such extended actions. Under quantum gauge transformation the extended action remains form invariant along with a shift in gauge parameters. These natural shift in gauge parameters get resemblance with those which appear through proper renormalization [1]. Since these gaugeon fields are unphysical and therefore, one needs to remove them. For this purpose, we have inserted a subsidiary condition of Gupta-Bleuler type for Yang-Mills theory which removes the unphysical gaugeon modes. But the Gupta-Bleuler type restriction has certain limitations. This situation is improved in the Higgs model where we have enlarged the configuration space by incorporating the ghost fields corresponding to gaugeon fields in the effective action. Now, such an enlarged action posses both the BRST symmetry as well as quantum gauge symmetry. In this enlarged Higgs action the unphysical gaugeon modes are removed by more acceptable Kogo-Ojima type condition.
In this work we have considered the Yang-Mills theory and Higgs model to investigate the quantum gauge freedom through Yokoyama gaugeon formalism in the framework of generalized BRST (FFBRST) transformation. We have generalized the BRST symmetry by making the infinitesimal transformation parameter finite and field-dependent. Such a generalized BRST transformation is symmetry of the action only but not of the generating functional of the Green's functions. We have shown that for a particular finite field-dependent parameter the Jacobian of path integral measure under generalized BRST transformation generates the gaugeon mode in the effective action in more rigorous way. We have established the results in both the Yang-Mills and Higgs theories with explicit calculations. Further implications and aspects of present investigations in certain string theory, M-theory and gravity theory will be more interesting.