# Renormalization of multicritical scalar models in curved space

## Abstract

We consider the leading order perturbative renormalization of the multicritical \(\phi ^{2n}\) models and some generalizations in curved space. We pay particular attention to the nonminimal interaction with the scalar curvature \(\frac{1}{2}\xi \phi ^2 R\) and discuss the emergence of the conformal value of the coupling \(\xi \) as the renormalization group fixed point of its beta function at and below the upper critical dimension as a function of *n*. We also examine our results in relation with Kawai and Ninomiya’s formulation of two dimensional gravity.

## 1 Introduction

The multicritical scalar models with \(\phi ^{2n}\) interaction are the simplest and most straightforward generalization of the \(\phi ^4\) model. Much like the \(\phi ^4\) field theory captures the critical properties of an universality class of models that includes the ferromagnetic Ising Hamiltonian, the \(\phi ^{2n}\) field theory can be thought as describing a generalization in which the Ising’s spin domains of plus or minus sign are potentially replaced by *n* distinct vacuum states which become degenerate at the critical temperature.

*all*the models \(\phi ^{2n}\) are nontrivial; the continuation to two dimensions is particularly relevant because they are known to interpolate with the unitary minimal models arising as representations of the infinite dimensional Virasoro algebra [11, 14].

For the most part the renormalization of the multicritical models generalizes the one of the \(\phi ^4\) model, but the leading contributions to the critical exponents are determined by multiloop computations in which the number of loops increases with *n* [2]. Likewise the \(\phi ^4\) model, the \(\phi ^{2n}\) interactions describe critical theories that are Gaussian for \(d>d_n\) and logarithmic at \(d=d_n\), but have non-trivial critical exponents for \(d<d_n\). A common practice is to compute such critical exponents in the \(\epsilon \)-expansion, in which one introduces the constant \(\epsilon =d_n-d\) and uses it to parametrize the displacement of the critical point from the Gaussian theory at \(d=d_n\) [9].

The only multicritical model that has nontrivial exponents in \(d=3\) is the \(\phi ^4\) one unless one includes the multicritical non-unitary models \(\phi ^{2n+1}\) [15], as we shall briefly do later. Specifically, \(\phi ^{3}\) and \(\phi ^{5}\) have upper critical dimensions \(d=6\) [17] and \(d=\frac{10}{3}\) respectively, but they require the tuning of an imaginary-valued magnetic field at criticality [18, 19, 20]. It is important to mention that the \(d=2\) realizations of these models are all “far away” in a perturbative sense from their Gaussian points even though \(d_{n}\rightarrow 2\) for \(n\rightarrow \infty \) [2]. Nevertheless, the simple existence of the sequence of multicritical theories provides a very interesting and valuable link between purely field-theoretical realizations and CFT representations [3, 21].

One natural and potentially interesting generalization of the above discussion is the study of the renormalization of the \(\phi ^{2n}\) models in *curved space*. Generically, the renormalization of a model in a curved background requires additional care to preserve covariance and further conditions to avoid new and unwanted infinities [22]. The extra work is often a gateway to extra information on the theory under consideration [23]. If the multicritical models are coupled with a background geometry, simple dimensional analysis reveals that there is a new non-minimal marginal interaction with the curvature: \(\frac{1}{2}\xi \phi ^2 R\). One expects that in curved space the perturbative construction should thus accommodate for some mixing between the \(\phi ^{2n}\) and \(\phi ^2 R\) operators regardless of *n*. In other words, the nonminimal interaction \(\phi ^2 R\) holds a special status in that it is always canonically marginal.

*conformal value*

More generally, one would be tempted to extend the above statement to any dimension

Educated guess:The critical point of the coupling \(\xi \) at the upper critical dimension, which emerges as fixed point of the renormalization group, is the conformal value (3).^{1}

*below*the upper critical dimension, having expressed the desire of analytically continuing these models to \(d=2\). In this case, we would want to know the conditions under which the conformal value (3) is always the critical value for \(\xi \) even when the \(\phi ^{2n}\) interaction is non-Gaussian below \(d_c\). For this purpose it is instructive to recall the investigation by Brown and Collins [22], in which it is shown that at the leading order our educated guess is true in the special case of the \(\phi ^4\) model, but beyond the leading order one has to exploit the freedom of subtracting additional finite parts proportional to the leading counter terms [23]. An analog renormalization condition has also been adopted for the \(\phi ^3\) model [27] in \(d=6\), and it plays an important role in preserving conformal invariance in [28, 29]. Notice that, strictly speaking, the said two examples have not been concerned with the analytic continuation below the upper critical dimensions \(d=4\) and \(d=6\), while our interest is to bring the multicritical models down to \(d=2\) which does require continuation. However, assuming that we have the same freedom in changing the renormalization condition, we can state a conjecture.

In this paper we consider the leading renormalization and \(\epsilon \)-expansion of all the infinitely many multicritical models \(\phi ^{2n}\) (and some other generalizations as well) in curved space using the formalism of functional perturbation theory [2, 3]. With the leading results we can show that the conformal value (3) of \(\xi \) is indeed the critical value at the leading order in \(\epsilon \), thus proving the educated guess of this introduction. While we leave the above general conjecture open, we stress that the educated guess is proven for an infinite number of theories. Interestingly, our computation is genuinely new in that the structure of the counterterms and their renormalization does not come from a straightforward generalization of the \(\phi ^4\) case. On the contrary, we see the case \(n=2\) as quite the exception which we have to deal with separately.

General conjecture:The critical point of the coupling \(\xi \), which perturbatively is determined as an \(\epsilon \)-expansion in \(\epsilon =d_n-d\), can always be the conformal value (3) thanks to an opportune renormalization condition.

The paper is organized as follows: In Sect. 2 we study divergences, counterterms, and renormalization group beta functions for all the \(\phi ^{2n}\) models. We discuss separately the cases \(n=2\), \(n=\infty \) and the nonunitary models \(\phi ^{2n+1}\). We elaborate briefly on the utility of our results in reproducing some well known formula of 2*d* gravity in the limit of large central charge. In Sect. 3 we show how the conformal value of \(\xi \) emerges as fixed point of its beta function. We also elaborate more on the stronger conjecture expressed in this introduction. Finally in Sect. 4 we draw some conclusion and give a prospect for future investigations. The appendices are dedicated to technical details on the covariant renormalization in curved space. In particular, Appendix A discusses the Seeley–de Witt representation of the covariant Green function, and Appendix B briefly describes an algorithm by Jack and Osborn for the computation of the poles of dimensionally regulated covariant Feynman diagrams in curved space.

## 2 Renormalization

*d*dimensions. The straightforward bare action is

*n*below the upper critical dimensions (1). If we parametrize \(V(\phi )=\frac{\lambda }{(2n)!}\phi ^{2n} + \dots \), the upper critical dimension is the one for which the canonical dimension of \(\lambda \) is zero, and the perturbative expansion is controlled by powers of \(\lambda \) itself. Below the upper critical dimensions, these perturbative expansions are known to lead to a sequence of universality classes often referred to as

*minimal models*because they interpolate with the minimal conformal theories arising as representations of the Virasoro algebra in \(d=2\) [11].

A simple dimensional analysis reveals that if we take \(F(\phi )=\frac{\xi }{2}\phi ^2 +\dots \), the coupling \(\xi \) is *always* dimensionless and thus it is expected to play a role in the perturbative expansion when promoting the minimal models to curved space. In other words, the \(\phi ^{2n}\) and \(\phi ^2 R\) operators are expected to mix because of statistical or quantum mechanical fluctuations. For the above reasons we are interested in renormalizing the path integral in \(d=d_n\) dimensions with \(V(\phi )\) and \(F(\phi )\) restricted to be polynomials of order 2*n* and 2 respectively, so to include all the relevant and naively marginal operators of the models. We do it by adopting dimensional regularization which corresponds to analytically continuing the dimensionality to \(d=d_n-\epsilon \).

In the following subsection we consider first the renormalization of the general \(\phi ^{2n}\) universality class for \(n\ge 3\), while the case \(n=2\) is deferred for later. The reason for this is that the case \(n=2\) is special when it comes to the renormalization of the function \(F(\phi )\). In particular, the results for the general \(\phi ^{2n}\) case often cannot be continued to \(n=2\) because the subleading correction to the propagator is powerlaw for each \(d=d_n\) with \(n\ge 3\), but it is logarithmic in \(d=d_{n=2}=4\). If the analytic continuation is performed anyway, there is thus an additional “unbalanced” singularity which is seen as an additional \(1/(n-2)\) pole in the beta functions.

### 2.1 \(\phi ^{2n}\) Universality class

The leading quadratic contribution to the path integral (8) is not a one loop contribution for all \(n\ge 3\) models, but rather it involves \((r-1)\)-loops, which is a marked distinction from the more familiar analyses of \(\phi ^4\) and Yang–Mills theory below the upper critical dimension \(d=4\). To highlight this fact let us consider the first element of this family, which is \(\phi ^6\) for \(n=3\) and which has been already renormalized in curved space in [12]: the leading contributions to the renormalization of the couplings come from two loop diagrams and in general contributions come from every other loop order [13].

*n*. Using the methods described in Appendix B and dimensional analysis, it is possible to infer that \(1/\epsilon \) poles arise for the cases \(r=n\) and \(r=2n-1\), corresponding to \((n-1)\)- and \((2n-2)\)-loop diagram respectively likewise flat space [2, 3]. In the case \(r=n\), the contribution arises solely from

*r*lines of the leading \(G_0(x,x')\) term of the Green function. In the second case the diagram can be either composed by \(2n-1\) lines of \(G_0(x,x')\), or by \(2n-2\) lines of \(G_0(x,x')\) and one of \(G_1(x,x')\). In practice, this makes for three multiloop diagrams that must be evaluated by the methods described in Appendix B. The diagrams are depicted in Fig. 1. We have that in \(d=d_n-\epsilon \) the three diagrams evaluate to

*Z*(0) allows for the determination of the anomalous dimension of the renormalized field. At the upper critical dimension we find

### 2.2 \(\phi ^4\) Universality class

The four dimensional case is special for three main reasons. Firstly, diagrams and counterterms leading to the renormalization are not directly obtained as the analytic continuations to \(n=2\) of the results of Sect. 2.1. Secondly, the subleading correction to the Green function in four dimensions is logarithmic. This means that in the \(\epsilon \)-expansion an additional divergence must be subtracted from the propagator as we show in (54). The difference in the behavior of the subleading part of the propagator is the reason why a \(\frac{1}{n-2}\) pole appears in the third diagram of (11). Thirdly, a simple dimensional analysis reveals that operators quadratic in the curvatures have the same canonical dimension of the operators \(\phi ^4\) and \(\phi ^2 R\), and hence must be renormalized together for consistency.

*c*and

*e*can be treated as numbers, even though the right hand side suggests otherwise.

In order to make this section on \(\phi ^4\) more self consistent, we briefly discuss some critical property of the above system. This discussion anticipates some points that are made later in the development of Sect. 3. One can see that at the leading order the critical value for the nonminimal coupling \(\xi =F''(0)\) is \(\xi =\frac{1}{6}\) as one would naively expect from continuing the general conformal value (3) to \(d=4\), thus proving the educated guess of the introduction for the special case \(n=2\).

*not*a fixed point to order \(\epsilon \). However, the freedom highlighted in [23] of redefining the potential \(U(\phi ,R)\) by a copy of the one loop counterterms can be exploited to ensure that (23) is the fixed point at NLO. The redefinition is a change of the renormalization conditions which thus defines and links the metric and the field. We refer to [23] for a more complete and detailed explanation of the results reported in this section.

### 2.3 \(\phi ^\infty \) Universality class: the Sine–Gordon model

*R*might in principle be hidden in the mass \(m^2=m^2(R)\). In this way we have ensured that the result agrees both with the assumption that this solution generalizes the Sine–Gordon universality to curved space, and with the expectation that the nonminimal coupling \(\xi \) should be zero at the critical point.

### 2.4 2*d* Gravity at large-*c*

As a brief intermezzo we believe that it is interesting to show the relevance of the results of Sect. 2.3 in reproducing some well-known result of two dimensional quantum gravity coupled to conformal matter. Let us recall that in exactly two dimensions the path integral of gravity can be determined by integrating the conformal anomaly [31], which leads to a renormalization procedure linked to a nonlocal action known as the Polyakov action [25]. This action is especially relevant because the spacetime integral of the Einstein term is a topological invariant in two dimension, and hence it cannot govern the dynamic of the model.

However for general *d* (and specifically for \(d=2-\epsilon \)) the Einstein term is not a topological invariant, and therefore it has been argued by Kawai and Ninomiya that it should be possible to reproduce the results based on the Polyakov action by just renormalizing the Einstein action in \(d=2-\epsilon \) and then taking the limit \(\epsilon \rightarrow 0\) [32]. The validity of this argument was shown through the course of several papers, which ultimately lead to the two loop renormalization of the Einstein action in \(d=2-\epsilon \). For more details we refer to [33] and references therein; notice however that in the literature of 2*d* gravity it is often chosen \(d=2+\epsilon \), therefore the replacement \(\epsilon \rightarrow -\epsilon \) is necessary when comparing results.

*c*distinct conformally coupled fields \(\phi _i\) in

*d*dimensions

*c*is often referred to as “central charge” and it counts the effective number of matter degrees of freedom.

^{2}Using this normalization the mode \(\psi \) of the metric enjoys a Weyl invariant action, which is in form analog to any of those of the fields \(\phi _i\), if not for an overall negative sign which makes \(\psi \) an unstable “scalar” degree of freedom. The idea of [36] is to transform (27) into

*G*.

*c*the fluctuations induced by the fields \(\phi _i\) dominate over those of \(g_{\mu \nu }\) too. In other words, for large-

*c*it should be necessary to integrate only the loops of \(\phi _i\), but this is exactly the multifield generalization of what we have done in Sect. 2.3 upon the identification

*L*and

*G*. Notice that in the large-

*c*limit we are dropping any parametric dependence on the mode \(\psi \) to highlight the connection with the previous section.

*G*and \(L(\phi _i)\). In order to separate the two beta functions we have to impose that \(L(0)=1\) along the flow. Additionally we impose that all fields \(\phi _i\) are coupled in the same way so that it will be sufficient to denote each one of them by \(\phi \). We find

*a*and

*b*have Gaussian solutions, thus setting all couplings except for

*G*at the respective fixed points we obtain

*c*limit of the exact leading result in which the constant

*A*takes the value \(A=\frac{25-c}{24\pi }\) [32].

Notice that the general Euclidean result hinges on our ability of solving the problem of the instability of the conformal mode, which in [36] is “Wick” rotated \(\psi \rightarrow \mathrm{i}\psi \). While several solutions have been proposed there is no definite answer, nor general consensus on how to approach the problem. In fact, proposals to solve the problem without a Wick rotation of the dilaton mode have received renewed attention recently [37]. This problem can be framed in the more general discussion of finding the universality class of quantum gravity and exploring the corresponding conformal theory [38]. Here we would like to mention another less explored yet interesting possibility that was outlined in [39]: the path integral of 2*d* gravity could be “defined” starting with the path integral of a fluid 2*d* membrane embedded in *D* bulk dimensions (which is essentially a non-critical Nambu-Goto string) and analytically continuing to \(D\rightarrow 0\). In the membrane path integral the correct counting of the degrees of freedom involves the propagation of modes of the extrinsic curvature, which play a role analog to the gauge fixing ghosts.

### 2.5 \(\phi ^{2n+1}\) Universality class

The results of Sect. 2.1 can in part be generalized to the tower of multicritical *non-unitary* models \(\phi ^{2n+1}\) [15]. The first model of such tower would be \(\phi ^3\) whose curved-space renormalization has been studied in [16]. While it can be seen that the quadratic leading term in the renormalization of the potential is replaced by a more involved cubic one, it turns out that the leading renormalization of the wavefunction and the non-minimal coupling function is contained in the same diagrams with the opportune change of the number of internal lines.

A simple rule of thumb to test the validity of the functional RG equations involves the replacement \(n\rightarrow n+\frac{1}{2}\) in the system (15): the beta function \(\beta _v\) ceases to make sense signaling that it should be replaced with a cubic term, but both \(\beta _z\) and \(\beta _f\) are still meaningful and in fact they are the correct beta functions. If the leading cubic flow of the potential, which is given in [15], is included, then it is trivial to generalize the system (15) to the entire sequence of \(\phi ^{2n+1}\) models.

We checked explicitly that in the special case \(\phi ^3\) the resulting system, which corresponds to the Lee-Yang universality class with a nonminimal coupling to the curvature, coincides with the one given in the appendix of [30]. Notice that the cubic case should be treated with more care than we do, because the complete renormalization requires counterterms for all the cubic invariants coming from the metric [27], and therefore it is as special as the quartic case described in Sect. 2.2. The general next-to-leading renormalization (occurring at two loops) of \(\phi ^3\) in curved space appeared for the first time in [27]. The renormalization of the model is actually known with position-dependent couplings [29], a result which is used to show that a natural six dimensional generalization of the Zamolodchikov’s *c*-function does *not* always increase monotonically with the flow [28].

## 3 Criticality

*at the upper critical dimension*

More interestingly, we can use (40) to explore the infrared limit \(\mu =0\). One can see trivially that the second term in the denominator of \(\lambda (\mu )\) drops for \(\mu =0\) and therefore we have \(\lambda (0)=\lambda ^*\). Slightly less trivial is to show that for \(\mu \rightarrow 0\) the integral appearing in the exponential of \(\xi (\mu )\) diverges logarithmically implying that the second term drops; we thus have \(\xi (0) = \xi _n\). These results are in line with the expectation that the nontrivial fixed point (37)–(38) is of infrared nature in that it controls the large scale behavior of the model near criticality.

## 4 Conclusions

We have considered the leading order renormalization of the multicritical scalar models with \(\phi ^{2n}\) interaction in curved space. Our analysis shows that for almost all values of *n* one has to consider counterterms for the self interaction as well as for the nonminimal interaction of the form \(\phi ^2 R\), while some additional counterterms based on curvature invariants are needed in the special case \(n=2\). The counterterms have been obtained from a computation of the \(\frac{1}{\epsilon }\) poles of dimensionally regulated covariant Feynman diagrams of \((n-1)\)-loops for the self interaction, and \((2n-1)\)-loops for the self energy and the nonminimal interaction.

Our result generalizes the renormalization of the \(\phi ^4\) model in curved space, which we have considered as a special case, but it also shows that the general case functions rather differently. Specifically, the structure of the counterterms for the nonminimal coupling displays a discontinuity for \(n=2\), which corresponds to \(\phi ^4\). We have deduced a set of functional beta functions which describes the scale dependence of a self-interaction potential and a generalized nonminimal interaction with the scalar curvature.

We have used the perturbative renormalization group flow to determine standard perturbative beta functions for the two canonically marginal couplings: \(\lambda \) of the self-interaction \(\phi ^{2n}\) and \(\xi \) of the nonminimal interaction \(\phi ^2 R\). The RG system clearly shows that at the leading order the scale invariant fixed point of the nonminimal coupling \(\xi \) coincides with its dimension-dependent conformal value \(\xi _c\) evaluated at the upper critical dimension. This result is in agreement with an educated guess enunciated in the introduction. Importantly, the leading critical value for the coupling \(\xi \) is an ultraviolet attractive feature of the renormalization group flow.

We have also discussed a more general conjecture for which at the next-to-leading order the \(\epsilon \)-expansion of the fixed point value of \(\xi \) matches the expansion of the conformal value \(\xi _c\) below the upper critical dimension. Based on similar and already available results for the \(\phi ^4\) [22] and \(\phi ^3\) models [27, 28, 29], we argue that one has to either follow a modified version of the prescription of Brown and Collins [22], or alternatively to subtract normally while exploiting the freedom of redefining the renormalization group flow at the next-to-leading order using the counterterms of the leading order [2]. In other words, one might want to *find* the appropriate renormalization condition which ensures the validity of the conjecture for the nonminimal coupling. We believe that this condition plays an important role, especially if it is necessary to describe the model in a conformal or Weyl-invariant way.

A clearer understanding of the status of the conjecture and the renormalization condition can only be achieved by building on the results of this paper and studying the next-to-leading order contributions to the renormalization group flow, which is thus an important future prospect for this computation.

## Footnotes

- 1.
- 2.
We find that the best recent review of this formulation appeared in [35], in which it has been named unimodular Dirac gravity, or alternatively unimodular dilaton gravity. The gauge group \(Diff^*\) of the formulation comes from the breaking of a semidirect product of diffeomorphisms and Weyl transformations which is itself isomorphic to the diffeomorphisms group \(Diff \ltimes Weyl\rightarrow Diff^*\simeq Diff\), but acts on \(\psi \) and \(g_{\mu \nu }\) in a nonstandard way.

## Notes

### Acknowledgements

We are grateful to H. Gies, R. Percacci, I. L. Shapiro, G. P. Vacca and A. Wipf for insights and discussions which stimulated the analysis of this paper. RM and OZ acknowledge support from the DFG under Grants Gi 328/7-1 and Za 958/2-1 respectively.

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