## Conformal invariance

We study a class of Lorentz violating quantum field theories that contain higher space derivatives, but no higher time derivatives, and become renormalizable in the large N expansion. The fixed points of their renormalization-group flows provide examples of exactly “weighted scale invariant” theories, which are noticeable Lorentz violating generalizations of conformal field theories. We classify the scalar and fermion models that are causal, stable and unitary. Solutions exist also in four and higher dimensions, even and odd. In some explicit four dimensional examples, we compute the correlation functions to the leading order in 1/N and the critical exponents to the subleading order. We construct also RG flows interpolating between pairs of fixed points.

JHEP 0802 (2008) 051 | DOI: 10.1088/1126-6708/2008/02/051

arXiv:0801.1216 [hep-th]

I 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 $N_f \lesssim 11N_c/2$ 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.

JHEP 0310 (2003) 045 | DOI: 10.1088/1126-6708/2003/10/045

arXiv:hep-th/0309251

I discuss several issues about the irreversibility of the RG flow and the trace anomalies $c$, $a$ and $a’$. First I argue that in quantum field theory: $i$) the scheme-invariant area $\Delta a’$ of the graph of the effective beta function between the fixed points defines the length of the RG flow; $ii$) the minimum of $\Delta a’$ in the space of flows connecting the same UV and IR fixed points defines the (oriented) distance between the fixed points; $iii$) in even dimensions, the distance between the fixed points is equal to $\Delta a =a_{UV}-a_{IR}$. In even dimensions, these statements imply the inequalities $0 \leq \Delta a \leq \Delta a’$ and therefore the irreversibility of the RG flow. Another consequence is the inequality $a \leq c$ for free scalars and fermions (but not vectors), which can be checked explicitly. Secondly, I elaborate a more general axiomatic set-up where irreversibility is defined as the statement that there exist no pairs of non-trivial flows connecting interchanged UV and IR fixed points. The axioms, based on the notions of length of the flow, oriented distance between the fixed points and certain “oriented-triangle inequalities”, imply the irreversibility of the RG flow without a global a function. I conjecture that the RG flow is irreversible also in odd dimensions (without a global a function). In support of this, I check the axioms of irreversibility in a class of $d=3$ theories where the RG flow is integrable at each order of the large $N$ expansion.

Class.Quant.Grav. 21 (2004) 29-50 | DOI: 10.1088/0264-9381/21/1/003

I study some classes of RG flows in three dimensions that are classically conformal and have manifest $g \rightarrow 1/g$ dualities. The RG flow interpolates between known (four-fermion, Wilson-Fischer, $\phi_3^6$) and new interacting fixed points. These models have two remarkable properties: $i$) the RG flow can be integrated for arbitrarily large values of the couplings g at each order of the $1/N$ expansion; $ii$) the duality symmetries are exact at each order of the $1/N$ expansion. I integrate the RG flow explicitly to the order ${\cal O}(1/N)$, write correlators at the leading-log level and study the interpolation between the fixed points. I examine how duality is implemented in the regularized theory and verified in the results of this paper.

Nucl.Phys. B658 (2003) 440 | DOI: 10.1016/S0550-3213(03)00174-3

I review my explanation of the irreversibility of the renormalization-group flow in even dimensions greater than two and address new investigations and tests.

I study various properties of the critical limits of correlators containing insertions of conserved and anomalous currents. In particular, I show that the improvement term of the stress tensor can be fixed unambiguously, studying the RG interpolation between the UV and IR limits. The removal of the improvement ambiguity is encoded in a variational principle, which makes use of sum rules for the trace anomalies $a$ and $a’$. Compatible results follow from the analysis of the RG equations. I perform a number of self-consistency checks and discuss the issues in a large set of theories.

J.Math.Phys. 43 (2002) 2965-2977 | DOI: 10.1063/1.1475766

I derive a procedure to generate sum rules for the trace anomalies $a$ and $a’$. Linear combinations of $\Delta a = a_{UV}-a_{IR}$ and $\Delta a’ = a’_{UV}-a’_{IR}$ are expressed as multiple flow integrals of the two-, three- and four-point functions of the trace of the stress tensor. Eliminating $\Delta a’$, universal flow invariants are obtained, in particular sum rules for $\Delta a$. The formulas hold in the most general renormalizable quantum field theory (unitary or not), interpolating between UV and IR conformal fixed points. I discuss the relevance of these sum rules for the issue of the irreversibility of the RG flow. The procedure can be generalized to derive sum rules for the trace anomaly $c$.

JHEP 0111:033 (2001) | DOI: 10.1088/1126-6708/2001/11/033

A flow invariant is a quantity depending only on the UV and IR conformal fixed points and not on the flow connecting them. Typically, its value is related to the central charges a and c. In classically-conformal field theories, scale invariance is broken by quantum effects and the flow invariant $a_{UV}-a_{IR}$ is measured by the area of the graph of the beta function between the fixed points. There exists a theoretical explanation of this fact. On the other hand, when scale invariance is broken at the classical level, it is empirically known that the flow invariant equals $c_{UV}-c_{IR}$ in massive free-field theories, but a theoretical argument explaining why it is so is still missing. A number of related open questions are answered here. A general formula of the flow invariant is found, which holds also when the stress tensor has improvement terms. The conditions under which the flow invariant equals $c_{UV}-c_{IR}$ are identified. Several non-unitary theories are used as a laboratory, but the conclusions are general and an application to the Standard Model is addressed. The analysis of the results suggests some new minimum principles, which might point towards a better understanding of quantum field theory.

Class.Quant.Grav. 18 (2001) 4417-4442 | DOI: 10.1088/0264-9381/18/21/304

I study a class of interacting conformal field theories and conformal windows in three dimensions, formulated using the Parisi large-$N$ approach and a modified dimensional-regularization technique. Bosons are associated with composite operators and their propagators are dynamically generated by fermion bubbles. Renormalization-group flows between pairs of interacting fixed points satisfy a set of non-perturbative $g \leftrightarrow 1/g$ dualities. There is an exact relation between the beta function and the anomalous dimension of the composite boson. Non-Abelian gauge fields have a non-renormalized and quantized gauge coupling, although no Chern-Simons term is present. A problem of the naive dimensional-regularization technique for these theories is uncovered and removed with a non-local, evanescent, non-renormalized kinetic term. The models are expected to be a fruitful arena for the study of odd-dimensional conformal field theory.

JHEP 0006 (2000) 042 | DOI: 10.1088/1126-6708/2000/06/042

I discuss the properties of the central charges $c$ and $a$ for higher-derivative and higher-spin theories (spin 2 included). Ordinary gravity does not admit a straightforward identification of c and a in the trace anomaly, because it is not conformal. On the other hand, higher-derivative theories can be conformal, but have negative $c$ and $a$. A third possibility is to consider higher-spin conformal field theories. They are not unitary, but have a variety of interesting properties. Bosonic conformal tensors have a positive-definite action, equal to the square of a field strength, and a higher-derivative gauge invariance. There exists a conserved spin-2 current (not the canonical stress tensor) defining positive central charges $c$ and $a$. I calculate the values of $c$ and $a$ and study the operator-product structure. Higher-spin conformal spinors have no gauge invariance, admit a standard definition of $c$ and $a$ and can be coupled to Abelian and non-Abelian gauge fields in a renormalizable way. At the quantum level, they contribute to the one-loop beta function with the same sign as ordinary matter, admit a conformal window and non-trivial interacting fixed points. There are composite operators of high spin and low dimension, which violate the Ferrara-Gatto-Grillo theorem. Finally, other theories, such as conformal antisymmetric tensors, exhibit more severe internal problems. This research is motivated by the idea that fundamental quantum field theories should be renormalization-group (RG) interpolations between ultraviolet and infrared conformal fixed points, and quantum irreversibility should be a general principle of nature.

Class.Quant.Grav. 17 (2000) 2847-2866 | DOI: 10.1088/0264-9381/17/15/301