REVIEW
Quantum Field Theory in the Large N Limit: a review
High Energy Physics - Theory · Submitted 13 Jun 2003 · 207 references
Pith's verdict: A 2003 review that still earns its citations: the model-equivalence derivations are on the page, and the authors are candid about where the 1/N method runs out of road.
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Referee report
The paper
The paper claims
A single bookkeeping trick — replacing $N$ coupled fields with one self-averaging composite field — solves $O(N)$ and $U(N)$ quantum field theories in the large-$N$ limit and reveals that theories renormalizable in different dimensions can,
The reading
What carries the argument
The mechanism is the auxiliary-field (Hubbard–Stratonovich-type) identity $1 = N$ times the delta function inserted for the constraint $O(N)$, followed by a Gaussian integration over the vector field and a steepest-descent evaluation of the resulting functional integral. The Gaussian integration produces a trace-log term, $O(N)$ times the logarithm of the inverse propagator of the vector field in the presence of the multiplier; the multiplier's saddle-point value is the mass of the elementary quanta, and its propagator, computed from the bubble diagram, carries the anomalous dimension that makes the $1/N$ expansion renormalizable in every dimension. This is the object that does the work: it converts a strongly coupled vector theory into a weakly fluctuating scalar problem, and it is what allows the review to move between the linear $(O(N))$ theory, the non-linear $O(N)$ model, the $O(N)$ model, the Gross–Neveu model, and their supersymmetric versions without re-deriving anything from scratch. A parallel section shows that a Gaussian variational ansatz reproduces the same saddle-point equations in the large-$N$ limit, giving an independent handle on the same physics.
Extended reading notes
Core claim
The central claim is an equivalence, valid to all orders in the $1/N$ expansion, between the $O(N)$-symmetric $(O(N))$ field theory and the $O(N)$ non-linear $O(N)$ model. The derivation runs through the auxiliary-field representation: inserting $1 = N$ times a delta function for $O(N)$ and integrating over the original vector field turns the partition function into a functional integral over a Lagrange-multiplier field $O(N)$ and a composite field $O(N)$, with the vector field appearing only quadratically. In the critical domain, the $O(N)$ term in the effective action has dimension $4-d$ and is irrelevant for $d<4$, so it can be dropped; shifting $O(N)$ by its expectation value and integrating it back out returns the original vector field subject to a fixed-length constraint, which is exactly the non-linear $O(N)$ model. The same argument, applied to a gauged complex vector, relates the $O(N)$ model to the abelian Higgs model, and applied to fermions relates the Gross–Neveu four-fermion theory to the Gross–Neveu–Yukawa theory. Along the way the review solves the general $U(phi^2)$ scalar theory at leading order in $1/N$, obtaining critical exponents $beta = 1/2$, $nu = 1/(d-2)$, $eta = 0$, $delta = (d+2)/(d-2)$, the scaling equation of state, and the leading correction-to-scaling exponent $omega = 4-d$, all in arbitrary dimension $2<d<4$.
Load-bearing premise
The consistency of the large-$N$ results with ordinary renormalization-group behaviour at fixed dimension requires the regularization-dependent constant $a(d)$ to be positive, and the review itself states that for $d<4$ this positivity is not assured and that for simple lattice regularizations in $d=3$ the sign is arbitrary.
Editorial extensions
If this is right
- Critical exponents and the scaling equation of state for $O(N)$-symmetric systems become calculable in every dimension $2<d<4$, not just near $d=4$ or near $d=2$, so the two epsilon expansions can be connected through a common answer.
- The $(O(N))$ theory and the non-linear $O(N)$ model must give identical universal quantities; a scheme-dependence in the cutoff is the only place where their apparent difference in number of parameters can hide.
- The Gross–Neveu and Gross–Neveu–Yukawa models must produce the same long-distance physics, including the mass ratio $m_sigma/m_psi = 2$ at leading order, which the review verifies independently in $d=2$ and $d=4$.
- Chiral symmetry breaking in the Gross–Neveu model is always present in two dimensions and appears above a critical coupling $G_c = 1/Omega_d(0)$ in higher dimensions, with the mass vanishing as a power of $G - G_c$ for $d<4$.
- Finite-size and finite-temperature crossovers between a $d$-dimensional theory and its $(d-1)$-dimensional reduction become tractable, since the same gap equation with quantized momenta or Matsubara frequencies replaces the infinite-volume one.
Reading between the lines
- If the equivalence holds to all orders in $1/N$, then any lattice simulation that measures, say, the $O(N)$ model's correlation-length exponent at $N=10$ in $d=3$ is implicitly a test of the $(O(N))$ theory's exponent at the same $N$ and $d$ — a cross-check that does not require simulating both theories.
- The review's insistence that the $1/N$ series is divergent but Borel summable for $d<4$ suggests that the practical value of the method is as an asymptotic expansion whose first few terms are reliable, not as a convergent scheme; a reader should expect the $1/N^2$ and $1/N^3$ coefficients quoted here to be useful only for large $N$.
- The equivalence argument leans on dropping the $O(N)$ term as irrelevant for $d<4$; in exactly $d=4$ that term is marginal and only logarithmically suppressed, which is precisely where the review says the non-linear $O(N)$ model must be replaced by the $(O(N))$ theory to be understood perturbatively — a hint that the correspondence is cleanest strictly below four dimensions.
- The variational derivation reproducing the same saddle-point equations suggests the large-$N$ results are the mean-field sector of a more general variational principle, so any improvement scheme (e.g. going beyond Gaussian trial states) would generate the $1/N$ corrections systematically rather than by diagrammatic brute force.
Original abstract
We review the solutions of O(N) and U(N) quantum field theories in the large $N$ limit and as 1/N expansions, in the case of vector representations. Since invariant composite fields have small fluctuations for large $N$, the method relies on constructing effective field theories for composite fields after integration over the original degrees of freedom. We first solve a general scalar $U(\phib^2)$ field theory for $N$ large and discuss various non-perturbative physical issues such as critical behaviour. We show how large $N$ results can also be obtained from variational calculations.We illustrate these ideas by showing that the large $N$ expansion allows to relate the $(\phib^2)^2$ theory and the non-linear $\sigma$-model, models which are renormalizable in different dimensions. Similarly, a relation between $CP(N-1)$ and abelian Higgs models is exhibited. Large $N$ techniques also allow solving self-interacting fermion models. A relation between the Gross--Neveu, a theory with a four-fermi self-interaction, and a Yukawa-type theory renormalizable in four dimensions then follows. We discuss dissipative dynamics, which is relevant to the approach to equilibrium, and which in some formulation exhibits quantum mechanics supersymmetry. This also serves as an introduction to the study of the 3D supersymmetric quantum field theory. Large $N$ methods are useful in problems that involve a crossover between different dimensions. We thus briefly discuss finite size effects, finite temperature scalar and supersymmetric field theories. We also use large $N$ methods to investigate the weakly interacting Bose gas. The solution of the general scalar $U(\phib^2)$ field theory is then applied to other issues like tricritical behaviour and double scaling limit.
The compiled record
The paper's own sentences, arranged, kept beside the review and separate from it. Each label is drawn from a closed vocabulary; the number beside it is the probability the classifier gave that label. Numbers in the tables are the authors', checked by code. No sentence in this record was written by a model.
The abstract, sentence by sentence
We review the solutions of O(N) and U(N) quantum field theories in the large $N$ limit and as 1/N expansions, in the case of vector representations. Since invariant composite fields have small fluctuations for large $N$, the method relies on constructing effective field theories for composite fields after integration over the original degrees of freedom. We first solve a general scalar $U(\phib^2)$ field theory for $N$ large and discuss various non-perturbative physical issues such as critical behaviour. We show how large $N$ results can also be obtained from variational calculations.We illustrate these ideas by showing that the large $N$ expansion allows to relate the $(\phib^2)^2$ theory and the non-linear $\sigma$-model, models which are renormalizable in different dimensions. Similarly, a relation between $CP(N-1)$ and abelian Higgs models is exhibited. Large $N$ techniques also allow solving self-interacting fermion models. A relation between the Gross--Neveu, a theory with a four-fermi self-interaction, and a Yukawa-type theory renormalizable in four dimensions then follows. We discuss dissipative dynamics, which is relevant to the approach to equilibrium, and which in some formulation exhibits quantum mechanics supersymmetry. This also serves as an introduction to the study of the 3D supersymmetric quantum field theory. Large $N$ methods are useful in problems that involve a crossover between different dimensions. We thus briefly discuss finite size effects, finite temperature scalar and supersymmetric field theories. We also use large $N$ methods to investigate the weakly interacting Bose gas. The solution of the general scalar $U(\phib^2)$ field theory is then applied to other issues like tricritical behaviour and double scaling limit.
background contribution method result interpretation main claim
| # | sentence | role | the authors' own work | states the main conclusion | strength |
|---|---|---|---|---|---|
| s0 | We review the solutions of O(N) and U(N) quantum field theories in the large $N$ limit and as 1/N expansions, in the case of vector representations. | contribution 0.92 | 0.62 | 0.15 | asserted 0.91 |
| s1 | Since invariant composite fields have small fluctuations for large $N$, the method relies on constructing effective field theories for composite fields after integration over the original degrees of freedom. | method 0.96 | 0.14 | 0.11 | asserted 1.00 |
| s2 | We first solve a general scalar $U(\phib^2)$ field theory for $N$ large and discuss various non-perturbative physical issues such as critical behaviour. | contribution 0.88 | 0.85 | 0.07 | asserted 0.99 |
| s3 | We show how large $N$ results can also be obtained from variational calculations.We illustrate these ideas by showing that the large $N$ expansion allows to relate the $(\phib^2)^2$ theory and the non-linear $\sigma$-model, models which are renormalizable in different dimensions. | contribution 0.40 | 0.89 | 0.12 | asserted 1.00 |
| s4 | Similarly, a relation between $CP(N-1)$ and abelian Higgs models is exhibited. | result 0.81 | 0.81 | 0.04 | asserted 1.00 |
| s5 | Large $N$ techniques also allow solving self-interacting fermion models. | contribution 0.59 | 0.42 | 0.05 | asserted 0.81 |
| s6 | A relation between the Gross--Neveu, a theory with a four-fermi self-interaction, and a Yukawa-type theory renormalizable in four dimensions then follows. | result 0.88 | 0.76 | 0.06 | asserted 0.99 |
| s7 | We discuss dissipative dynamics, which is relevant to the approach to equilibrium, and which in some formulation exhibits quantum mechanics supersymmetry. | contribution 0.88 | 0.42 | 0.04 | asserted 0.72 |
| s8 | This also serves as an introduction to the study of the 3D supersymmetric quantum field theory. | contribution 0.39 | 0.36 | 0.03 | asserted 0.94 |
| s9 | Large $N$ methods are useful in problems that involve a crossover between different dimensions. | contribution 0.32 | 0.26 | 0.06 | asserted 0.87 |
| s10 | We thus briefly discuss finite size effects, finite temperature scalar and supersymmetric field theories. | contribution 0.98 | 0.69 | 0.04 | asserted 0.93 |
| s11 | We also use large $N$ methods to investigate the weakly interacting Bose gas. | contribution 0.93 | 0.78 | 0.04 | asserted 0.89 |
| s12 | The solution of the general scalar $U(\phib^2)$ field theory is then applied to other issues like tricritical behaviour and double scaling limit. | contribution 0.85 | 0.76 | 0.07 | asserted 1.00 |
What was examined, and what was not
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Figures
Showing 3 of the paper's 9 figures. See all figures in the PDF ↗ Figures reproduced from arXiv:hep-th/0306133 by the authors.
References
How later papers used it
196 later papers cite it
2 later papers by other authors are listed below: 2 used its method or tool.
Uses its method or tool 2 of 21 papers with this label
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We will now calculate the partition function and hence free energy of this interacting Bosonic system in thermal equilibrium, using a large $N$ approximation [ref] .
Renyi Entropy of Interacting Thermal Bosons in Large $N$ Approximation (2020) · $𝒰(N)$ model and large-N field theory · 0.96
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Following this paper's analysis for the Gross-Neveu-Yukawa model, integrated out only (N-1) fermions to obtain the fermionic contribution to the action.
Transition between algebraic and $\mathbb{Z}_2$ quantum spin liquids at large $N$ (2018) · 0.94 · paraphrased: the licence does not allow quoting
How the literature uses it
138 arXiv papers cite this in 217 sentences
Papers citing it · 105 sentences without a confident label
Uses its method or tool 11 papers
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Renormalization of the saddle point condition then can be achieved by the non-perturbative renormalization conditions [ref]
What if $\phi^4$ theory in 4 dimensions is non-trivial in the continuum? · Calculation · 0.90
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[ref] We first introduce an auxiliary field $α(x)$ to rewrite the action with source as [equation] In the large $N$ limit, let $Δm_0^2$ be the saddle point value of $i α$, which is determined by [equation] $ϕ^N = √(N) v$ gives [equation] Eqs. ([ref]) and ([ref]) are Eqs. (2.7) and (2.8) of Ref. [ref] .
Analytic form of the effective potential in the large N limit of a real scalar theory in four dimensions · The large $N$ limit of the $O(N)$ linear sigma model · 0.90
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More specifically, we study quenches of scalar fields on a de Sitter spacetime using the non-perturbative large-$N$ approximation [ref] .
Non-linear effects in early Universe cosmology · Section · 1.00
Uses its data or values 3 papers
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In $d=2+$, the Gross-Neveu model has a UV fixed point [ref] ; g^* =2πN , note that here $N=2 Ñ$, where the extra $2$ accounts for the dimension of spinors.
On Instantons in Gross-Neveu and Gross-Neveu-Yukawa models · Instanton solutions · 0.97
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We also list the best available BMW results [ref] and some results obtained by other methods [ref] that can be considered the `world's best' values, as well as experimental values for the Ising model [ref] .
Effect of the quartic gradient terms on the critical exponents of the Wilson-Fisher fixed point in $O(N)$ models · Short overview on the present status of the determination of the critical exponents of the WF FP in the functional RG fr · 0.90
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While they are available as functions of $d$ [ref] , we will just state them for the physically interesting case $d=3$: Δ_ϕ_i=1/2+ 4/3 π^2 N- 256/27 π^4 N^2+ … , Δ_ϕ^2 = 2 - 32 /3 π^2 N + 32 (16- 27 π^2)/27 π^4 N^2+ … .
TASI Lectures on Large $N$ Tensor Models · Some Applications of the Vector and Matrix Large $N$ Limits · 0.90
Compares with it 4 papers
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These results are in agreement with results in [ref] .
$\epsilon$-Expansion in the Gross-Neveu Model from Conformal Field Theory · Anomalous dimension of $ψ$, $ψ̅$ and $ψ̅ψ$ · 0.99
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[equation] which again agrees with the general analysis presented in <ref>, upto some normalization and contact terms which can be attributed different regulation used in [ref] .
Revisiting AdS/CFT at a finite radial cut-off · Large $N$ limit of $O(N)$ Wilson-Fisher model · 0.99
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Nonetheless, the theory exhibits a remarkable similarity with the well-known $O(N)$ vector model [ref] that we elaborate on in more detail below, and offers a new perspective on the nascent NN-QFT correspondence that may be fruitfully developed further.
The edge of chaos: quantum field theory and deep neural networks · Introduction · 0.89
Cites it as support 2 papers
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The explicit solution of this model in the large $N$ limit [ref] or via integrability [ref] confirms that the theory is gapped and no SSB occurs.
Spontaneous symmetry breaking on surface defects · Examples · 0.99
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The existence of a critical transition for finite values of $ε$, and in particular in three dimensions, and for sufficiently large values of $N$ is confirmed by nonperturbative (with respect to the couplings) computations in the large-$N$ limit [ref] .
Three-dimensional Abelian and non-Abelian gauge Higgs theories · Three-dimensional Abelian Higgs field theory in the large-$N$ limit · 0.97
Cites it as background 75 papers
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We consider the supersymmetric action [ref] for the Wess-Zumino model in $(2+1)d$ in the fundamental of $O(2N)$, with $2N$ real scalars and $2N$ real fermions.
Large $N$ Wess-Zumino model at finite temperature and large chemical potential in $3d$ · Wess-Zumino model with chemical potential($μ$) and large $μ$ · 0.81
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In this section we review the relevant pieces in the large $N$ analysis of the $O(N)$ model as described in [ref] (see also the review [ref] and more recently [ref] ), which will also serve to establish some notation.
Notes on Resonances and Unitarity from Celestial Amplitudes · Effective Description of the O(N) Model · 0.98
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The Lagrangian is [ref] [equation] where $ℛ$ is the Ricci scalar.
The large charge expansion at large N · $$ model · 0.93
Discussion
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