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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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arXiv IDhep-th/0306133 (version 1)
Pith IDOVKRWYGG, Pith's permanent record of this paper and its review
CitationJean Zinn-Justin, Moshe Moshe. "Quantum Field Theory in the Large N Limit: a review." arXiv:hep-th/0306133 (2003). https://pith.science/paper/OVKRWYGG
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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

This review sets out to show that a whole family of quantum field theories with $N$ fields in a vector representation becomes solvable when $N$ is taken large, and that the solution is not a toy: it reproduces critical exponents, phase diagrams, and bound-state spectra in every dimension between two and four, where ordinary perturbation theory has nothing to say. The organising idea is a probability-style argument: at large $N$ the invariant composite field $O(N)$ built out of the vector, $O(N)$-invariant $O(N)$, has small fluctuations, so it can be treated as the dynamical variable and the original fields integrated out. The authors' most consequential claim is that this procedure relates pairs of theories that look unrelated — the $(O(N))$ theory and the non-linear $O(N)$ model, the $O(N)$ model and the abelian Higgs model, the Gross–Neveu model and a Yukawa theory — showing they share the same long-distance physics despite being renormalizable in different dimensions. If the claim holds, dimensional crossovers (finite size, finite temperature, dimensional reduction) become calculable rather than merely conjectured, and the large-$N$ expansion becomes a check on the $4-d$ and $d-2$ expansions rather than a rival to them. The review is explicit that the method is a technique with its own limits, and it repeatedly insists that the $1/N$ expansion must be checked to be both infrared finite and renormalizable before its results can be trusted.

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

Editorial extensions of the paper, not claims the author makes directly.

  • 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.

Compiled by classifier-1.13-20260917 via OpenRouter, question bank abstract-p1-20260923, 2026-09-23: 1 request, 9,568 input tokens, $0.0004 reported.

What the abstract itself states, as probabilities: claims first 0.02 · states limitation 0.13 · human participants 0.01 · controlled comparison 0.01 · randomized assignment 0.01 · states quantitative result 0.04 · compares with prior results 0.31 · mentions code or data availability 0.01.

The abstract, sentence by sentence

arXiv listing abstract v1. Results and the main claim are underlined; .

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

What the classifier reports for each sentence. The number is the probability it gave the label.
#sentencerolethe authors' own workstates the main conclusionstrength
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 kind of paper the classifier reports this is: review 1.00 · evidence from literature 0.49.

What was examined, and what was not

sources read
  • arXiv listing abstract v1
not examined
  • full text
  • figures and tables
  • references and citing sentences
  • relations to other papers
levels run
  • P abstract
not run
  • S sections
  • T body sentences
  • C citations
  • R cross-paper relations

Every quoted sentence on this record is the authors'. Every label is from Pith's closed vocabulary, with the probability shown beside it. Every number in the measurements table is the authors', checked by code. Record as JSON

Figures
Figure 6. Here µ − µc = 1 (sets the mass scale) and u/uc= varies between 1.6 and 1.2. There are two degenerate O(N) symmetric SUSY vacua with ϕ = 0 at mψ = mϕ = M+ = (µ − µc)/(u/uc + 1) and at mψ = mϕ = −M− = (µ − µc)/(u/uc − 1). 0 < u < uc and µ > µc, the system has a non-degenerate O(N) symmetric ground state with bosons and fermions of mass M = M+. As µ − µc changes sign (0 < u < uc fixed), two degenerate ground states appear. Either M = 0 and ϕ 2 = −N(µ−µc)/u or the system stays in an O(N) symmetric ground state
Figure 7. Region II of
Figure 9. Region IV : The energy density W(m, ϕ) = 1 N E(m, ϕ) as given in Eq. (6.49) as a function of the boson mass (m) and A, where A 2 = ϕ 2 /uc. Here µ−µc = −1 , u/uc = 0.2. As seen here there are two distinct degenerate phases. One is an ordered phase (ϕ 6= 0) with a massless boson and fermion, the other is a symmetric phase (ϕ = 0) with a massive (m = |M−|) boson and fermion. with a mass |M−| for the bosons and fermions. Similarly, when one goes from µ < µc to µ > µc at u > uc = 4π the O(N) symmetry is restored but there are two degenerated ground states to choose from M = M±. In

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

Evidence: substantial. 196 of the citing papers share no author with it, and 33 of those carry a confident label from Pith's citation classifier.

Pith read 14 of the 312 citing sentences, choosing only those whose citation markers point at this paper alone, and wrote the paraphrases from them. The summary, the counts, the evidence level, the links and the quotes come from code; the summary counts the papers listed below. A later paper is listed under a label when Pith's citation classifier gives one of its sentences that label at probability 0.8 or more (0.95 for support), Pith's reading finds that the citing paper itself did it, and a second check confirms the paraphrase against the sentence. This section lists uses only. It does not list criticism or disagreement, so their absence here says nothing about how the paper has been received. Papers that share an author with this one are listed apart, under "By the same authors". A quote is the citing authors' own sentence, shown when their paper's licence is CC BY, CC BY-SA or CC0. JSON

Uses its method or tool 2 of 21 papers with this label
How the literature uses it

138 arXiv papers cite this in 217 sentences

Each citing sentence is taken from the citing paper's LaTeX source and typed by Pith's classifier. A label appears at probability 0.8 or more (0.95 for support), a contest at 0.7 or more; the quoted words are the citing authors'. How each label was checked · JSON

Papers citing it · 105 sentences without a confident label

Uses its method or tool 11 papers
Uses its data or values 3 papers
Compares with it 4 papers
Cites it as support 2 papers
  • 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

  • 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

Discussion

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