The Erwin Schr¨odinger International Boltzmanngasse 9 ESI Institute for Mathematical Physics A-1090 Wien, Austria Self–Dual Noncommutative φ4–Theory in Four Dimensions is a Non–Perturbatively Solvable and Non–Trivial Quantum Field Theory Harald Grosse Raimar Wulkenhaar Vienna, Preprint ESI 2428 (2013) June 17, 2013 Supported by the Austrian Federal Ministry of Education, Science and Culture Available online at http://www.esi.ac.at Self-dual noncommutative φ4-theory in four dimensions is a non-perturbatively solvable and non-trivial quantum field theory Harald Grosse1 and Raimar Wulkenhaar2 1 Fakult¨at f¨ur Physik, Universit¨at Wien Boltzmanngasse 5, A-1090 Wien, Austria 2 Mathematisches Institut der Westf¨alischen Wilhelms-Universit¨at Einsteinstraße 62, D-48149 M¨unster, Germany Abstract We study quartic matrix models with partition function [E,J] = 2 λ 4 Z dM exp(trace(JM EM 4 M )). The integral is over the space of Her- mitean -matrices,− the− external matrix E encodes the dynamics, λ> 0 R N ×N is a scalar coupling constant and the matrix J is used to generate correlation functions. For E not a multiple of the identity matrix, we prove a universal al- gebraic recursion formula which gives all higher correlation functions in terms of the 2-point function and the distinct eigenvalues of E. The 2-point func- tion itself satisfies a closed non-linear equation which must be solved case by case for given E. These results imply that if the 2-point function of a quartic matrix model is renormalisable by mass and wavefunction renormalisation, then the entire model is renormalisable and has vanishing β-function. As main application we prove that Euclidean φ4-quantum field theory on four- dimensional Moyal space with harmonic propagation, taken at its self-duality point and in the infinite volume limit, is exactly solvable and non-trivial. This model is a quartic matrix model, where E has for the same spectrum as the Laplace operator in 4 dimensions. Using theN theory → ∞ of singular integral equations of Carleman type we compute (for and after renormalisa- tion of E,λ) the free energy density (1/volume)N log(→ ∞[E,J]/ [E, 0]) exactly in terms of the solution of a non-linear integral equation.Z Z Existence of a solution is proved via the Schauder fixed point theorem. The derivation of the non-linear integral equation relies on an assumption which we verified numerically for coupling constants 0 <λ 1 . ≤ π MSC2010: 81T16, 39B42, 45E05, 47H10 Keywords: quantum field theory in 4 dimensions; noncommutative geometry; exactly solvable models; matrix models; Schwinger-Dyson equations; singular integral equations [email protected] [email protected] Contents 1 Introduction 1 1.1 Perturbative, axiomatic and algebraic quantum field theory......... 2 1.2 Euclideanquantumfieldtheory . .. 2 1.3 Solvablemodels................................. 3 1.4 Constructivemethods. 4 1.5 Noncommutativegeometry. 5 1.6 Matrixmodels.................................. 6 1.7 Outlineofthepaper .............................. 8 2 Ward identities in matrix models 10 2.1 Field-theoreticalmatrixmodels . ..... 11 2.2 Wardidentity .................................. 12 2.3 Expansionintoboundarycomponents. .... 13 3 Schwinger-Dyson equations 15 3.1 Strategy ..................................... 15 3.2 Schwinger-Dyson equations for a φ4-model: B =1cycle........... 16 3.3 An algebraic recursion formula for a real field theory . ........ 17 3.4 Thegenusexpansion .............................. 19 3.5 Graphical solution of the recursion formula in the planarsector . 23 4 4 Self-dual noncommutative φ4-theory 28 4.1 Definitionofthematrixmodel. .. 28 4.2 Renormalisation of the two-point function . ....... 29 4.3 Integralrepresentation . ... 31 4.4 TheCarlemanequation. 32 4.5 Themasterequation .............................. 36 4.6 Higher correlation functions and effective coupling constant......... 42 4.7 Miscellaneousremarks . 45 5 Conclusion and outlook 46 A Correlationfunctionswithtwoboundarycomponents 48 A.1 Two cycles of odd length: Schwinger-Dyson equations . ........ 48 A.2 Two cycles of even length: Schwinger-Dyson equations . ........ 50 A.3 Remarks on functions with more than two cycles. ..... 52 4 A.4 The B = 2 sector of noncommutative φ4-theory ............... 52 B Perturbative expansion 55 Acknowledgements 58 References 58 1 Introduction A rigorous construction of quantum field theories in four dimensions was not very suc- cessful so far. In this paper we show that for φ4-theory on four-dimensional Moyal space 1 with harmonic propagation, taken at critical frequency and in the infinite volume limit, much more is true: The model is exactly solvable. We know that this is a toy model in so far as classical locality and Poincar´esymmetry are not realised and the Minkowskian continuation of that Euclidean model needs to be investigated. On the other hand, the model – carries an action of an infinite-dimensional symmetry group, – is invariant under a duality transformation between position and momentum space [LS02], – is almost scale invariant [DGMR07], – is known to have a realisation as a matrix model (with non-constant kinetic term), – has perturbatively an infinite number of divergent but renormalisable Feynman graphs [GW05b]. Each renormalised Feynman graph has subleading logarithmic terms which make the perturbatively renormalised correlation functions divergent at large energy. The model and its solution touch many aspects of quantum field theory which we recall in the next subsections. The reader in a hurry may jump to Sec. 1.7. 1.1 Perturbative, axiomatic and algebraic quantum field theory Starting with the Lamb shift in the 1940s and culminating in the experimental tests of the Standard Model, perturbatively renormalised quantum field theory is an enormous phenomenological success. However, this success lacks a mathematical understanding. The perturbation series is at best an asymptotic expansion which cannot converge at 2 1 physical coupling constants such as the electron charge e 137 . In addition there are physical effects such as confinement which seem out of reach≈ for perturbation theory. Therefore, the development of a mathematical foundation of quantum field theory that permits non-trivial examples is one of the most urgent challenges in mathematical physics. In the early 1950’s, G˚arding and Wightman gave a rigorous mathematical founda- tion to relativistic quantum field theory by casting the unquestionable physical principles (locality, covariance, stability, unitarity) into a set of axioms. These ideas were published years later [Wig56, WG64, SW64]. The difficulty to provide non-trivial examples to these axioms motivated the development of equivalent or more general frameworks such as Al- gebraic quantum field theory and Constructive/Euclidean quantum field theory. Algebraic quantum field theory shifts the focus from the field operators to the Haag-Kastler net of algebras assigned to open regions in space-time [HK64]. Fields merely provide coordinates on the algebra. Over the years this point of view turned out to be very fruitful [Haa92]. One important advantage over the axiomatic setup is the natural possibility to describe quantum field theory on curved space-time [BF09]. 1.2 Euclidean quantum field theory Of central importance for us is the Euclidean approach. Wightman functions admit an analytic continuation in time. At purely imaginary time they become the Schwinger functions [Sch59] of a Euclidean quantum field theory. Symanzik emphasised the powerful Euclidean-covariant functional integral representation [Sym64], which yields a Feynman- Kac formula of the heat kernel [Kac49]. In this way the Schwinger functions become the 2 moments of a statistical physics probability distribution. This tight connection between Euclidean quantum field theory and statistical physics led to a fruitful exchange of con- cepts and methods, most importantly that of the renormalisation group [WK74]. It is sometimes possible to rigorously prove the existence of a Euclidean quantum field theory or of a statistical physics model without knowing or using that this model derives from a true relativistic quantum field theory. This is, for instance, the case for the model constructed in this paper. Sufficient conditions on the Euclidean model which guarantee the Wightman axioms were first given by Nelson [Nel71]. These conditions based on Markov fields turned out to be too strong or inconvenient. Shortly later, Osterwalder and Schrader established a set of axioms [OS73, OS75] by which the Euclidean quantum field theory is equivalent to a Wightman theory. The most decisive axiom is reflection positivity which yields existence of the Hilbert space and a positive energy Hamiltonian. The Euclidean approach together with the Osterwalder-Schrader axioms turned out to be the key to construct relativistic quantum field theories in dimension less than four. Two successful methods were developed: The correlation inequality method [Gin70, GRS75] relies on positivity and monotonicity of Schwinger functions and is suitable for bosonic theories. The phase space expansion method [GJS74, GJ87, Riv91] works both for bosonic and fermionic theories. It uses lattice partitions and iterated cluster and Mayer expansions to control the utraviolet limit of the theory. It can also typically establish that the Schwinger functions built constructively are the Borel sums of their ordinary perturbative series. The cluster expansion [GJS74] is also used to prove a mass gap in the spectrum of the Hamiltonian. 1.3 Solvable