Measures on Banach Manifolds, Random Surfaces, and Nonperturbative String Field Theory with Cut-Offs
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MEASURES ON BANACH MANIFOLDS, RANDOM SURFACES, AND NONPERTURBATIVE STRING FIELD THEORY WITH CUT-OFFS JONATHAN WEITSMAN Abstract. We construct a cut-off version of nonpertubative closed Bosonic string field theory in the light-cone gauge with imaginary string coupling constant. We show that the partition function is a continuous function of the string coupling constant, and conjecture a relation between the formal power series expansion of this partition function and Riemann Surfaces. 1. Introduction Recent work of Schnabl [11], confirming conjectures of Sen [12], has given strong evidence for string field theory as a candidate for nonperturbative string theory. This may indicate that string field theory provides a framework for the advances in nonperturbative string theory which have influenced both mathematics and physics. In view of this fact, and of the important role of non- perturbative string theory in mathematics, it seems reasonable to ask to what extent string field theory can be understood mathematically. In this paper we will show that in the simple case of the light cone closed Bosonic string field theory, and once appropriate cut-offs have been introduced, such a mathematical interpretation is possible.∗ 1.1. Free string field theory for the closed Bosonic string: Formal path integrals. Free field theory for the closed Bosonic string in light cone gauge was described by Kaku and Kikkawa y [7]. The string field Ψ(t; `; φ) is formally a complex-valued function of time t 2 R, string length 1 d d ` 2 (0; 1), and a map φ : S ! R , which may be thought of as a loop or \string" in R 1 d z (more properly φ should be considered as a distribution on S with values in R ). In view of the Schrodinger representationx L (S0(S1; ) ⊗ d; dµp ) 'F; 2 R R −(d2=dx2)+m2 (where m > 0; see Appendix A) which identifies functions on distributions on S1 with elements of Fock space F; we may formally think of the string field as a distribution Ψ(t; `) on R × [0; 1) with values in some completion of Fock space. In these terms a cut-off version of Kaku and Kikkawa's [7] formal free string path integral is given by arXiv:0807.2069v1 [math-ph] 14 Jul 2008 D E Z R L1 R 1 d L d` −∞ dt Ψ(t;`); i dt −H`;m Ψ(t;`) (1.1) dΨ e 0 F ; where H`;m is the Hamiltonian on Fock space of mass m > 0 (see Appendix A) and L0;L1; with 0 < L0 < L1 < 1; correspond to the maximum and minimum allowed string lengths. 2000 Mathematics Subject Classification. Primary . Supported in part by NSF grant DMS 04/05670. June 15, 2021. ∗For another mathematical study of string field theory, see Dimock [3]. yIn the light cone gauge this string length appears as the momentum conjugate to the light cone coordinate X+; see [7]. z d+1;1 This model corresponds to a gauge-fixed theory of strings propagating in R ; see e.g. [10]. xIn this paper all vector spaces are complex unless specifically denoted otherwise. 1 2 JONATHAN WEITSMAN To try to interpret an expression of the type (1.1) as a measure, we compute correlation functions. 0 0 1 0 If we take t; t 2 (−∞; 1), g; g 2 C ([L0;L1]), and v; v 2 F, and set Z L1 D E Φv;g;t(Ψ) = g(`) Ψ(t; `); v d`; F L0 then the expression (1.1) gives rise to the formal computation D E R L1 R 1 d Z d` dt Ψ(t;`); i −H`;m Ψ(t;`) L0 −∞ dt (1.2) Φv;g;t(Ψ) Φv0;g0;t0 (Ψ) dΨ e F L1 Z D 0 E = v; e−i(t−t )H`;m v0 θ(t − t0)g(`)g0(`) d`; F L0 where the function θ : R ! R is defined by ( θ(x) = 0 if x < 0; θ(x) = 1 if x ≥ 0: 1.2. Free string field theory for the closed Bosonic string: Mathematical results. In Section 2 we will prove the following result, which may be viewed as a mathematical version of α −α H (1.2). Choose α > 0 and let F = e L0;m F: α Theorem 1. There exists a Banach space B ⊃ H1(R) ⊗ L2([L0;L1]) ⊗ F and a Gaussian prob- 0 0 1 ability measure µ on B such that if v; v 2 F are eigenvectors of H`;m, g; g 2 C ([L0;L1]), and 0 t; t 2 R, the function Φv;g;t : H1(R) ⊗ L2([L0;L1]) ⊗ F ! C, given by Z L1 D E Φv;g;t(f ⊗ h ⊗ w) = f(t) d` h(`)g(`) w; v F L0 extends to an element Φv;g;t 2 Lp(B; dµ) for all p ≥ 1. Furthermore, L1 Z Z D 0 E 0 −|t−t j H`;m 0 (1.3) dµ Φv;g;t Φv0;g0;t0 = d` g(`) g (`) v; e v : F B L0 Remark 1.4. Note that (1.3) differs from (1.2) by the absence of the function θ(t − t0) and by the familiar \Wick rotation" i(t − t0) ! (t − t0). In physical language we have replaced the first order Minkowski action by a second order Euclidean action. This would seem to be necessary to get a reasonable field theory limit; furthermore the first order theory of Kaku and Kikkawa [7] does not produce any vacuum-to-vacuum diagrams in either the free or interacting theories. Our main technique for producing the measure µ is the abstract Wiener space construction of L. Gross [6] (see Appendix B). The path integral description of quantum field theory relates the measure µ to quantities asso- 1 ciated with two dimensional quantum field theory on the Riemann surface S × R as follows. Let 1 0 1 d S` denote the circle of length `, and let dν`;C be the Gaussian measure on S (S` × R; R) ⊗ R with 2 −1 0 1 0 0 1 1 d covariance C = (−∆ + m ) . Then if g; g 2 C ([L0;L1]), t; t 2 R, and f; f 2 C (S ; R) ⊗ R , 1 1 d we may define for 2 C (S` × R; R) ⊗ R Z L1 Z D 1 E Φf;g;t( ) = d` g(`) (s; t); p f(s=`) ds 1 d L0 S` ` R 1 1 d Then the map Φf;g;t : C (S` × R; R) ⊗ R × [L0;L1] ! C extends to a function Φf;g;t 2 Lp(dν`;C × dm[L0;L1]) for all p ≥ 1; NONPERTURBATIVE STRING FIELD THEORY 3 where m[L0;L1] is Lebesgue measure on [L0;L1]: And we have (1.5) q −|t−t0j − 1 (d2=dx2)+m2 Z L1 Z Z L1 `2 1 0 D e 0E d` Φf;g;t Φf 0;g0;t0 dν`;C = d` g(`) g (`) f; f : 0 1 d q L (S1; )⊗ d L0 S (S × ; )⊗ 2 L0 1 2 2 2 2 R R ` R R R − `2 (d =dx ) + m in line with (1.3). 1.3. Interacting String field theory: Formal path integrals. Interacting string field theory is obtained from a cubic function on the space B. We first describe a version of the formal construction 1 d of [7]. We imagine a string { that is, a function φ : S` ! R { breaking up into two loops 1 d φ1 : S`1 ! R 1 d φ2 : S`2 ! R where `1 + `2 = `. In mathematical terms, we have a projection `1;`2 1 d 1 d 1 d (1.6) π` : L2(S ; R) ⊗ R ! (L2(S ; R) ⊗ R ) ⊕ (L2(S ; R) ⊗ R ); giving rise to a map{ `1;`2 (1.7) π` : F ! F ⊗ F and, if we imagine for a moment that the string field Ψ is a function with values in Fock space, we formally obtain a cubic interaction term Z 1 Z L1Z L1 D `1;`2 E (1.8) I(Ψ) = dt d`1 d`2 Ψ(`1; t) ⊗ Ψ(`2; t); π` Ψ(`1 + `2; t) : F −∞ L0 L0 A version of the partition function of the interacting string field theory of [7] is given formally by D E Z R L1 R 1 d L d` −∞ dt Ψ(t;`); i dt −H`;m Ψ(t;`) +λ Re I(Ψ) (1.9) Z(λ) = dΨ e 0 F : The expression (1.9) gives rise to a formal power series, each term of which corresponds to a k directed trivalent ribbon graph Γ whose edges are labeled by two variables te 2 R, `e 2 [L0;L1]. Formally, then, X jAut(Γ)j Z 1 Z 1 Y Z L1 Z L1 Y Z(λ) ∼ λjvert (Γ)j ::: dt ::: d` f (ft g; f` g); jvert (Γ)j! e e Γ e e Γ −∞ −∞ e2e(Γ) L0 L0 e2e(Γ) where jvert (G)j is the number of vertices in Γ and jAut(Γ)j is the order of the group of automor- phisms of Γ : Kaku and Kikkawa's [7] arguments lead one to expect that 2 −d=2 fΓ(fteg; f`eg) ∼ det(−∆Γ;fteg;f`eg + m ) ; where ∆Γ;fteg;f`eg is the Laplacian on a Riemann surface formed by replacing each edge e of the ribbon graph Γ by a tube of length te and width `e, and gluing the corresponding tubes to form a {Note that since Fock space is constructed from the symmetric product of square-integrable loops, which are not continuous, the problem of \gluing" two loops of lengths `1 and `2 to form a loop of length `1 + `2; which is a cause of concern in the physics literature, does not arise.