The S-matrix of superstring field theory

Journal of High Energy Physics, Nov 2015

We show that the classical S-matrix calculated from the recently proposed superstring field theories give the correct perturbative S-matrix. In the proof we exploit the fact that the vertices are obtained by a field redefinition in the large Hilbert space. The result extends to include the NS-NS subsector of type II superstring field theory and the recently found equations of motions for the Ramond fields. In addition, our proof implies that the S-matrix obtained from Berkovits’ WZW-like string field theory then agrees with the perturbative S-matrix to all orders.

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The S-matrix of superstring field theory

HJE The S-matrix of superstring eld theory Sebastian Konopka 0 1 2 0 elds. In addition , our proof implies 1 Theresienstra e 37 , D-80333 Munchen , Germany 2 Arnold Sommerfeld Center , Ludwig-Maximilians Universitat We show that the classical S-matrix calculated from the recently proposed superstring eld theories give the correct perturbative S-matrix. In the proof we exploit the fact that the vertices are obtained by a eld rede nition in the large Hilbert space. The result extends to include the NS-NS subsector of type II superstring eld theory and the recently found equations of motions for the Ramond that the S-matrix obtained from Berkovits' WZW-like string eld theory then agrees with the perturbative S-matrix to all orders. Superstrings and Heterotic Strings; String Field Theory; Gauge Symmetry Homotopy algebras and string eld theory 1 Introduction 2 3 4 5 2.1 2.2 3.1 3.2 3.3 5.1 5.2 5.3 The minimal model The minimal model of an A1-algebra The minimal model and Siegel gauge The minimal model and the S-matrix Evaluation of the minimal model Variations Closed type II-superstring Equations of motion for the Ramond elds Relation to Berkovits' WZW-like theory 6 Conclusions A Strong deformation retracts and cohomomorphisms 1 3 eld theory was proposed [2]. However, its construction was highly formal and turned out to give divergent results due to collisions of local operators on the world-sheet and therefore required regularization [3]. The modi ed string eld theory was proposed in [4, 5] and dealt with the problem by using a modi ed kinetic term. But, it is not clear whether this eld theory reproduces the correct particle spectrum. Recently, a new regularization in terms of small Hilbert space elds and smeared picture changing operators was given [6]. If the latter formulation de nes a valid open superstring eld theory, its S-matrix must necessarily coincide with the usual perturbative string S-matrix calculated in the formalism of picture changing operators [7, 8] or in terms of integrals over { 1 { supermoduli space [9, 10]. In this paper we prove such equivalence to the former formalism at tree-level or genus 0. Let us now outline the main ingredients of this proof for open string eld theory. At the perturbative level bosonic string eld theory provides a de nition of the Polyakov pathintegral for arbitrary matter part with c = 26. This means that its tree-level perturbation series gives rise to a regularized version of integrals over the whole moduli space of punctured discs. The Feynman perturbation series of planar tree-level diagrams in Siegel gauge coincides with the usual description of the color-ordered amplitude as an integral over the positions of all but three punctures. On the other hand the vertices of open string eld theory satisfy the axioms of a cyclic A1 algebra. At the algebraic level, the connection between the S-matrix and the A1 algebraic structure is established through the so-called minimal model. For any A1 structure there exists an A1 structure on the cohomology H (Q) in such a way that this induced structure is A1-quasi-isomorphic to the original one. An explicit formula for the minimal model and the A1-quasi-isomorphisms is formulated in terms of sums over all planar tree diagrams [11] and we argue that the matrix elements of the induced maps coincide with the color-ordered S-matrix. The open superstring eld theory action for the NS-sector in [6] was found by requiring the vertices to be in the small Hilbert space and that they constitute a cyclic A1 algebra. The solution was eventually obtained through a eld rede nition in the large Hilbert space from a free theory. The eld rede nition was constructed by integration of a pair of M = [M; ] M = [ ; ]; where t was a deformation parameters, s a formal parameter counting the so-called picture de cit and was an arbitrary function of M(s; t). In this paper, we show that at the level of the S-matrix this eld rede nition leads to the needed insertions of picture changing operators (PCO) at the external legs. One essential feature of the proof is that it only requires the above two equations. The proof itself is divided into three steps. In the rst step we nd an explicit expression of the minimal model using homological perturbation theory. Next, we argue that the products of the minimal model are identical to the perturbative, color-ordered S-matrix elements. We do this by showing that they satisfy a recursion relation that generates all planar tree diagrams. Finally, we evaluate the minimal model of open superstring eld theory and relate it to the minimal model of the underlying bosonic string products. From which the postulated equivalence of the S-matrix of superstring eld theory with the perturbative S-matrix calculated in the PCO formalism follows. The outline of this paper is as follows: in section 2 we review the reformulation of bosonic string eld theory and open NS-superstring eld theory from [6] in homotopy algebraic language. In section 3, we discuss some mathematical properties of the minimal model and nd an explicit expression through an application of the homological perturbation lemma. Section 4 contains the key result of this paper. We apply the previously described techniques to evaluate the minimal model of open NS-superstring eld theory. { 2 { Quite interestingly, the proof can be adapted to all other superstring eld theories based on homotopy algebraic methods. This includes the extension to the classical closed NS-NS superstring [12], the heterotic NS string [12], the equations of motion for the complete classical open superstring, closed superstring and heterotic string [13]. As the arguments are very similar, we only discuss the extension to the closed NS-NS superstring in section 5.1 and the extension to the equations of motion of the complete open superstring in section 5.2. From there it should be clear that the extension to the remaining cases is straightforward. We also comment on the implications of our results on the S-matrix of Berkovits' WZW-like superstring eld theory in section 5.3. Finally, we present our conclusions. 2 Homotopy algebras and string eld theory In this section we review the cobar description of the Chern-Simons-like formulation of the open NS-superstring given in [6]. We start our short review with Witten's bosonic SFT and explain the modi cations necessary to obtain a description of the open NS-superstring. Afterwards, we review the rst quantized S-matrix and the concept of a minimal model associated to an A1-algebra as well as its connection with the color-ordered S-matrix. 2.1 From Witten's bosonic OSFT to cyclic A1 algebras The action of Witten's open bosonic string eld theory [ 1 ] is formulated in terms of the world-sheet BPZ inner product h ; i, the world-sheet BRST operator Q and a binary product . These algebraic operations act on the Hilbert space of the underlying world-sheet CFT and form a di erential graded algebra (DGA). Furthermore h ; i is an invariant, graded-symmetric bilinear form of ghost number 3. The action of the bosonic string reads then as 1 2 S = h ; Q i + go 3 h ; where go denotes the open string coupling constant. In the following we set go = 1. The string eld is an element in the CFT Hilbert space at ghost number 1. The action enjoys a gauge invariance, where the gauge parameter is an arbitrary element in the CFT Hilbert space of ghost number 0. It turns out [11] that gauge-invariance of S is equivalent to (Q; ) forming a DGA, i.e. they verify the following axioms, a; b; c 2 H: ; i ; = Q + Q2 = 0 Q(a b) = Qa b + ( 1)a a Qb; (a b) c = a (b c): (2.1a) (2.1b) (2.1c) Here we introduced the symbol ( 1)a, which denotes the Grassmann parity of the state a that is equal to the ghost number mod 2. With these physical conventions is an odd quantity. Let us further recall that Q carries ghost number 1 and carries no ghost number. { 3 { One way to study the space of DGAs (and A1-algebras) on a xed vector space is through their coalgebra representation. First one performs a shift in the Grassmannality on the Hilbert space in such a way that all non-essential signs in (2.1) are eliminated and then one treats the axioms as component equations on the tensor algebra T H. Let us do this in greater detail, see [6] for a more detailed review: we work with Z-graded vector spaces so that we consider the same vector space but with di erent gradings as di erent objects. Given a Z-graded vector space H = L k2Z Hk, we can de ne a new Z-graded vector space by setting (H[ 1 ])k = Hk+1. If we forget about the grading, H and H[ 1 ] are identical. The identity map I : H ! H can be lifted to a map s : H[ 1 ] ! H. s is called the suspension and carries degree 1. The latter property is important as it tells us that s is an anticommuting object. For example we have that (s and we can introduce a new string eld s)(s 1 s 1) = = s 1 I. Moreover, s is invertible is even and carries degree 0. As s is invertible, we can express the DGA axioms equivalently on H[ 1 ], where M1 = s 1Qs and M2 = s 1 (s s). We can think of these multilinear equations as linear equations on suitable tensor products of H[ 1 ]. More precisely, we introduce linear maps on the tensor algebra T (H[ 1 ]), Mk : T (H[ 1 ]) ! T (H[ 1 ]); k = 1; 2 via characterized in terms of a coalgebra structure on T (H[ 1 ]). A coalgebra structure on a vector space A is speci ed by linear maps : A ! A 0 A and : A ! C. These maps are called coproduct and counit respectively and are subject to the axioms ( ( On T (H[ 1 ]) the coproduct and counit are de ned as, a1; a2; : : : 2 H[ 1 ], (a1 (a1 a2 a2 X r+s=n an) = 0 (1 ) = 1; an) = (a1 a2 ar) 0 (ar+1 ar+2 an); { 4 { (2.2) where the sum over r and s includes the values r = 0 and s = 0, in which case the corresponding product on the right has to be interpreted as 1 Moreover, the tensor product on T (H[ 1 ]) is written as , while the tensor product in T (T (H[ 1 ])) is denoted as 0. A ( rst order) coderivation M is then a linear map M : A ! A that ful lls a compatibility condition with M = (M 0 IA + IA 0 M) : It can be shown [11] that every coderivation can be written uniquely as a sum M = P1 k=0 Mk, where Mk is of the form (2.2) with Mk : H[ 1 ] k ! H[ 1 ]. See [14] for a detailed proof. We will call Mk the k-string product in M or simply the k-product and write coderivations always in bold face. Another important property of coderivations is their closure under taking (graded) commutators. We will also need the notion of a cohomomorphism F : A ! A0 that is de ned as a linear map intertwining the two coalgebra structures, 0 F = (F 0 F ) = 0F : Any cohomomorphism between tensor coalgebras T (H[ 1 ]) and T (H0[ 1 ]) is completely characterized by its projections fk = 1F k : H[ 1 ] k ! H0[ 1 ], where k and k are the canonical projections k : T (H[ 1 ]) (H[ 1 ]) k and inclusion maps k : (H[ 1 ]) k , ! T (H[ 1 ]). The most general form of a cohomomorphism is then given by [14] F = 1 X X n=0 r1;r2;:::;rn fr1 fr2 frn = (2.3) 1 n=0 X( 1F ) n: With this new terminology we see that a DGA structure on H is equivalent to a degree 1 coderivation M on T (H[ 1 ]) that only has a non-trivial 1- and 2-product and is square zero M2 = 12 [M; M] = 0. A square zero coderivation is also called a codi erential. From an algebraic point of view it seems very natural to relax the condition that M contains 1- and 2-products and just consider arbitrary degree 1 codi erentials. We obtain a generalization of a DGA called a weakly homotopy associative algebra. If the 0-product (or tadpole) vanishes, i.e. M0 = 0, the algebraic structure is called a strongly homotopy associative algebra or A1 algebra. The rst few axioms of an A1 algebra are M1M3 + M3(M1 I 2 + I M1 I + I 2 The rst two axioms are identical to the DGA axioms, but the associativity condition is only enforced up to M1-exact terms. The conditions at higher order impose coherence conditions onto the higher products. Witten's bosonic OSFT is also equipped with an invariant bilinear form h ; i. The invariance follows from the cyclicity of the action and states that for a; b; c 2 H, hQa; bi + ( 1)aha; Qbi = 0 of the cyclicity conditions (2.4) to arbitrary strongly homotopy associative algebras is now straightforward and the resulting algebraic structure is called a cyclic A1 algebra. A rich source of cyclic A1 algebras is provided by solutions to classical BV master equations, if functions on eld space are required to be non-commutative but cyclic polynomials. Indeed, it can be shown that a cyclic A1 structure is equivalent to a solution of a classical BV master equation fS; Sg = 0 [11]. It has been shown in [15] that for any cyclically invariant decomposition of the moduli space of punctured disks, the corresponding string eld theory action satis es a classical BV-master equation. Hence, the products Mk together with the BPZ inner product de ne a cyclic A1 algebra. For the bosonic open string in nitesimal deformations of the cyclic A1 structure modulo eld rede nitions are classi ed by physical closed string states [16], so that at least perturbatively the study of the cyclic BV-complex is equivalent to the world-sheet perspective. In the rest of the paper we work in the suspended Hilbert space H[ 1 ] only and we drop the su x [ 1 ]. Occasionally, we use Q instead of M1 to make some formulas look more familiar. 2.2 Homotopy algebraic description of superstring eld theory The main aim of [6] has been to formulate a consistent eld theory of interacting open superstrings in the small Hilbert space. However, due to presence of an additional conserved charge called picture its formulation was not straightforward. Instead of constructing a eld theory geometrically, a di erent approach was chosen: the primary goal was to nd a nontrivial solution to the classical BV-master equation such that the string eld lives in the small Hilbert space and carries ghost number 1 and picture number 1. Moreover, it was required that the kinetic term be given by the world-sheet BRST operator Q to ensure that the linearized equations of motion have the correct solution space. At the conceptual level, the formulation of classical perturbative open superstring theory in terms of picture changing operators (PCOs) was rewritten as a eld theory. The complete answer was formulated as the result of a recursive algorithm, which takes as input the bosonic vertices together with a contracting homotopy of [ ; ] acting on coderivations. In order to express the recursive structure most clearly, the so-called picture de cit, a new grading on the space of coderivations was introduced: we say that a coderivation Mn has picture de cit def(Mn) = n 1 pic(Mn), where pic(Mn) denotes the picture number of Mn. With this new grading, the restriction on picture number on the state space was lifted. A generic { 6 { coderivation can be expanded in terms of picture de cit as where def(M[k]) = k. In order to keep track of the various homogeneous components M[k], we introduced a formal variable s of picture de cit 1 and used an identi cation Coder(T H) = (Coder(T H) C[s])[0] under which M = The physical vertices that enter the action come then from a coderivation M[0] with picture de cit 0. By construction, the coderivation M has now picture de cit 0. With these ingredients, the recursive algorithm is obtained from integrating the ow of a vector eld Mjt=0 = M0 = Q + sM0;2 + s2M0;3 + : : : M = is an arbitrary coderivation solving the last equation. In [6], a particular contracting homotopy for [ ; ] was used to solve the last equation for . In [6] it was shown that this recursive algorithm always yields the nal string products in M[0] up to a given number of inputs in a nite number of steps and that changing the duration t of the ow yields is equivalent to a change in the coupling constant. We want to emphasize that our proof works for arbitrary families M(s; t) that solve equation (2.6) and (2.7) and does not depend on any particular choice of homotopy. Let us review the particular contracting homotopy for [ ; ] used in [6] to solve equation (2.7). For unrelated reasons, it will also be useful (but not essential) in computing the S-matrix. Given that the cohomology of 0 is trivial, we take a contracting homotopy (which must be of the sum of a BPZ-even operator and an -closed BPZ-odd operator that we set to zero). For a coderivation N constructed from a single k-product Nk, we de ne N as 1( { 7 { (2.5) (2.6) (2.7) X where X = [Q; ] is a picture changing operator used in [6]. The essential idea behind cohomology theories is that certain quantities interest can be represented in many di erent ways. Such quantities could be geometrical, topological invariants in mathematics or scattering matrix elements in physics. The various representations of that data are called models for the cohomology theory. When modelled with the help of dg-chain complexes, they typically include lots of auxilliary data and encode the physical information in the cohomology of some di erential Q together with some additional algebraic structure on that cohomology, like the gauge-invariant \S-matrix". The calculation of the gauge-invariant data living on the cohomology can be done in various models. Some of them lead to nice interpretations, while some of them allow for easy calculations. A1 algebras are special cases of this idea: every A1 algebra M induces an A1algebra structure M~ on the cohomology H (Q), the so-called minimal model. We need a little more terminology. Given two A1 algebras on H and H0 described by coderivations M and M0, we can de ne an A1-morphism F : (H; M) ! (H0; M0) as a cohomomorphism F : T H ! T H0 that intertwines both structures, F M = M0F . F is called an A1isomorphism if it is invertible as a cohomomorphism. Let us denote by fk = 1F k the component maps of F . The component f1 must the satisfy f1Q = Q0f1: Consequently f1 is a chain map and, therefore, gives rise to a map H (Q) ! H (Q0). If the latter map is invertible, F is called an A1-quasi-isomorphism. An A1-algebra is called minimal if Q = 0. The important minimal model theorem states that any A1-algebra is isomorphic to a minimal A1-algebra and that this minimal model characterizes the A1 algebra M completely, i.e. any two A1 algebras with A1-isomorphic minimal models are quasi-isomorphic [11, 17]. A nice review is [ 18 ]. In the following we want to motivate the construction of the minimal model structure M~ via homological perturbation theory. In general there are two ways to de ne an algebraic structure on H (Q). The rst approach takes arbitrary representatives for each cohomology class and de nes a cohomology class by specifying some Q-closed vector. Then, one has to show that by changing the representative by a Q-exact piece modi es the answer only by a Q-exact piece. Alternatively, one can make a xed choice of representative for each cohomology class and de ne the structure on them. The drawback of the latter construction is that it depends on the particular choice of representative and is not manifestly { 8 { independent of it. Since one does not expect that the algebraic structures are independent of the choice of representative, one should at least require that two di erent choices give rise to isomorphic algebraic structures. In our case at hand, the rst method only works for 2-product so that we need to resort to the second method for the higher products. Let us now see how the rst two products of the minimal model structure are constructed explicitly. In the rst approach the induced 2-product M~ 2 is obviously de ned through the product M2: M~ 2 = M2: the canonical projection p : ker(Q) Qy such that we have H This is a good choice because Q is a derivation of this product so that the product of two Q-closed vectors is again Q-closed and shifting the representative by a Q-exact piece shifts the product by a Q-exact term. Thus, we have de ned a binary product on cohomology M~ 2 : H (Q) H (Q) ! H (Q). Since M2 is associative up to homotopy, the induced product is completely associative. At this point we already have obtained an A1-algebraic structure on the cohomology. Unfortunately, this new structure is not the minimal model because it is not A1-quasi-isomorphic to the original structure in general. Finding the 3-product is a little bit more complicated. The naive guess M~ 3 = M3 does not work, since it does not map Q-closed states into Q-closed states. Unless the induced 2-product on cohomology is trivial, there is no way to de ne the 3-product such that all Q-exact states decouple, so that the rst method fails and we have to resort the the second. Choosing a representative for each cohomology class means that we select a section i : H (Q) ,! H of H (Q). Consequently we can nd a homotopy (3.1a) (3.1b) HJEP1(205)87 Our choice of binary product can then be expressed as I = p i; I QyQ QQy = i p P: M~ 2 = p M2 i 2: Using the A1-relations together with (3.1) one can show that this is indeed an associative product. Playing around a little bit, one discovers that the following product M~ 3 = p M3 + M2(( QyM2) I + I ( QyM2)) i 3 satis es [M~ 3; M~ 2] = 0. Hence, M~ 2 and M~ 3 satisfy the rst two non-trivial A1 relations. As an example consider the DGA of di erential forms on a compact manifold X, the socalled de Rham complex. The cohomology theory that it models is purely topological and is known as the cohomology of X. The cohomology classes are in one-to-one correspondence with the homology H (X) through Poincare duality. The induced associative product M~ 2 is then the cup product. If X is orientable, we can also endow the DGA with an invariant symplectic form so that we obtain a cyclic DGA. The symplectic form is given by integration over X. In this case the product M~ 2 also calculates intersection numbers { 9 { between cycles. The higher products M~ k; k give re ned topological information about X [ 18 ]. 3 correspond to the Massey products and Guessing the higher order products is quite cumbersome and we want a systematic way to construct them. The answer is given in terms of the homological perturbation lemma [ 19 ]. Our goal is to construct M~ together with a pair of mutually inverse A1quasi-isomorphisms from/to the original A1-structure. This means that we look for A1morphisms p : (H; M) ! (H (Q); M~ ) and i : (H (Q); M~ ) ! (H; M) such that pi = IT H and ip = IT H, where the last requirement means that ip is homotopic to IT H, i.e. there is a homotopy H : T H ! T H such that ip = IT H + MH + HM HJEP1(205)87 If M = Q there are no induced products and the problem is easily solved in terms of Qy, p and i from (3.1). The appropriate choices are on T H (3.2a) (3.2b) (3.2c) ( Qy) P s pM = M~ p iM~ = Mi: H = p = i = M~ = 0: X I r r;s 0 X p r; They satisfy important compatibility conditions with the coalgebra structures and T Hp, The homological perturbation lemma allows us to modify this solution of the \free" problem into a solution of the complete problem. Let V be a graded vector space and d some di erential on V . The homological perturbation lemma [ 19 ] gives then a collection of formul that allow for calculating the cohomology of a perturbed di erential d + , where is small in an appropriate sense. For our purposes one may think of V = T H and of d as the codi erential Q representing the free theory. We treat then the di erence Mint = M Q as a perturbation in the sense of HPT. In order to state the perturbation lemma, we need a little bit more terminology. Let (V; d) and (W; D) be two chain complexes and let p : V ! W and i : W ! V be chain maps, s.t. pi = I and P^ = ip = I + hd + dh for some linear map h : V ! V called the homotopy. This collection of data (V; W; d; D; p; i; h) is called homotopy equivalence data and (W; D) is said to be a deformation retract of (V; d). Let now be a perturbation of the chain complex (V; d), i.e. (d + )2 = 0. should be small in the sense that (1 exists. In our context = Mint represents the interaction part of the theory and is proportional to some coupling constant, so that the inverse exists at least perturbatively in the coupling constant. The main statement of the perturbation lemma is now that it is possible to deform the rest of the homotopy equivalence data in such a way that one retains valid homotopy equivalence data. More precisely, If the homotopy h satis es additional properties, then (W; D) is called a strong deformation retract of (V; d). Applying the homological perturbation lemma to the case at hand means replacing In particular, the new di erential on M~ on T H (Q) reads as M~ = p(1 MintH) 1Minti: (3.5) Since (T H (Q); 0) is a strong deformation retract of (T H; Q), one can show that M~ is a coderivation and so de nes a minimal A1 structure on H (Q) and that p and i are cohomomorphisms. See appendix A for further details. 3.2 The minimal model and Siegel gauge In string eld theory the S-matrix is usually calculated in Siegel gauge. The various terms in the S-matrix calculated with the Siegel gauge propagator have a nice geometric interpretation in terms of disks obtained by glueing strips between the vertices and integrating over their lengths. A standard result in bosonic string eld theory implies that one obtains the correct perturbative S-matrix [ 20, 21 ]. In this section we assume that the string background is at Minkowski space or contains some uncompacti ed directions. The construction of the minimal model required the choice of contracting homotopy Qy of the Hilbert space onto the cohomology. While such an operator always exists, it is not necessarily equal to the Siegel gauge propagator. To see this, we make the choice Qy = Lb00 (1 this it follows that the physical projector P is given by e 1L0 ). From P = 1 [Q; Qy] = e 1L0 : Although P is a projection operator, its image also contains unphysical states as QP 6= 0 and P is not a projection operator onto H (Q) but onto a larger vector space. We obtain a deformation retract of the original Hilbert space onto the image of P . At the end of section 3.1 we argued that if we start with a strong deformation retract, we obtain an A1 algebraic structure on Hp = P H together with a pair of A1-quasi-isomorphisms. Actually, we can relax these assumptions a little bit and require only that Qyi = pQy = (Qy)2 = 0. The di erential on Hp is QP . Application of the homological perturbation lemma means that we set (3.6) (3.7) This gives then the induced A1-structure S(M) as S(M) = QP^ + P^Mint(1 We call S(M) the almost minimal model and call the actual minimal model M~ the algebraic minimal model whenever these distinctions are relevant. Since S(M) and M are A1-quasiisomorphic to each other, by the minimal model theorem the minimal model will be M~ in both cases. As discussed in the next section, the maps in S(M) are given by sums over planar tree-level diagrams with propagators Lb00 . Calculation of the minimal model for S(M) requires us to choose a contracting homotopy for QP . The projector P puts the states onto the mass-shell. This means that for operator O1 and O2 that are obtained by restricting space-time momentum preserving operators on H to operators on Hp we have the identity O1P O2 = 0 for states with generic momentum. Thus, only diagrams with no internal lines contribute generically to the minimal model maps, but the only such diagrams are the vertices of S(M), which are identical to the perturbative S-matrix and coincide with the minimal model generically. Consequently, we can calculate the S-matrix either in Siegel gauge or using a complete gauge- xing, but obtain the same answers. 3.3 The minimal model and the S-matrix From a physical point of view the relevant information contained in the equations of motion are the observables and their expectation values. Observables are functions of the elds , but we are not interested in arbitrary such functions, but only those that are gauge invariant. Moreover, we identify two gauge invariant functions if their di erence vanishes on-shell, i.e. is proportional to the equations of motion. The equivalence classes are the observables and can be thought of as functions on the moduli space of solutions modulo gauge-equivalence. The S-matrix measures then the obstruction for this moduli space to be smooth at = 0 [ 22 ]. The most popular method for calculating the S-matrix perturbatively is through the use of Feynman diagrams. This approach, however, is not necessarily the best method for our purposes because the combinatorics for large n-point amplitudes is rather involved. Instead we use homological perturbation theory (HPT) which hides this di culty and gives easy to manipulate formul for the S-matrix. Using HPT to generate the full Feynman perturbation series is not new, see for example [23{25]. In the previous section we constructed the almost minimal model S(M) of M. The claim is that its products are identical to the color-ordered S-matrix elements of a string eld theory with vertices encoded in the codi erential M. More precisely, we claim that the color-ordered S-matrix S for Q-closed states i can be written as S( 1; 2; : : : ; n+1) = ( 1) 1 !( 1 1S(M)n( 2; 3; : : : ; n+1)); or more abstractly as where P^ is an extension of the physical projector to the tensor algebra, ! the symplectic form. The almost minimal model S(M) is gauge-invariant in the sense that Let us now justify these formulas. The products in S(M) are identical to the color-ordered S-matrix elements of the underlying eld theory d + since they are A1-quasi isomorphic and thus have identical moduli spaces [11]. In the L1-case this has been shown by Kontsevich in [17] in his proof of deformation quantization. However, there is a more elementary way to verify this claim. The essential contribution to S(M) is given by P^Mint(1 We want to interpret this expression as a sum over planar rooted tree diagrams. To this end, we need to introduce a set of Feynman rules. A planar rooted tree diagram is a planar graph of genus 0 with a distinguished external line that we call its root. A Feynman 1 ; 2; : : : ; n is obtained from a planar rooted tree with n external 1. Assign the state ( 1) 1 !( 1 I) to the root, 2 to the rst leg next to the root in (3.8) (3.9) clockwise order and so on. the number of edges connected to it. 2. To the vertex connected to the root assign the multilinear map Mk, where k + 1 is 3. To each other vertex of valence k + 1 assign the operator QyMk. 4. Compose these multilinear maps according to the shape of the diagram. M2( QyM2( 2; 3) QyM2( 4; 5))) nology we consider equation (3.9) and show that it is equal to the sum over all Feynman diagrams as just de ned. Let us de ne two maps on the tensor algebra,1 agrees with (3.9). The map A is a cohomomorphism and, consequently, is determined by its component map 1A : T H ! H. Using the explicit form (2.3) of a cohomomorphism and the de nition of the maps Mk in equation (2.2), one easily deduces the following pair of equations, 1A = P + ( Qy) 1 X Mk(P + ( Qy) 1 ) k: X Equation (3.10) provides us with a recursive algorithm for the restrictions of 1 to n inputs. The reason being that the sum on the right hand side starts at k = 2 and so involves restrictions of 1 to at most n 1 inputs. A graphical representation of equation (3.10) can be found in gure (2). From the graphical representation it follows that 1 is constructed from a sum over all planar tree-level Feynman diagrams. Equation (3.10) is recognized as the classical Dyson-Schwinger equation once one identi es Qy with the propagator and Mk as interaction vertices of an action. The important property (3.8) that encodes the gauge-invariance of the S-matrix follows straightforwardly, [Q; S(M)] = (1 MintH) 1 [Q; MintH]Mint(1 HMint) 1 + [Q; Mint] = S(M)P^S(M) = 0; where in the last step we used that fact that internal lines are generically o -shell. Finally, we discuss the cyclicity properties of the S-matrix obtained from the almost minimal model for a cyclic A1-structure. Since the operator Q is BPZ-odd, any contracting homotopy Qy can be written as the sum of a BPZ-even operator and a BPZ-odd and 1At rst one might think that one could replace HMint with QyMint in the next formula, but this leads to the wrong combinatorics. For example for the 5-point function it generates too many tree diagrams with two 3-vertices. The projections P play an important role in establishing the identi cation with the S-matrix. and small solid circles elementary vertices Mk. Internal lines correspond to Qy. External lines pointing to the right represent physical states. Q-closed operator , [Q; ] = 0. In order to calculate the deviation from cyclicity, we consider the sum, !( 1 P + P 1 ) = X ! Mk(P + ( Qy) 1 ) k P k 2 = X ! ( Qy) 1 k 2 = 0. Moreover, if = [Q; R] for some operator R, the gauge-invariance (3.8) tells us that the S-matrix is still cyclic, so that one can relax the requirement that Qy has to be BPZ-even. 4 Evaluation of the minimal model In this section we apply the formalism for tree-level perturbation theory explained in the previous section to open superstring eld theory. The original formulation can be found in [6] and we review the relevant main ingredients and then express its S-matrix in terms of the usual perturbative S-matrix. Equations (2.6) and (2.7) are the only two ingredients used in the evaluation of the almost minimal model along with a choice of propagator Qy. We claim that the following equation holds X where T denotes some coderivation whose particular form is not relevant for the S-matrix. Before proving formula (4.1), we deduce the announced equivalence to the usual superstring S-matrix. To this end we recall that the S-matrix elements of our theory are calculated from the physical vertices M[0], that is from the string products that are proportional to s0. As the formula for the almost minimal model (3.6) does not involve any operations on s, we conclude that the coe cient of s0 in S(M), S(M)[0], must be identical to the S-matrix of our eld theory. Let us consider the n-product S(M)n. Since picture de cit (3.11) (4.1) is additive when composing multilinear maps, the highest power in s of S(M)n must be n 1. This is precisely the case when each vertex has maximal possible picture de cit. But these vertices are identical to the bosonic vertices, so that S(M)[n 1] must calculate the bosonic S-matrix elements. In order to convert the S(M) into a real S-matrix, we need to convert its output into an input using the symplectic form on the small Hilbert space !S = !(I ), where the right-hand side is expressed in terms of the large Hilbert space symplectic form !. The S-matrix now reads as This linear functional has to be evaluated on vectors in the small Hilbert space that are generalized solutions to the equation Q = 0. Moreover, this functional also has an expansion in terms of the variable s. Let us now look at the various coe cients of s in (4.1). The right-hand side is a Q-exact and -exact coderivation, so that we nd on H (Qj ), !(I where we used the compatibility of ! with both Q and . Thus, we can deduce from (4.1) the recursion relation, k In order to solve this hierarchy of di erential equations, we x the number of external states to n + 1 and consider Sn+1 = S n+1 and we nd that it satis es the following di erential equations, . . 1 n + 1 r+s=n X In the last equation we obtain zero, because all vertices have maximal possible picture de cit. These equations can be integrated, if we use the initial conditions Sn[n+] 1(0) = Snb+os1 and Sn[k+] 1(0) = 0 for k < n, where Snb+os1 denotes the bosonic (n + 1)-S-matrix element. The result is, Sn[0+]1(t) = tnSnb+os1Xn 1 : Moving around the picture changing operators X does not change the S-matrix because of equation (3.8). We can therefore distribute the PCOs such that each external leg has at most one X. Hence, the S-matrix S[0] can be calculated by taking all but two vertex operators in the 0-picture and the remaining two in the 1-picture. The functional Sbos then inserts these vertex operators at the boundary of a disc and integrates over the possible positions, essentially by the validity of the usual bosonic string eld theory construction. Consequently, S[0] is identical to the perturbative string S-matrix, as claimed. It remains to prove (4.1). Before we consider the completely general case, we concentrate on the 3-product in (4.1) for which the proof can be carried out by hand. In this case S(M)3 reads as S(M)3 = P^(M3 + M2( QyM2 Note that is equation is between polynomials in s, so that it actually represents multiple M2( QyM2 I + I QyM2) I + I QyM2) + M2 Qy M2 I + I Qy M2 ^ P + M2( Qy[ ; 2 ] I + I Qy[ ; 2]) P^ = P^ [ ; 3 + 2 ( QyM2 I + I QyM2) + M2( Qy 2 I + I Qy 2)] P^: In order to proceed, we apply [Q; ] to the second equality and use the property [ ; ] + [ ; ] = I to nd, Q; ( QyM2 I + I QyM2) = [Q; [ ; + M2( Qy 2 I + I Qy 2)]P^ where the ellipsis corresponds to some irrelevant terms contributing to T. Rearranging the latter equation a little bit yields, X During the calculation we made use of equation (3.7), which allows us to drop terms involving the physical projector P between operators when evaluating this expression on physical states, because generically the internal lines will be o -shell. Thus, for n = 3, (4.1) follows. The general case can be derived analogously. The starting point is equation (3.6). The homotopy H is constructed from the propagator Qy and from the physical projector P via formula (3.2c). It follows then straightforwardly that MH(1 MintH) 1MintP^ + P^(1 MP^ = P^(1 = P^(1 MintH) 1[M; ]H(1 MintH) 1MintP^ + P^(1 MintH) 1[M; ]P^ MintH) 1[M; ](1 M(1 = P^(1 MintH) 1[ ; ](1 where in the last step we used the fact that the interaction term, the physical projector and the homotopy H commute with the coderivation . Note that is a coderivation. Now, we solve (4.6) for modulo -exact terms using the contracting homotopy . We nd that = In order to produce a PCO instead of a on the right-hand side, we calculate the commutator with the coderivation Q. Calculating the commutator of Q with operators of the form (1 A) 1 is easy, once one recognizes that the Leibniz rule for [Q; ] implies that A) 1] = (1 A) 1[Q; A](1 A) 1. In our case we have A = MintH and, hence, [Q; ] = P^(1 MintH) 1[Q; MintH] P^ + P^ [Q; HMint](1 Note that M2 = 0 implies that [Q; Mint] = Mi2nt and that H is a homotopy from I to P^, see (3.3). Therefore, the rst and the second commutators yield [Q; MintH] = Mint (1 MintH) [Q; HMint] = (1 HMint) Mint + P^Mint: MintP^; Using these results, we can simplify (4.8) further and arrive at the identity [Q; ] = P^(1 MintH) 1[M; ](1 HMint) 1P^ = Using equation (4.7) together with the gauge-invariance of the S-matrix (3.8), we nally deduce a relation of the form X [Q; [ ; ]]; (4.10) from which the main equation (4.1) follows. This concludes the proof of equivalence of the superstring eld theory proposed in [6] with the ordinary perturbative string S-matrix for open superstrings in the NS-sector. In the previous section we presented a proof of the equivalence of open superstring eld theory to usual perturbative string theory in the NS-sector. However, the homological perturbation theoretical proof is applicable to some other, closely related physical systems: the action of NS-NS sector of closed type II-superstring theory [12] and the extension to the R-sectors at the level of the equations of motion [13]. In both cases the construction is obtained by integrating the ow generated by an exact homological vector eld on the formal manifold of homotopy algebraic structures. Now, in both cases the fundamental equation (4.1) still holds true, X From the proof in section 4 this follows quite trivially, because we only assumed that 5.1 Closed type II-superstring On the world-sheet of a closed type II superstring we have holomorphic and antiholomorphic degrees of freedom. On-shell NS-NS vertex operators in Siegel gauge take the form V = Vm(z; z)c(z)c(z) ( (z)) ( (z)), where Vm denotes a superconformal matter primary eld of dimension ( 1 ; 12 ). The type II-worldsheet is a super Riemann surface with two 2 odd directions and the world-sheet theory now comes with a holomorphic and an antiholomorphic picture number, both of which have to add up to 2 in order to obtain a non-vanishing correlator. In [12] a holomorphic and an antiholomorphic picture de cit together with formal variables s and s were introduced. A generic coderivation L can then be identi ed with L = X skslL[k;l]; k;l 0 where L[k;l] has holomorphic picture de cit k and antiholomorphic picture de cit l. Closed string products are graded-symmetric, hence, the underlying homotopy algebraic structure is an L 1 algebra instead of an A1-algebra. However, it is possible to take the universal envelope of an L1-algebra [26] and obtain an A1-algebra to which the usual construction can be applied. Alternatively, one can think of the construction in the dual geometric picture and skip the universal enveloping algebra completely. Eventually, two vector elds and were introduced, [ ; ] = L = [L; ] [ ; ] = L = [L; ] (5.1) (5.2) The equations (5.2) were then solved using the special contracting homotopy for or that was built using the zero-modes of the - or - elds. This was required to preserve the constraint b0 = L 0 = 0 on the closed-string state space. However, in the following we do not require this choice for and . The closed string products are then obtained by integrating the ow of (some, possibly t-dependent) linear combination of and starting at a point where L = Lbos coincides with the closed string vertices of closed bosonic string eld theory [21] and the so-obtained closed string products are all related by a eld rede nition in the small Hilbert space.2 It therefore su ces to consider the special case for which we integrate + : Let us remark that our argument also works for arbitrary t-dependent linear combinations for , but for simplicity we restrict to this special choice. The main equation (5.1) now (5.3) The rest of the argument is very similar to the one given in section 4. We will only work out the details for the four-point S-matrix elements here. The closed string S-matrix elements are calculated from S(L) using the symplectic form !S = !(I 0 0c0 ), where ! denotes the BPZ-inner product for the world-sheet theory formulated in the large Hilbert space. The S-matrix is then the restriction of the functional S S = !S(I S(L)) to the relative cohomology H (Qj ; ). Equation (5.3) decomposes into a system of differential equations in the deformation parameter t by reading o the coe cients of terms homogeneous in s and s: @ S[0;0](t) = S[1;0](t)X + S[0;1](t)X 4 [1;0](t) = 2S4[2;0](t)X + S[1;1](t)X 4 [0;1](t) = S[1;1](t)X + 2S4[0;2](t)X 4 [1;1](t) = 2S4[2;1](t)X + 2S4[1;2](t)X [2;0](t) = S[2;1](t)X [0;2](t) = S[1;2](t)X [2;1](t) = 2S4[2;2](t)X [1;2](t) = 2S4[2;2](t)X 2This can be seen by showing that the commutator [ ; ] is a eld rede nition. In fact, [ ; ]L = [L; ] with = [ ; ] + [L; of L1-structures modulo eld rede nitions [ ; ] = 0 and the endpoint of the ow of a + b depends only on the duration in or -direction. In the last equation we used the fact that the highest picture de cit for 3-products in this construction is [2; 2] so that there are no source terms of the last di erential equation. Indeed, the functional S[2;2] is identical to the S-matrix calculated from bosonic CSFT described by the initial vertices. It is clear that this system of equations can be integrated directly and we can express the S-matrix S[0;0] in terms of the bosonic CSFT-S-matrix Sbos;4 = S[2;2](0) and picture changing operators X and X located at the punctures, 4 4 S[0;0] = Sbos;4X2X2: 4 Moreover, if the external states are on-shell, we can move the PCOs arbitarily and may adjust them such that all external states are in the (0; 0) picture except for two that are in the ( 1; 1) picture. Equations of motion for the Ramond elds Formulating the dynamics of the Ramond string elds in the small Hilbert space using an action principle is still an open problem. Finding covariant equations of motion is a somewhat simpler problem and was solved recently using homotopy algebraic methods [13]. In this subsection we brie y describe this construction but using a more condensed notation that makes the connection to the methods used here more clearly without obscuring the overall picture by details. Furthermore, we only discuss the validity of the resulting equations of motion for the open superstring obtained from the stubi ed bosonic open string products. The extension to the closed type II superstring and the heterotic string contains no new conceptual ideas and we leave the details to the enthusiastic reader. The string eld = NS + R now takes values in the CFT state space HNS HR, where the NS eld is at picture 1 and the R eld is at picture . The nal result of the construction of [13] is a homological vector eld on the non-commutative manifold whose function ring is given by T H . The zeros of this vector eld coincide with the solutions to the equations of motions. Let us recall that if this vector eld came from an action S, then it could be written as a normal vector to the surface S = const: using 1 2 a symplectic form to identify tangent with cotangent vectors. However, no consistent truncation of the open string state space is known so that the BPZ inner product reduces to a symplectic form in the R-directions. If such a product would exist, then one could try to nd an integrating factor such that the vector eld actually becomes a gradient vector eld of some function. However, even if no action for a set of equations of motion is known, there are still some questions that can be answered. For example, one can ask about the structure of the space of solutions modulo gauge transformations. The cotangent complex of such an algebraic variety is characterized by the classical S-matrix, when the equations of motion arise from an action. Thus, we can still calculate the S-matrix for the R-equations of motion. Equality to the S-matrix of perturbative string theory is a necessary criterion for calling the equations from [13] equations of motions for the open superstring as the universal deformations should coincide. In [27] it was shown that one may calculate the classical S-matrix directly from a pseudo-action. The obstruction to the existence of smooth directions in the moduli space of solutions is measured by the minimal model of the homological vector eld [ 22 ]. The minimal model is an A1 structure on H (Qj ) that is obtained by restricting S(M) to the cohomology. We may contract the minimal model structure with a non-degenerate symplectic form on H (Qj ) to obtain a linear functional S, which is the classical S-matrix of the equations of motion. In order to de ne said symplectic structure, we need to introduce an inverse picture changing operators Y that is required to be BPZ-even and a homotopy inverse of X. We now introduce an operator O by The sought for symplectic form !~ is now in terms of the large Hilbert space BPZ-inner product !, HJEP1(205)87 O = NS + Y R: !~ = !(I O): It is readily checked that !~ is Q-closed and, hence, descends to a non-degenerate pairing on H (Qj ). The S-matrix for a homological vector eld M is then S = !~(I 1S(M)): (5.4) The main di erence to the construction for the pure NS-subsector is that we now have two component elds NS and R which carry di erent picture number. Thus, the required number of PCO insertions will depend on the sector of the inputs to a vertex. In general, if we substitute a pair of NS string elds with a pair of R string elds, the number of required PCOs reduces by one. In order to keep track of these requirements, in [13] a new auxiliary quantum number, Ramond number, was introduced and the corresponding formal variables was called u, i.e. all coderivations are formal power series in u with coe cients of ur having Ramond number r. A product Mn has Ramond number r if and only if it vanishes unless it has 2r or 2r + 1 number of Ramond inputs. Moreover, the notation Mnj2r was introduced to denote the restriction to Ramond number r. Notice that Ramond number is additive under taking commutators of coderivations so that u indeed counts Ramond number. Denote now by Nk the bosonic open string products and de ne a codi erential M(0) M(0) = Q + uN2j2 + sN2j0 + s2(N3j0 + uN4j2) + = 1 X n=0;r=0 snurNn+r+1j2r: We use M(0) as a starting point for the usual deformation equations (2.6), (2.7) and integrate the ow. The equations of motion are then the Maurer-Cartan equations for the codi erential M at the end of the ow, when restricted to picture de cit 0. By construction the products in M[0] produce string elds in picture 1 or We are now ready to evaluate the S-matrix (5.4) in the same way as in section 4. The functional S is then equal to the bosonic S-matrix with vertex operators inserted in the correct picture if the output of S(M) is an NS-state. If it is an R-state, we can use one of the PCOs to remove the Y operator at the output and we still obtain the perturbative string S-matrix. Let us see how 1 2 . this works for the four-point amplitude of two R-states R1 and R2 with two N S-states N S1 and N S2. The relevant component of S4[0] has Ramond number 0 and is given in terms of the bosonic S-matrix S(Mbos) as S4[0](R1; R2; N S1; N S2) = !(R1 Y (X X S(Mbos)(R2; N S1; N S2)) = !(XY R1 S(Mbos)(XR2; N S1; N S2)) = Sbos(R1; XR2; N S1; N S2): This concludes our discussion of the validity of the construction in [13] as valid Ramond equations of motion. Relation to Berkovits' WZW-like theory Our result has further implications. In [28] it was shown that the CS-like formulation of open super string eld theory from [6] is related to a gauge- xed version of Berkovits' WZW-type super string eld theory through a eld rede nition. Now, since the S-matrix is invariant under eld rede nitions up to a similarity transformation, our result states that the S-matrix of Berkovits' WZW-type formulation agrees with the usual perturbative super string S-matrix. Previously [29{31] some checks in this direction were performed, but remained restricted to the four-point and ve-point S-matrix elements. Equivalence of CS-like heterotic string eld theory and its WZW-like formulation has been studied recently in [32]. 6 We showed the equivalence of perturbative string theory with the super string eld theories based on the small Hilbert space. This equivalence requires that the solution space of the linearized equations of motion coincides with the physical string spectrum and that the S-matrix around a given vacuum agrees with that of perturbative string theory. The rst requirement was true by construction and we only had to show the second. In doing so, the special form of the cohomological vector eld encoding the equations of motion was crucial: it allowed us to relate the S-matrix of the underlying bosonic string eld theory to the real S-matrix by a sequence of descent equations (4.4) without employing complicated combinatorial arguments involving Feynman diagrams or worldsheet diagrams. Despite the progress at the algebraic level, there are still some open questions to address. For example, it would be interesting to see if and how the algebraic construction and properties arise from the world-sheet point of view. Since the formulation is entirely in terms of the small Hilbert space expressing the interaction vertices in terms of integrals over the moduli space of super Riemann surfaces should be easier than in the large Hilbert space formulations. However, even though formulated in terms of small Hilbert space elds, we still use the bosonized - -ghosts. A rst step towards a geometric formulation would be to reformulate the construction in terms of operators manifestly built from modes of the and -ghosts and to nd a geometric interpretation of the descent equations. Quantization of the theory necessitates an action principle. However, since only equations of motion are known for the Ramond string elds, the rst step must be to reformulate them in terms of an action principle. In turn we would need to nd a suitable symplectic form of picture number 1 on the space of (o -shell) Ramond elds. Most likely such a construction would require a constraint on the Hilbert space, similar to the construction of closed string eld theory. Some proposals [33, 34] in this direction exist, but it is not clear if these constraints do not alter the spectrum of physical states. Acknowledgments The author would like to thank Ted Erler for critically reading the manuscript and Ivo HJEP1(205)87 Sachs for helpful comments. This project was supported in parts by the DFG Transregional Collaborative Research Centre TRR 33, the DFG cluster of excellence Origin and Structure of the Universe. A Strong deformation retracts and cohomomorphisms In this appendix we want to elaborate on some claims made in section 3 according which the perturbed projections p0 and inclusions i0 are cohomomorphisms and the perturbed di erential D0 is actually a coderivation. We claimed that the property pQy = Qyi = (Qy)2 = 0, which implies that we are working with a strong deformation retract, is su cient. The key equations of this proof are the following two identities, (h 0 ip + I 0 h)( 0 I + I 0 ) k (i 0 i) = (p 0 p) (h 0 ip + I 0 h)( 0 I + I 0 ) k = X X r+s=k;r;s 0 r+s=k;r;s 0 ((h )r 0 (h )s)(i 0 i); (p 0 p)((h )r 0 (h )s): This equation can be proven by mathematical induction. The proof in both cases is very similar, so that we only sketch it for the rst identity. Indeed, the case k = 0 is obvious. So suppose that the above equation is true for some k 0. Then it follows that (h 0 ip + I 0 h)( 0 I + I 0 ) k+1 (i 0 i) ( s;0(h )r+1 0 (h )s + (h )r 0 (h )s+1))(i 0 i) ((h )r 0 (h )s)(i 0 i): This concludes the proof. One immediate corollary is that i0 is a cohomomorphism, i0 = (1 h ) 1i = X (h 0 ip + I 0 h)( 0 I + I 0 ) k (i 0 i) = (i0 0 i0) : X X r+s=k;r;s 0 r+s=k;r;s 0 X r+s=k+1;r;s 0 1 k=0 Similarly, one can prove that p0 is a cohomomorphism. One further simple consequence is that D0 is a coderivation, D0 = (D 0 I + I 0 D) + (p 0 p)( 0 I + I 0 )(i0 = (D0 0 I + I 0 D0) : Open Access. 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Sebastian Konopka. The S-matrix of superstring field theory, Journal of High Energy Physics, 2015, 187, DOI: 10.1007/JHEP11(2015)187