New worldsheet formulae for conformal supergravity amplitudes

Journal of High Energy Physics, Jul 2018

Abstract We use 4d ambitwistor string theory to derive new worldsheet formulae for tree-level conformal supergravity amplitudes supported on refined scattering equations. Unlike the worldsheet formulae for super-Yang-Mills or supergravity, the scattering equations for conformal supergravity are not in general refined by MHV degree. Nevertheless, we obtain a concise worldsheet formula for any number of scalars and gravitons which we lift to a manifestly supersymmetric formula using four types of vertex operators. The theory also contains states with non-plane wave boundary conditions and we show that the corresponding amplitudes can be obtained from plane-wave amplitudes by applying momentum derivatives. Such derivatives are subtle to define since the formulae are intrinsically four-dimensional and on-shell, so we develop a method for computing momentum derivatives of spinor variables.

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New worldsheet formulae for conformal supergravity amplitudes

HJE New worldsheet formulae for conformal supergravity Joseph A. Farrow 0 1 Arthur E. Lipstein 0 1 0 Durham , DH1 3LE , United Kingdom 1 Department of Mathematical Sciences, Durham University We use 4d ambitwistor string theory to derive new worldsheet formulae for tree-level conformal supergravity amplitudes supported on re ned scattering equations. Unlike the worldsheet formulae for super-Yang-Mills or supergravity, the scattering equations for conformal supergravity are not in general re ned by MHV degree. Nevertheless, we obtain a concise worldsheet formula for any number of scalars and gravitons which we lift to a manifestly supersymmetric formula using four types of vertex operators. The theory also contains states with non-plane wave boundary conditions and we show that the corresponding amplitudes can be obtained from plane-wave amplitudes by applying momentum derivatives. Such derivatives are subtle to de ne since the formulae are intrinsically four-dimensional and on-shell, so we develop a method for computing momentum derivatives of spinor variables. Scattering Amplitudes; Conformal Field Models in String Theory; Supergrav- - ity Models 1 Introduction Review 2 3 4 5 B C D Non-plane wave examples D.1 Non-plane wave scalar D.2 Non-plane wave graviton 1 Introduction Plane wave graviton multiplet scattering 3.1 Supersymmetric formula Non-plane wave graviton multiplet scattering Conclusion A BRST quantization Derivation of Berkovits-Witten formula Momentum derivatives Starting with the discovery of the Parke-Taylor formula for tree-level gluon amplitudes [ 1 ], the study of scattering amplitudes has suggested intruiging new ways of formulating quantum eld theory. Building on the work of Nair [2], Berkovits and Witten [3, 4] proposed a worldsheet model for N = 4 super-Yang-Mills (SYM) known as twistor string theory, whose correlation functions give rise to an elegant formula for tree-level amplitudes in terms of integrals over curves in twistor space [5]. It turns out however that twistor string theory also contains conformal supergravity (CSGR) in its spectrum [6], so it is di cult to extend this formula to loop-level. These ideas were then extended by Skinner to N = 8 supergravity (SUGRA) [ 8 ] following the discovery of a gravitational analogue of the Park-Taylor formula by Hodges [7]. This framework was subsequently extended to a broad range of theories after Cachazo, He, and Yuan (CHY) proposed worldsheet formulae for tree-level scattering amplitudes of non-supersymmetric gauge and gravitational theories in any dimension [ 9 ]. These formulae are supported on solutions to the scattering equations, which were previously discovered in the context of ordinary string theory [ 10, 11 ]. The worldsheet theory underlying the CHY formulae was constructed by Mason and Skinner and is called ambitwistor string theory [12]. In the case of 10d supergravity, this model is critical and can be extended to loop-level [13, 14]. { 1 { Using a model known as 4d ambitwistor string theory, it is possible to obtain manifestly supersymmetric formulae for tree-level scattering amplitudes in 4d N = 4 SYM and N = 8 SUGRA supported on re ned scattering equations [15, 16]. In this approach the scattering equations are split into two sets, which we refer to as left and right. In general, the number of states in the left set is related to the Grassmann degree of the superamplitude. Furthermore, in the case of SYM and SUGRA, it is also tied to the MHV degree of the amplitude, which is related to the number of negative helicty supermultiplets being scattered. In particular, an Nk 2MHV amplitude describes the scattering k negative helicity multiplets. These formulae are simpler than twistor string formulae in that they do not contain integrals over moduli of curves in twistor space, and are also simpler than the CHY formulae in general dimensions in that the solutions to the re ned scattering equations are split into di erent MHV sectors.1 Moreover, the formulae arising from 4d ambitwistor string theory are intimately related to Grassmannian integral formulae for N = 4 SYM and N = 8 SUGRA obtained using on-shell diagrams [20{23]. Like the Berkovits-Witten twistor string, the 4d ambitwistor string for N = 4 SYM contains CSGR in its spectrum, in particular a certain non-minimal version of N = 4 conformal supergravity (in the minimal version there is no coupling between the Weyl tensor and scalar elds of the model [24]). Although CSGR is not unitary, it is nevertheless of theoretical interest for several reasons. For example, it is renormalizable and can be made UV nite if coupled to N = 4 SYM [25, 26]. Furthermore, it is possible to obtain classical Einstein gravity with cosomological constant by imposing Neumann boundary conditions on conformal gravity [27] (see also [28, 29]), and this approach was used to deduce twistor string formulae for scattering amplitudes of Einstein supergravity in at space [30, 31]. Given the large amount of symmetry in CSGR, we expect its scattering amplitudes to have simple mathematical properties. The purpose of this paper is therefore to investigate this structure using 4d ambitwistor string theory. CSGR amplitudes were previously studied in [6, 32{36]. More recently, these amplitudes were shown to arise from taking the double-copy of super-Yang-Mills with a (DF )2 gauge theory [37]. An ambitwistor string description of the (DF )2 theory was subsequently found in [38] and used to deduce a CHY formula for conformal gravity amplitudes in general dimensions. In this paper, we use 4d ambitwistor string theory to derive compact new worldsheet formulae for CSGR amplitudes supported on re ned scattering equations. In contrast to the worldsheet formulae for N = 4 SYM and N = 8 SUGRA, we nd that the number of particles in the left set is not generally tied to the MHV degree. Nevertheless, the formulae we obtain are very simple, allowing us to generalise previous results in many ways. For example, we obtain a simple formula for scalar-graviton amplitudes with any number of particles in the left set. If only two particles are in the left set, this formula reduces to the one previously derived by Berkovits and Witten. More generally, the formula can be readily evaluated numerically and we match it against results obtained from Feynman diagrams and double copy techniques developed in [37] up to 8 points with any number of particles 1In particular, the number of solutions to the re ned scattering equations is given by the Eulerian number n 3 for an n-point Nk 2MHV amplitude [17{19]. In contrast, the scattering equations in general k 2 dimensions have (n 3)! solutions. { 2 { in the left set. Moreover, we generalize this to a manifestly supersymmetric formula using four types of vetex operators which describe states in either the left or right set and either the positive or negative helicity graviton multiplet. Since the equations of motion for conformal gravity are fourth order in derivatives, they also admit non-plane wave solutions of the form A xeik x. We nd that the vertex operators for such states are very simple and give rise to scattering amplitudes which can be obtained by taking momentum derivatives of plane wave amplitudes, and are therefore well-de ned at least in a distributional sense. Vertex operators for non-plane wave states were previously proposed in [33]. Whereas previous vertex operators were de ned only for A2 = 0, our vertex operators are de ned for any A and appear to be more compact. Nevertheless, computing non-plane wave amplitudes using 4d ambitwistor string theory turns out to be subtle for several reasons. First of all, in order to compute worldsheet correlators we introduce source terms in the path integral leading to deformed scattering equations. The nal result is then obtained by taking functional derivatives with respect to the sources and setting them to zero, which returns to the original scattering equations. Second of all, since the formulae are manifestly four-dimensional and on-shell, we develop a prescription for taking momentum derivatives of spinor variables. The structure of this paper is as follows. In section 2, we review some facts about 4d ambitwistor string theory that will be relevant in this paper. In section 3, we derive worldsheet formulae for scattering amplitudes of graviton multiplet states with plane-wave boundary conditions. In section 4, we generalize these formulae to non-plane wave states, and in section 5 we present our conclusions and future directions. There are also four appendices. In appendix A, we review the BRST quantization of 4d ambitwistor string theory and show that the vertex operators presented in this paper are BRST invariant. In appendix B, we show that our worldsheet formula for scalar-graviton amplitudes reduces to the Berkovits-Witten result when there are only two particles in the left set. In appendix C, we describe a method for taking momentum derivatives of on-shell variables. Finally, in appendix D, we use this method to compute several examples of non-plane wave amplitudes. 2 Review In this section, we will review the 4d ambitwistor string theory describing N = 4 SYM and CSGR. The Lagrangian for this model is given by (2.1) where the worldsheet coordinate is a complex number supertwistor space L = 1 2 ZA = B 0 0 ~ 1 I A _ C ; WA = B ~ _ CA ; @ ~I and Lj is the Lagrangian for a current algebra, the details of which will not be important. Note that ; _ = 1; 2 are spinor indices and I = 1; 2; 3; 4 is an R-symmetry index, and { 3 { the corresponding worldsheet elds are bosonic and fermionic, repsectively. Furthermore, recall that a 4d null momentum can be written in bispinor form as follows: ki _ = i ~i_ where i labels the particle number. The worldsheet elds ; ~ can therefore be thought of as parametrizing an on-shell momentum, although their relation to the external spinors of the amplitude will be made precise later on. The scattering amplitudes will ultimately be expressed in terms of inner products hiji = i j two-index Levi-Civita symbol. and [ij] = ~ _ ~ i j _ _ , where _ is the model have holomorphic conformal weight 12 . Note that the model has a GL(1) symmetry Z; 1W . We will gauge this symmetry as well as the Virasoro symmetry (which contains an SL(2) subgroup). The physical states of the model then correspond to the BRST cohomology. In contrast to ordinary string theory, the spectrum of 4d ambitwistor string only contains eld theory degrees of freedom, notably N = 4 SYM and CSGR without an in nite tower of massive higher-spin states. In appendix A we describe the BRST quantization of this model in more detail. Field theory scattering amplitudes are then obtained from correlation functions of vertex operators corresponding to physical states. Each vertex operator is described by a pair of complex numbers = 1t (1; ), which correspond to homogeneous coordinates on the Riemann sphere at tree-level. For example, an integrated vertex operator encoding the N = 4 SYM multiplet (which consists of a gluon, six scalars, and eight fermions) has the following form: Vi( ) = the vertex operator V~ by complex conjugating V and Fourier transforming back to We de ne the left set L as the set of particles with V~ vertex operators, and the right set R as those with V vertex operators. To compute an n-point Nk 2MHV superamplitude in space. N = 4 SYM is then obtained from the worldsheet correlator DV~1 : : : V~kVk+1 : : : Vn E integrated over the locations of the vertex operators. Particular component amplitudes can then be extracted by integrating out the appropriate variables. At tree-level, we may take all the vertex operators to be integrated and use the SL(2) GL(1) GL(2) symmetry to x the coordinates of two vertex operators to be i = (1; 0) and j = (0; 1). Note that the remaining integral over worldsheet coordinates is localized by the delta functions in the vertex operators which encode the scattering equations. The scattering equations are then re ned according to how many particles are in the left and the right set. { 4 { As we mentioned above, the 4d ambitwistor string also describes CSGR. The spectrum of this theory contains the following graviton multiplets: = h 1 2 3 4 + I J K IJK + I J AIJ + I I + + = + 1 2 3 4 + I J K L IJKL + I J AKL IJKL + I JKL IJKL + h+; (2.2) where h refers to helicity 2, IJK ; IJK to helicity 3=2, AIJ ; AIJ to helicity 1, I ; I to helicity 1=2, and refer to spin-0 states. Note that the spin-1 states above are distinct from the gluons of N = 4 SYM. Also note that the graviton multiplets can have plane wave eik x boundary conditions or non-plane wave A xeik x boundary conditions because the equations of motion are fourth order in derivatives. We will present vertex operators corresponding to the graviton multiplet states in the next sections and demonstate their BRST invariance in appendix A. In contrast to the worldsheet formulae for N = 4 SYM and N = 8 SUGRA, the scattering equations for CSGR are not in general re ned by MHV degree since the left set can contain states from both graviton multiplets. Note that the CSGR spectrum also contains gravitino multiplets consisting of states with helicities and plane-wave boundary conditions. The scattering amplitudes for these states can be computed using the techniques we develop in this paper, although we leave a detailed analysis for future work. For more details about spectrum of CSGR in the context of the Berkovits-Witten model, see [6, 33]. 3 Plane wave graviton multiplet scattering In this section, we will consider scattering amplitudes for graviton multiplets with plane wave boundary conditions in CSGR. First we derive a concise worldsheet formula for scalar-graviton amplitudes, and then we lift it to a supersymmetric formula. We denote left set vertex operators with V~, and right set with V . The scattering equations will then be re ned by how many states are in the left set, which will not in general correspond to the MHV degree. The vertex operators describing gravitons and scalars are given by: (3.1) We verify the BRST invariance of these vertex operators in appendix A. Let be the set of positive/negative scalars, and G be the set of positive/negative helicity gravitons, so that G Z dt h~i ~( ) Y rg2G+ h+ Vrg + { 5 { integrated over the locations of the vertex operators (modulo GL(2)). The correlator can be easily computed using the path integral formalism by combining the exponentials in the vertex operators with the action to obtain the modi ed Lagrangian ~ tegrated out yielding delta functionals which localize the functional integrals over the remaining elds onto solutions of the equations of motion which are uniquely solved by ( r) where (ij) = ( i j ) =titj . The amplitude is then given by the following worldsheet integral: h; An;jLj = Z d 2 n GL(2) (SE) Y lg2G Hlg Y ~ Fl Y Fr Y rg2G+ ~ Hrg where d2 n = in=1d idti=ti3, (SE) = Y 2 ~ l For jLj = 2, the scattering equations have only one solution, and it can be found analytically. As we show in appendix B, on the support of this solution (3.3) reduces to the Berkovits-Witten result i2 +[H+ j=1;j6=i hiji hixii hiyii where xi and yi are arbitrary reference spinors. For jLj > 2 we have veri ed (3.3) (and its extension to include fermions and spin one states) numerically by matching it against results obtained using Feynman diagrams and color-kinematics duality [37] up to eight points with any number of particles in the left set.2 In order to do so, new techniques were developed for numerically solving the scattering equations which will be reported on in [39]. 2We thank Henrik Johansson for providing numerical results derived from color-kinematics duality against which to compare our worldsheet formula. { 6 { (3.2) (3.3) (3.4) (3.5) The scalar-graviton vertex operators in (3.1) can be lifted to the following supersymmetric vertex operators: V~ ( ) = V~+( ) = V ( ) = V+( ) = i 2j4 ~ i where the V~ These vertex operators encode all states in the positive/negative helicity graviton multiplets (denoted with a ), which can occur in the left/right set (denoted with/without a tilde). Hence, CSGR amplitudes are computed using four types of vertex operators, in contrast to N = 4 SYM and N = 8 SUGRA which have only two types of vertex operators. Note that vertex operators can be obtained by complex conjugating the V vertex operators and Fourier transforming back to then obtained by computing a correlator of vertex operators integrated over the worldseet (modulo GL(2)). As before, one can integrate out the ( ; ~) elds localizing ( ; ~) onto the solutions in (3.2). In the supersymmetric case, we can similarly integrate out the elds, localizing the ~ eld onto the following solutions to the equations of motion: t ~( ) = X r2R r : ( r) An n-point Nk 2MHV amplitude with jLj particles in the left set is then given by the following worldsheet formula: where k = j j + j ~ j and n Ak;jLj = Z d 2 n GL(2) (SE) Hl Y l 2~ Y l+2~+ ~ Fl+ Y Fr Y r+2 + H~r+ ; l2L (SE) = Y 2j4 ~ l long as jLj is preserved. For the special case where k = jLj, a formula in terms of integrals over curves in twistor space was previously conjectured in [35], and it would be interesting { 7 { (3.6) to see how this formula is related to (3.7). Component amplitudes can be extracted by integrating out the appropriate variables. For example, the scalar-graviton amplitudes in (3.3) are obtained by integrating out ( l)4 for l 2 L, and setting ( r) = 0 for r 2 R. In a similar way, one can also obtain component amplitudes with fermions and spin-1 states. 4 Non-plane wave graviton multiplet scattering The fourth order equations of motion for conformal gravity lead to a second set of graviton multiplet states with boundary conditions A xeik x. Note that if A k = 0, then this is actually a solution to the second order wave equation. Following from this we nd that vertex operators for non-plane wave states have a free vector index which will be contracted We propose the following vertex operators describing non-plane wave gravitons ~ _ Vh ( ) = V~ +_ ( ) = V _ ( ) = _ Vh+ ( ) = Z dt t 2 Z Z ! ~ ( )~i_ t~( ) eith~( )ii 2 ( i Since the ( ; ~) elds appear outside the exponentials, when computing correlation functions we cannot simply combine them with the action and integrate them out as before. On the other hand, if we add source terms for these elds, then we can compute a di erent correlator where they only appear in the exponentials and obtain the original correlator by taking functional derivatives with respect to the sources and setting them to zero afterwards. In more detail, we add the following source terms to the Lagrangian: Z tJ ( 0) 0 ; t~( ) = ~ r ( r) + Z tJ ( 0) 0 : (4.2) which are uniquely solved by X tl l ( l2L and consider a correlator of vertex operators like the ones in (4.1), but without ( ; ~) terms outside the exponenti als. This correlator can then be evaluated by combining the exponentials of the vertex operators with the action and integrating out ( ; ~) giving rise to delta functionals which localize the ( ; ~) elds onto solutions of the following equations of motion: X tr ~r ( r) + J~; ~ J ~ J { 8 { r2R X r2R ( ) = Z 0 ~( 0) ; ~( ) = Z 0 ( 0) We can then restore the terms with ( ; ~) outside the exponentials by taking functional derivatives with respect to (J; J~) and setting them to zero afterwards (note that after setting the sources to zero, the scattering equations will no longer be deformed). From the explicit form of ( ; ~) in (4.2), we conclude that correlators with non-plane wave vertex operators can be evaluated by making the following substitutions for ( ; ~) outside of the exponentials in (4.1): These formulae are familiar from canonical quantization. Note that the non-plane wave graviton vertex operators in (4.1) contain singular terms which cancel out. In the rst vertex operator for example, a pole arises in the rst term after making the replacement in (4.3), but this is precisely cancelled by the pole which arises from ( ) in the second term. Indeed, looking at (3.2), we see that the residue of the second pole is i, which precisely cancels the residue of the rst pole. We describe this in more detail in appendix D, where we also work out some examples at n points with up to two non-plane wave states, and show that amplitudes with non-plane wave states can be obtained by acting on the plane-wave amplitudes with a momentum derivative for each non-plane wave state. This could have been anticipated from the LSZ reduction formula by noting that a non-plane wave solution can be written as a momentum derivative of a plane-wave solution: A x eik x = A eik x; where k is understood to be o -shell prior to taking the derivative. Since the amplitudes are manifestly 4d and on-shell, we must de ne a prescription for taking momentum derivatives of on-shell quantities. We de ne such a prescription in appendix C, and use it to derive the following formulae which are su cient to di erentiate any little group invariant function of spinor brackets: where is an arbitrary spinor and is a reference spinor which parametrizes an o -shell extension of the momentum k. Another subtlety about non-plane wave amplitudes is that they can be expressed in many di erent ways. For example, using momentum conservation to remove the dependence on the momentum of a particular leg, amplitudes with a single non-plane wave state can be written with a derivative acting only on the momentumconserving delta function, although the expressions we obtain from worldsheet calculations will generally not be of this form for amplitudes with more than three legs. On the other hand, amplitudes with non-plane wave states are well-de ned in a distributional sense. In particular, if we multiply a non-plane wave amplitude by a test function, integrate over { 9 { ~ _ V ( ) = ~ _ V+ ( ) = V _ ( ) = V+ _ ( ) = Z dt t 2 Z Z tdt Z dt t 2 states in the left set. 5 Conclusion momentum space, and perform integration by parts, then we are left with derivatives of the test function times a plane-wave amplitude which is unambiguous. Finally, let us point out that non-plane wave scalar-graviton amplitudes can be supersymmetrized using the following vertex operators, which are the non-plane wave analogues of (3.6): i _ ( ) ! 2j4 ~ i HJEP07(218)4 Once again, the superamplitude will depend on both the MHV degree and the number of In this paper we investigate tree-level scattering amplitudes of graviton multiplets in CSGR using 4d ambitwistor string theory. This model has the same spectrum as the BerkovitsWitten twistor string (notably N = 4 SYM and a non-minimal version of N = 4 CSGR) but gives rise to scattering amplitudes in the form of worldsheet integrals supported on re ned scattering equations which are split into two sets, referred to as left and right. In contrast to the 4d ambitwistor string formulae for N = 4 SYM and N = 8 SUGRA, we nd that the scattering equations for CSGR are in general not re ned by MHV degree so the amplitudes are labelled by the MHV degree as well as the size of the left set. On the other hand, we are able to obtain very simple formulae for scattering amplitudes which generalize previous results in several ways. We obtain a compact formula describing the scattering of any number of scalars and gravitons with any number of particles in the left set. If two particles are in the left set, the worldsheet integrals can be solved analytically reproducing the results of Berkovits and Witten. If more than two particles are in the left set, the worldsheet integrals can be evaluated numerically and we match the results against those obtained using Feynman diagrams and the double copy approach developed in [37] up to 8 points with any number of particles in the left set. An explicit algorithm for numerically solving the scattering equations and computing amplitudes with plane wave external states will be described in [39]. Moreover we generalize the scalar-graviton amplitudes to a supersymmetric formula using four types of vertex operators which describe states in the left or right set and the positive or negative helicity graviton multiplet. Since the equations of motion for CSGR are fourth order in derivatives, there are also graviton multiplets with non-plane wave boundary conditions of the form A xeik x. Amplitudes with such states are subtle to compute since this requires introducing sources in the worldsheet path integral which lead to deformed scattering equations, as well as developing a prescription for taking momentum derivatives of spinor variables. In the end however, we show that the amplitudes can be obtained by acting on plane wave amplitudes with momentum derivatives. There are a number of interesting open questions: Conformal symmetry is not manifest in our worldsheet formulae. As explained in [36], this is not surprising since chosing plane wave external states singles out 2-derivative solutions to the 4-derivative equations of motion, breaking conformal invariance. On the other hand, the underlying theory has conformal symmetry so it would be interesting to understand how it is realized at the level of amplitudes. Hidden conformal symmetry of gravitational amplitudes was recently explored in [ 40 ], so it would be interesting to see if the ideas developed in that paper can be applied to conformal gravity. As shown in [23], the 4d ambitwistor string formulae for N = 4 SYM and N = 8 SUGRA can be mapped into Grassmannian integral formulae which can be derived from a completely di erent approach involving on-shell diagrams. For N = 4 SYM, these formulae suggest a new interpretation of the amplitudes as the volume of a geometric object known as the Amplituhedron [ 41 ]. It would interesting to carry out an analogous mapping for CSGR amplitudes and see if they have a similar geometric interpretation. A double copy construction has recently been proposed for CSGR [37], which involves combining super-Yang-Mills with a certain non-supersymmetric (DF )2 gauge theory, and an ambitwistor string theory describing the latter in general dimensions was proposed in [38]. It would be interesting to try to formulate the (DF )2 theory using 4d ambitwistor string theory and obtain worldsheet formulae for the scattering amplitudes supported on re ned scattering equations. Classical Einstein gravity in dS4 can be obtained from conformal gravity by imposing Neumann boundary conditions which x external states to be of the Bunch-Davies form (1 + ik ) e ik +i~k ~x, where is the conformal time coordinate. These external states have also been used to compute three and four-point de Sitter correlators using Feynman diagrams and BCFW recursion and the results are consistent with holography [42, 43]. Note that the Bunch-Davies state is essentially a linear combination of plane-wave and non-plane wave states which are precisely of the form we have studied in this paper. We therefore hope that the techniques developed in this paper can be used to compute de Sitter correlators using worldsheet methods.3 In summary, we nd that 4d ambitwistor string theory reveals interesting new mathematical structure in the scattering amplitudes of CSGR, which appears to be very di erent 3Note that correlators in dS and AdS are related by analytic continuation [45]. A twistor string formula for N = 8 SUGRA in AdS4 was proposed in [ 44 ], although it is written in terms of external states which make it di cult to relate it to results obtained using other methods. from the structure previously found in N = 4 SYM and N = 8 SUGRA. We hope that exploring the directions described above will lead to a deeper understanding of gravitational amplitudes which can ultimately be applied to more realistic models. Acknowledgments We thank Simon Badger, Nathan Berkovits, and Paul Heslop for useful discussions, and especially Tim Adamo for providing comments on the manuscript and Henrik Johansson for sharing numerical results. AL is supported by the Royal Society as a Royal Society University Research Fellowship holder, and JF is funded by EPSRC PhD scholarship In this appendix, we will review how to BRST quantize the 4d ambitwistor string theory in (2.1). After gauging the GL(1) symmetry (Z; W ) ! Z; 1W and introducing a where a is a worldsheet gauge eld and L ! L + aZ W + eTmatter; 1 2 Tmatter = where Tj is the current algebra stress tensor. Note that this action is a ( ; ) ghost system with holomorphic conformal weights (1=2; 1=2). A general ( ; ) system with holomorphic conformal weights ( ; 1 ) has the stress tensor T = where = 1 for bosonic/fermionic statistics. The central charge can then be read o from the OPE of T with itself and is given by c = 2 We may gauge- x e = a = 0 using the Fadeev-Popov procedure by introducing ghost systems (b; c) and ~b; c~ with holomorphic conformal weights (2; 1) and (1; 0), respectively. The stress tensor for the ghosts is then given by Tghost = Tbc + T~bc~ where Using (A.1), the contribution of the ghosts to the central charge is cghost = 26 2 = We then de ne the BRST charge Q as follows: I Q = d (c (Tmatter + Tghost) + c~Z W ) : The key property that Q must satisfy is nilpotency, i.e. Q2 = 0. In order for Q to satisfy this constraint, the total central charge must vanish. The (Z; W ) system has zero central charge since the bosonic contributions cancel the fermionic ones, so the central charge of the current algebra must be cj = +28. The physical states of the theory correspond to the BRST cohomology. Hence, the corresponding vertex operators must be Q-closed, i.e. fQ; Vg = 0. The condition of Qclosure implies that the vertex operators must have holomorphic conformal weight w and GL(1) weight qV = 0. The conformal and GL(1) weights may in turn be read o from V = 1 the OPE of the vertex operator with T and Z W : T ( )V( 0) = wV V( ) ( 0)2 + : : : ; Z W ( )V( 0) = qV V( ) 0 + : : : where the ellipsis denote less singular terms. Let us verify that the vertex operators considered in this paper are Q-closed. Since the equations of motion for conformal gravity are fourth order in derivatives, the spectrum contains plane wave states of the form eik x as well as non-plane wave states of the form A xeik x. Moreover, in the ambitwistor string framework, vertex operators with opposite helicity are simply complex conjugates of each other and are therefore naturally divided into two sets, which we shall refer to as left and right sets (the scattering equations are then re ned by the number of states in each set). A plane-wave vertex operator in the right set is schematically of the form 2 ( i t ( )) eit ( ) ~i ; where ki = i ~i is the on-shell momentum. Using the incidence relation adapted to worldsheet elds ( ) = x ( ) (where x is a point in spacetime), we see that the exponential reduces to a plane-wave on the support of the delta function. Let us therefore consider an ansatz for a plane-wave vertex operator of the form 1. In practice, one can avoid a tedious (but straightforward) OPE calculation using the following rules for computing conformal and GL(1) weights: T : [Z] = [W ] = Z W : [Z] = 1 2 [ i] = h~ i where weights of t are xed by the consistency condition that [tZ] = 0 (for vertex operators in the left set, t will have opposite weights). We take external spinors have zero weight i.e. i = 0. Applying these rules to the vertex operator in (A.2), we nd that w where wj is the conformal weight of the current algebra. Q-closure then implies that and wj = 2 s, which implies that s 2.4 If s = 1, then the vertex operator reduces to which describes a gluon in N = 4 SYM. For s = 2, the vertex operator describes a graviton with plane wave boundary conditions: where we discarded the current algebra since wj = 0. To deduce the vertex operator for a scalar in the left set, consider the ansatz 1. Using the rules described above, one nds that t ( )) eit ( ) ~i (A.4) ing ansatz: following ansatz 0. If s = 1, then the vertex operator reduces to the gluon vertex operator in (A.3), but if s = 0 it describes a scalar with plane-wave boundary conditions Let us now turn our attention to non-plane wave states. Let us consider the follow2 ( i t ( )) eit ( ) ~i : Following an analysis very similar to the one for (A.2), we nd that s = = 2, so the vertex operator reduces to that of a non-plane wave graviton. Similarly, we nd that the A _ t ( )) eit ( ) ~i must satisfy s = 0 and = 1, and reduces to the vertex operator for a non-plane wave scalar. 4Note that if we do not impose the constraint qV = 0, i.e. we do not gauge the GL(1) symmetry of the worldsheet theory, then vertex operators with higher spin appear to be allowed. In this appendix, we will evaluate the worldsheet integral in (3.3) for the case jLj = 2. In this case, the worldsheet integral is straightforward to evaluate analytically. Let us take the left set to be L = f1; 2g and the right set to be R = f3; : : : ; ng. Using the GL(2) symmetry to set 1 = (1; 0) and 2 = (0; 1), the delta functions encoding the re ned scattering equations reduce to Y 2 ~ l l2L tl ~ ( l) = h12i2 4(P ); 2 ( r t r ( r)) = (1r)2 (2r)2 h12i (12) 1 r h21i hr1i 2 r hr2i h21i ; r 2 R: The remaining worldsheet integrals then localize onto the solution r = hr1i hr2i h21i ; h21i ; r 2 R: On the support of the re ned scattering equations, (3.3) then reduces to An;jLj=2 = 4(P ) h12i2 Y (1r)2(2r)2 h; Plugging in (B.1), we nd that the factor associated with each h+ leg is given by where we de ne the gravitational inverse soft factor for leg j as follows: r2R X [rgr](1rg)2(2rg)2 (rgr) h12i (12) = X [rgr] h1ri h2ri r2R hrgri h1rgi h2rgi = rg1;n2 ; where a and b are reference spinors. Using momentum conservation, it is possible to show that the inverse soft factor is independent of the choice of reference spinors so we will just refer to it as j;n. Similarly, we nd that the factor associated with each + leg is given by (B.1) (B.2) (B.3) (B.4) (B.5) HJEP07(218)4 where we checked the second equality numerically up to high multiplicity. Hence, we nd that for jLj = 2, (3.3) reduces to the formula of Berkovits and Witten; X [rr0](rr0)(12) r<r02R (l r)2(l r0)2 h12i = l ;n; We would like to be able to take derivatives of on-shell quantities with respect to o shell momenta. As written, this problem is not well speci ed as there are four degrees of freedom in an o shell momentum p but only three degrees of freedom in the spinor variables which parametrize an on-shell momentum after removing the little group redundancy. We work with real momenta such that ~ _ = . To make the problem well-de ned, we will introduce a fourth degree of freedom in addition to the three degrees of freedom in the spinor variables and write the o -shell momentum in terms of ( ; ) as follows: p _ = ~ _ + ~ _ (C.1) ~_ is a reference null vector which encodes an o -shell direction. Inverting these equations to solve for (p) is simple, and we nd that (p) = 2pp2 . We can also solve for (p) . To see this, contract with the spinor ~_ to arrive at ~_ p _ = [ ~~] . For real momenta it is clear that [ ~~] 2 = such that [ ~~] = e i p express it in terms of p as follows: p, hence there must exist some phase (p) p. Given that is de ned only up to an arbitrary phase, we can = = ~ [ ~~] : 1 pp0 + p3 p 1 p0 + p3 ! ip2 : p ei (p) ~_ p _ ! ei (p) ~_ p 1 ei (p) ~_ p _ 2 p p ~ _ p + i _ p + i (p) : p ! 0 1 (p) = 1 (p) 2 h i h i = = h i Note that for the choice = and = 0 we recover the well-known expression Di erentiating (C.2) with respect to the o -shell momentum then gives In general, di erentiating a function of spinor brackets which is not little-group invariant will result in a @ (p) term. Let us therefore consider momentum derivatives of the little-group invariants ~ _ a di erent external leg for example). We nd that and , where is an arbitrary spinor (which can come from h i 2 h i2 [ ~~] ; ~ _ (C.2) (C.3) (C.4) (C.5) (C.6) (C.7) where in the second line we used the Schouten identity, and in the third equality we used (C.4). Furthermore, using equation (C.1) we nd that This calculation involves only derivatives of vectors with respect to vectors, and hence we do not need equation (C.4). The result is p _ _ _ p 2 2 p h i ~ = ~ _ . o -shell direction, i.e. _ = 0 and p Note that the right-hand-side is a projection matrix which removes components along the D Non-plane wave examples In this section, we will work out examples of scattering amplitudes for non-plane wave states of the form A xeik x using the vertex operators proposed in section 4, and use the method described in appendix C to express them as momentum derivatives of plane wave amplitudes. D.1 Non-plane wave scalar We will rst calculate an amplitude with two plane wave negative helicity gravitons, a negative multiplet scalar with non-plane wave boundary conditions, and n 3 plane wave positive helicity gravitons. We then de ne the left set to be L = f1; 2g, the right set R to contain the remaining particles, and the set R0 = f4; : : : ng to be the set of positive helicity gravitons. The vertex operators for these states can be found in (3.1) and (4.1). After replacing ( ; ~) outside the exponential in the non-plane wave scalar vertex operator and taking functional derivatives according to (4.3), we obtain the following worldsheet formula: A(h h x h+ : : : h+) = A3 _ Z d2 n h12i2 GL(2) (12)2 Y ! (SE): (D.1) The rst term comes from acting with the functional derivatives on the delta functions brackets h~ ~ ( )i in the positive-helicity graviton vertex operators. imposing the scattering equations, and the second term comes from acting on the spinor We can evaluate the worldsheet integral analytically following the same procedure described in appendix B. Using the GL(2) symmetry to x 1 = (1; 0) and 2 = (0; 1) and converting the delta functions in the left set into a momentum conserving delta function, we see that the remaining terms do not depend on ~1 or ~2. Furthermore, the Jacobian from the scattering equation delta functions only contains angle brackets, so the @@~ will act only on the momentum conserving delta function. We can then simplify this part of the calculation as follows: 2 4(P ): After some further simpli cation using the Schouten identity, we obtain HJEP07(218)4 A(h h x h+ : : : h+) = h12i4 A3 _ 2R0 2R0 ! where the gravitational inverse soft factor ;n was de ned in (B.4). by applying a momentum derivative to a plane-wave amplitude as follows: Using the results in appendix C, it is not di cult to show that (D.3) can be obtained A(h h x h+ : : : h+) = h12i4 A3 Y 2R0 1 ;n 4(P )A : Clearly for n = 3, jR0j = 0 and the result holds. To show this for n > 3, let us compute the momentum derivative of the gravitational inverse-soft factor for leg j with respect to particle i where i 6= j and assume that the reference spinors do not depend on i. We then nd that ~j _ X [jk] hkai hkbi k6=j hjki hjai hjbi ~j _ hjai hjbi hiji2 habi hjii i + hiai hbii j ; ~ _ i i hjii a hibi + [ji] hiai _ hjii b ! (D.5) where we used equations (C.5) and (C.7) and chose the reference spinor to be ~_ = ~ _ . j Setting i = , j = 3, a = 1, and b = 2, we see that the second term in (D.3) contains the derivative of ;n, from which (D.4) follows. D.2 Non-plane wave graviton We now compute an amplitude with one negative-helicity non-plane wave graviton, A(hx h h+ : : : h+). As outlined in section 4, the vertex operator for hx has divergences (D.2) (D.3) (D.4) which cancel, and we show the details of this here. Following the same steps as in the previous section, we obtain the following worldsheet formula: A(hx h h+ : : : h+) = A + X r2R Y X r06=r2R r002R ! Y where the rst term comes from acting with the functional and ordinary derivative in hx on the delta functions imposing the scattering equations, the second term comes from acting with the functional derivative on the spinor brackets graviton vertex operators, and we have regulated divergent terms by taking a limit. h~r ~ ( r)i in the positive-helicity Cancelling the singular terms and carrying out the worldsheet integral as described in appendix B then gives A(hx h h+ : : : h+) = A 1 _ GL(2) (12) (12) Z d 2 n + X r2R h12i ~ 1 r _ (1r) Y X r06=r2R r002R r2R h1ri2 h2ri r02R;r06=r Y Using the results from appendix C to di erentiate the gravitational inverse soft factor as we did in the previous section, we arrive at the nal result that r2R r;1n 2 4(P ) : ! Following a similar calculation with two hx states we nd that A(hx hx h+ : : : h+) = h12i4 Y r2R r;1n 2 4(P ) ! where A1;2 are vectors in the wavefunctions of particles 1,2. Open Access. 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Joseph A. Farrow, Arthur E. Lipstein. New worldsheet formulae for conformal supergravity amplitudes, Journal of High Energy Physics, 2018, 74, DOI: 10.1007/JHEP07(2018)074