Static BPS black hole in 4d higher-spin gauge theory

We find exact spherically symmetric solution of 4d nonlinear bosonic higher-spin gauge theory, that preserves a quarter of supersymmetries of N=2 supersymmetric 4d higher-spin gauge theory. In the weak field regime it describes $AdS_4$ Schwarzschild black hole in the spin two sector along with non-zero massless fields of all integer spins.


Introduction
Nonlinear field equations of higher spin (HS) gauge fields were originally found for the 4d HS theory [1,2]. Then these results were extended to d = 3 [3] and any d [4]. In d ≥ 4, HS gauge theories describe interactions of infinite sets of propagating massless fields of lower and higher spins. The theory is manifestly general coordinate invariant, containing gravity as its part. In HS theory, spin two sources HS fields and vice versa. As a result, solutions of Einstein gravity are not necessarily solutions of the HS theory. HS interactions may significantly affect the theory in the strong field regime. Moreover, since the interval ds 2 = g mn dx m dx n associated to the s = 2 field g mn is not invariant under HS gauge symmetry, standard concepts of general relativity have to be reconsidered in the HS theory with unbroken HS symmetries.
The difficulty of finding solutions in HS theory is due to nonlocality of the field equations in the auxiliary noncommutative twistor space, rendered by the star-product operation. By field equations this nonlocality in the twistor space is mapped to space-time nonlocality of the HS field equations at the interaction level in accordance with the fact that HS interactions contain higher derivatives [5,6]. (For review and more references on HS gauge theories see [7].) As a result, the nonlinear HS gauge theory goes beyond the low energy limit typical for Einstein theory and its perturbative stringy corrections, having capacity to account strong field effects.
So far a very few exact solutions of the HS theory were available. The simplest one is the AdS d vacuum solution. BTZ black hole (BH) [8] also solves 3d nonlinear HS field equations [9]. The first nontrivial example of exact solution of 3d HS theory was found in [10], where it was shown that for non-zero HS curvature vacuum field B 0 = ν, the 3d HS field equations describe massive matter fields with the ν-dependent mass parameter. Some solutions of 4d HS equations were studied by Sezgin and Sundell [11,12]. More general class of solutions with non-vanishing HS fields, which reveal examples of algebraically special solutions of HS theory, was obtained by Iazeolla, Sezgin and Sundell in [13]. A large class of solutions of this type solves chiral models with (0, 4) or (2, 2) space-time signatures. In the noncommutative twistor sector they effectively amount to the 3d solution of [10], but their physical meaning is still not completely clear.
Since HS theory extends Einstein gravity, the natural question is what are counterparts of BH solutions in HS theory? In this paper we answer this question for the simplest case of a spherically symmetric charged BH. Namely, we present explicit coordinate free construction of the exact solution of the bosonic sector of N = 2 supersymmetric 4d HS theory of [2], characterized by a single free dimensionful parameter M (for a given cosmological constant) that determines a value of the BH electric charge e. In the weak field approximation, where the terms proportional to e 2 are neglected, the obtained solution reproduces Schwarzschild BH of mass M for s = 2 along with BH spin-s massless fields found in [14]. Beyond this limit, the obtained solution differs from Schwarzschild BH being supersymmetric, i.e., BPS. This in turn suggests that its electric charge is critical and hence nonzero.
Our construction is based on the unfolded formulation of Einstein BH developed in [15] where AdS 4 Killing symmetries play a distinguished role. The presented HS BH solution is constructed in terms of a Fock vacuum in the HS star-product algebra associated to a particular Killing symmetry of AdS 4 . This Ansatz exhibits dramatic simplification of the perturbative analysis of the nonlinear HS equations, reducing the problem to 3d HS equations. The same time, it makes the obtained solution supersymmetric.
The paper is organized as follows. In Section 2 we recall the formulation of AdS 4 BH of [15]. Nonlinear 4d HS bosonic equations are recalled in Section 3. In Section 4 we explain how BH and its HS extension arises in the free HS theory. The exact HS BH solution of the nonlinear HS field equations is obtained in Section 5. Its symmetries are found in Section 6. Conclusions and perspectives are discussed in Section 7. Conventions are summarized in Appendix.

AdS 4 Schwarzschild black hole
As is well-known, the metric of a BH of mass M in AdS 4 admits the Kerr-Schild form 1 [16] where η mn , (m, n = 0 . . . 3) is the background AdS 4 metric with negative cosmological constant −λ 2 , k m is the Kerr-Schild vector, that satisfies where D m and D m are BH and AdS 4 covariant derivatives, respectively, and Note, that it makes no difference in (2.1)-(2.3) whether indices are raised and lowered by either AdS 4 or full BH metrics. Although in this paper we proceed with the coordinate independent description, let us present explicit decomposition (2.1) in a coordinate system of [17] where k m has a particularly simple form Let AdS 4 be described by the zero-curvature equation where d = dx n ∂ ∂x n and W 0AB (x) = W 0BA (x) is the sp(4) connection 1-form. Indices A, B, . . . are raised and lowered by the sp(4) symplectic form identified with 4d charge conjugation matrix (see Appendix). In two-component spinor notation where 1-forms ω αβ and h αα describe the Lorentz connection and vierbein, respectively. AdS 4 possesses ten global symmetry parameters K AB = K BA valued in the o(3, 2) ∼ sp(4) Lie algebra, that satisfy where D 0 is the AdS 4 covariant differential (e.g., ) that squares to zero by virtue of (2.6). In terms of two-component spinors with where From (2.10) it follows in particular that v αα is an AdS 4 Killing vector in the local frame. According to [15], the Kerr-Schild vector of the BH metric (2.1) and its derivatives are expressed in terms of K AB (2.9) where are projectors, that satisfy The conjugated projectorsπ ± γβ satisfy analogous relations.) Note that, alternatively, one can use v + αα in (2.11) instead of v − αα [14]. A type of the BH (e.g., whether it is rotating or static) depends on the values of sp(4) invariants associated to K AB . The static case is characterized by the condition The function r (2.3) satisfies [15] 1 Einstein equations for the metric (2.1) are satisfied as a consequence of (2.8) and (2.11).
The Kerr-Schild BH solution admits a HS Kerr-Schild generalization for massless bosonic fields of all spins [14]

HS equations in four dimensions
To reproduce Kerr-Schild solution in HS theory let us recall the structure of nonlinear 4d HS equations. (For more detail see [18].) Starting from this section we set λ 2 = 1 for convenience. HS nonlinear equations in d = 4 are formulated in terms of 1-forms W (Z, Y |x) = dx n W n (Z, Y |x) and 0-forms B(Z, Y |x) that depend on space-time coordinates x n and auxiliary spinor variables Z A and Y A . In addition, the 1-form connection along Z-direction should be introduced S(Z, Y |x) = S α (Z, Y |x)dz α +Sα(Z, Y |x)dzα to be expressed via dynamical fields by the field equations. It is required that {dx n , dz α } = 0, {dx n , dzα} = 0. In this section, we consider the bosonic sector of the HS equations of [2], where the fields B and W are even functions of the oscillators (Z, Y ) and S is odd. The simplest version of HS equations with the topological fields factored out is The star-product in the auxiliary space of commuting variables (3.7) An integration contour in (3.7) is chosen so that 1 ⋆ f = f ⋆ 1 = f . Note that the star-product definition (3.7) differs from that of [18] by the absence of the imaginary unit factor in the exponential. From (3.7) it follows in particular An important property of the star-product (3.7) is that it admits left and right inner Klein and Note, that Klein operators, that act only on Y or Z variables, are delta-functions [19] (hence not entire) Also note thatf (z, y) = f (z, y) ⋆ δ(y) gives Fourier transform with respect to y-variablê f (z, y) = d 2 uf (z, u)e −uαy α . Analogous formulae hold for dotted spinors and for y ↔ z.

It follows then that
That the results are entire functions rather than distributions is because the star-product (3.7) is a specific normal star-product rather than Weyl star-product (for more detail see [18]). The Eqs.
with an arbitrary gauge parameter ǫ = ǫ(Z, Y |x). The vacuum solution for (3.1)-(3.5), that describes empty describes AdS 4 vacuum fields via Eq. (2.6) that acquires the form The variables Z A in the star-product (3.7), that play important role in the description of HS interactions, can be neglected at the free level. Following [20,18] a free HS field is described in the unfolded formalism by the 1-forms w(Y |x) and 0-forms (3.17) that encode, respectively, linearized HS potentials and field strengths along with towers of auxiliary fields. w(Y |x) and C(Y |x) are the parts of W (Z, Y |x) and B(Z, Y |x), that remain unrestricted by the equations (3.3)-(3.5). In the sector of 0-forms, free equations resulting from (3.2) have a form of covariant constancy condition in the twisted-adjoint modulẽ The linearized equations for HS gauge potentials, that result from (3.1), are [20] Since the twist operation in (3.18) can be realized asf (Y ) = δ(y) ⋆ f ⋆ δ(y) , any solution of the global symmetry equation (3.22) ǫ 0 (Y |x) generates a solution of (3.18) of the form where c 1 , c 2 are arbitrary constants. This formula manifests that adjoint and twisted adjoint covariant derivatives are related via Fourier transform of either y α orȳα variables.

Black hole solution in free HS theory
As shown in [15], a generic AdS 4 BH is completely determined by a chosen global symmetry parameter K AB of AdS 4 . Let us apply this idea to a HS generalization. Since K AB satisfies (2.8), it follows that for any f (ξ). By (3.23), a solution of the free HS equation (3.18) is generated by Generally, C (4.2) is not Hermitian yielding at the linearized level two different real solutions.
In the s = 2 sector each can be shown to correspond to generic AdS 4 -Kerr-NUT BH with mass m ∼ Re M and NUT charge n ∼ Im M for one and vice versa for another. In this paper we confine ourselves to the simplest static case of (2.14) with real M and f (ξ) = exp(ξ/2), The coefficient 1/2 in the exponential (4.3) is chosen so that, due to (2.14), F K satisfies The first order HS BH curvature C(Y |x) = MF K ⋆ δ(y) has the form According to [15], these are Petrov type-D Weyl tensors, that describe a Schwarzschild BH of mass M in the s = 2 sector along with a tower of Kerr-Schild fields (2.16) of all spins s. Note that the frame-like HS connections corresponding to (4.6), that carry equal numbers of undotted and dotted spinors, can be shown to be gauge equivalent to

BH Fock vacua
The key fact is that F K (4.3) generates a Fock vacuum of the star-product algebra, defining a subalgebra of a reduced set of oscillators. Indeed, let us introduce the projectors Π ±AB The creation and annihilation operators In the nonlinear case, we have to analyze corrections due to Z-dependence of the HS curvature and connection. F K is the only Z-independent element of the star-product algebra (3.7) that satisfies (5.3). More generally, let F be a space of elements f (Z, Y |x) that satisfy where φ is an arbitrary function and F is a subalgebra of the star-product algebra. Namely, where we have introduced the induced star-product * on the space of functions φ(A|x), that has the form The integral (5.7) is normalized in such a way that 1 * φ = φ * 1 = φ. Star-product (5.7) is associative and describes the normal ordering of the operators (Y −A − Z −A ) and (Y +A + Z +A ). Note that any function of the formF K = f (Z|x) ⋆ F K satisfies (5.3) and, hence, can be represented in the form (5.4). Indeed, using (3.7), one can easily check that For (anti)holomorphic functions, the integration can be performed further In the sequel we will work with (anti)holomorphic functions of (āα)a α , using relations The star-product (5.7) possesses the Klein operators K andK,

Ansatz and final result
The key observation is that the Eqs. (3.4), (3.5) can be solved exactly by the Ansatz B = MF K ⋆ δ(y) , (5.14) with some functions σ α (a|x),σα(ā|x) to be specified later. Within the induced star-product (5.7), this Ansatz reduces (3.4), (3.5) to two copies of 3d (anti)holomorphic deformed oscillators considered in [21,10] in the context of 3d HS theories. Indeed, introducing s α = a α + σ α (a|x) ,sα =āα +σα(ā|x) (5. where W 0 (Y |x) is the vacuum connection (3.15) and the additional terms manifest holomorphic factorization with respect to the variables (a α ,āα). Note, that (5.20) has no definite holomorphy properties in the (y α ,ȳα) variables because both a α andāα mix y α withȳα via (5.5). From (3.15) and (3.8) it follows that where the x-dependence of A A has been taken into account with the aid of (2.8) and (3.16) so that the differentiald in (5.21) only accounts the manifest x dependence, i.e.,dA = 0. Using and their complex conjugated, where The operator Q has the following properties inherited from D 0 Q f (a|x) * g(a|x) = Qf (a|x) * g(a|x) + f (a|x) * Qg(a|x) , (5.26) Remarkably, despite Q (5.25) contains second derivatives in the oscillators, it respects the chain rule (5.26). This is because the star-product (5.7) is x-dependent, so that, acting on the starproduct * ,d effectively compensates the terms that would spoil (5.26). Note that a similar construction was recently discussed in [23] in a different context of the definition of the ring of solutions of unfolded HS field equations (see Appendix of [23]). Since we consider purely bosonic problem, being an even function of a-oscillators, the connection (5.20) commutes to the Klein operators (5.11) and therefore solves (3. and the Kerr-Schild vector k αα is defined in (2.11). Let us stress, that it is the Ansatz (5.20), that effectively implied holomorphic factorization of the oscillators a α andāα, allowed us to integrate the equations on HS connection. Note also, that (5.28) and (5.29) do not correspond to the standard gauge choice of the HS perturbative analysis of [2,18] with S(Z, Y )| Z=0 = 0. Hence, to reproduce HS Kerr-Schild solution (4.7) at first order in M one has to apply an appropriate HS gauge transformation. In the first order, such a gauge transformation is with an arbitrary g 0 (Y |x).
An important property of (5.34) is that where Ω αα (x) is the sp(2) flat connection (except may be r = 0, cf. (5.32)) The observation (5.35), (5.38) suggests the exact solution of (5.23). Indeed, one may note that the first term in (5.38) is the linearized part of 1 8 Ω αα (s α * s α − a α * a α ). Using that bilinears of the deformed oscillators generate their sp(2) rotations, we obtain the exact solution of (5.23) in the following simple form Remarkably, the connection (5.40) in fact does not contain the O(M 2 ) terms, i.e., The simplest way to see this is to observe that from (5.23) it follows ∂ω 2 ∂a α = 1 2 [ω 1 , σ α ] * and hence, π − α γ ∂ ∂a γ ω 2 (a|x) = 0 , so that ω 2 = ω 2 (a − |x). On the other hand, it is easy to see that ω 2 (a|x) (5.41) is an entire function of the oscillators a ± such that Hence, it should be zero. Note that the straightforward verification of this fact, that was also completed, is not trivial, implying interesting integral identities. Note, that the r.h.s. of (5.44) is consistent with d 2 f 0 = 0. Indeed, d 2 f 0 = M 16 dτ αγ ∧ dτ γ α ∧ dτ αα which is a 3d volume 3-form for the "frame" E αα = dτ αα , that is however zero since τ αα τ αα = const. Note that f 0 contributes to the HS connection (5.20) via its real combination f 0 = f 0 +f 0 that can be chosen in the form (5.33). It is straightforward to verify that (5.33) satisfies (5.44) plus its complex conjugated. Thus, the final result for HS connection is (5.31).

Symmetries
Global symmetries of a 4d static BH include SO(3) spatial rotations and R 1 time translations both in asymptotically Minkowski and in AdS 4 (in fact, its universal covering) geometry. Infinitesimally, they form algebra su(2) ⊕ gl(1). A static Reissner-Nordström BH is in addition characterized by its electric charge e, reproducing Schwarzschild BH at e = 0. The critical value of charge e 2 = M 2 , that corresponds to the extremal BH, is characterized by the coincidence of two BH horizons [24] and by SUSY, being BPS [25]. Note that, at the free field level, the s = 1 field in (5.30) is just the Maxwell field strength of the AdS 4 Reissner-Nordström potential.
In this section we analyze global symmetries of the obtained static HS BH solution, showing in particular that (i) its space-time symmetry is su(2) ⊕ gl(1), (ii) it is supersymmetric, preserving a quarter of 4d N = 2 SUSY of the nonlinear HS model of [2], and (iii) it possesses infinite dimensional HS extension of (super)symmetries of (i) and (ii).
A maximal finite dimensional subalgebra of (6.2) is spanned by the bilinears of Y − and Y + and constants. In particular, it contains generators of su(2) ⊕ gl(1) that belong to the class (6.2). Since the algebra sp(4) of AdS 4 space-time symmetries is spanned by various bilinears of Y A , this su(2) ⊕ gl(1) describes space-time symmetries that remain unbroken. Hence, the obtained solution indeed describes a spherically symmetric static BH. Note that the Y -independent constant parameter in (6.2) describes a u(1) inner symmetry. The full set of parameters (6.2) describes an infinite dimensional HS algebra of global symmetries of the HS BH solution.

Supersymmetry
The solution (5.28)-(5.31) turns out to be supersymmetric, which is most easily seen from the embedding of the bosonic HS equations considered so far into the N = 2 supesymmetric nonlinear HS system of [2] (see also [18]) which has the form It is important to note that the system (6.4), (6.5) describes the doubled set of massless fields where all fields of integer and half-integer spins appear in two copies. Its bosonic sector consists of two independent subsystems described by the equations (3.1)-(3.5), which can be projected from the system (6.4), (6.5) by the projectors P ± = 1 2 (1 ± kk) , P ± P ± = P ± , P ∓ P ± = 0 , (6.7) that by (6.6) commute to boson fields, that are even in spinorial variables, but not to fermions. As such, the bosonic reduction of the system (6.4), (6.5) describes two parallel bosonic worlds. Each of them is not supersymmetric because P ± do not commute to fermions. Conventional N = 2 SUSY is achieved in the "diagonal world" described by the metric g nm = 1 2 (g +nm +g −nm ). A natural BH solution of the supersymmetric HS theory is a combination of BH solutions in each of the bosonic sectors, where, a priori, one can choose solutions with unrelated K ± AB . We wish however to consider the case where K + AB = −K − AB and the corresponding Fock vacua Consider the bosonic solution of the N = 2 supersymmetric HS system of the form (Note that the factor of i in the definition of B in Eq.(6.5) is enforced by the reality conditions that conjugate k andk [2].) The labels ± refer to the solutions in the sectors of P ± that may 2 Let us note that although the Fock vacua F ± K have ill-defined (infinite) mutual star-product this does not matter because, living in "parallel worlds" (i.e., being multiplied by P ± ), they never meet.
have different masses M ± . It is however convenient to demand the vacuum fields to coincide: In the case where M + or M − vanishes, the respective world is AdS 4 . A global symmetry parameter ǫ(Y ; kk|x) now depends on kk = P + − P − and should satisfy the conditions These are verified by AdS 4 covariantly constant ǫ that admit the following two representations with some ǫ ± lA (Y ) and ǫ ± rA (Y ). Indeed, for instance, the terms proportional to F + P + are annihilated by Y − from the left, Y + from the right and P − from the both sides.
Clearly, the bosonic global symmetry parameters (6.2) belong to the class (6.11). However, now the parameters that are odd in Y A are also allowed. In particular, global SUSY with an belongs to this class (recall that Y A anticommute to kk). This global SUSY is a quarter of the N = 2 SUSY with the supergenerators Q 1 A = Y A and Q 2 A = ikkY A [26] of the AdS 4 vacuum of the system (6.4), (6.5). Hence, the HS BH subalgebra corresponds to a 1 4 BPS state. This strongly indicates that the obtained HS BH solution should be extremal.
Covariantly constant solutions of (6.11) form an infinite dimensional HS superalgebra of global symmetries of the obtained BH solution.

Conclusion
The new exact solution of 4d bosonic HS theory announced in this paper is a HS generalization of a static BH in GR. At the free field level it contains Schwarzschild BH in the spin two sector. However, the contribution of HS fields is important in the strong field regime as is demonstrated by the remarkable fact that the nonlinear corrections cancel out for static BH in the full HS theory, reducing HS nonlinear equations to the free ones. This property is a HS analogue of the fact that the usual Kerr-Schild Ansatz reduces nonlinear Einstein equations to free Pauli-Fierz ones for a 4d BH. Let us stress that massless HS fields do not satisfy HS field equations in the BH background unless the HS interactions are switched on via the terms bilinear in the HS connections in the nonAbelian HS curvatures on the left hand side of (3.1).
In the proposed construction, static HS BH is described in terms of a Fock vacuum in the star-product algebra in the auxiliary twistor space. This Ansatz effectively projects 4d HS equations to 3d HS equations of [10] that describe 3d massive matter fields and can be solved with the aid of Wigner's deformed oscillators. The BH mass M coincides with the vacuum value B 0 = ν of [10] that sets the mass scale of 3d interacting massive fields. This reduction suggests an interesting duality between AdS 3 massive fields of a mass scale ν and 4d HS BH of mass M, ν = λGM, where −λ 2 is the cosmological constant and G is the Newton constant. More generally, the obtained results indicate that near BH fluctuations in HS theory describe a HS theory in the lower dimension thus providing a nontrivial dimensional compactification mechanism analogous to the brane picture in String Theory.
At the linearized level, where other fields do not contribute to the metric, the obtained solution reproduces AdS 4 Schwarzschild BH in the s = 2 similarly to the charged Reissner-Nordström BH that involves second order contribution of the electromagnetic field via its stress tensor. That the obtained solution exhibits leftover SUSY strongly indicates that it should be extremal. We believe that it can be further generalized to the NUT case using the phase ambiguity of 4d nonlinear HS equations of [2] as well as to the Kerr BH, extremal and not. More generally, a generalization to d dimensions is a challenging issue both at the free and at the nonlinear level. The preliminary analysis indicates that our method should work in various dimensions. It looks especially promising in the context of black rings [27] that exist in d ≥ 5.
An exciting problem for the future study is to explore physical properties of the obtained solution. Here, the key question is how do small fluctuations propagate in the HS BH background? Its analysis should shed light on such fundamental concepts in HS BH physics as horizons, trapped surfaces etc. Since HS theory is essentially nonlocal at the interaction level, involving higher derivatives for higher spins, the analysis of these issues should be done by new means beyond standard GR machinery. For example, in HS theory it is not granted that the geodesic motion has much to do with signal propagation in the strong field regime. Moreover, in the HS theory with unbroken HS symmetries it is not even clear how to define a metric tensor beyond the linearized approximation. The study of thermodynamical interpretation of the HS BH is also of primary importance. We plan to consider these problems in the future.

Appendix. Conventions
For any Lorentz vector ξ n , its two-component spinor counterpart is ξ αα = ξ n σ n,αα , where σ n,αα = (I αα , σ i,αα ) contains unity matrix I along with Pauli matrices. Latin indices are raised and lowered by Minkowski metric η mn . Spinorial indices are raised and lowered according to To distinguish between two types of projectors Π ±AB and π ± αβ we use the convention with lower and upper labels ± assigned to the objects projected by Π ±AB and by π ± αβ , respectively. For example,