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Higher-Dimensional Black Holes: Hidden Symmetries and Separation of Variables

In this paper, we discuss hidden symmetries in rotating black hole spacetimes. We start with an extended introduction which mainly summarizes results on hidden symmetries in four dimensions and introduces Killing and Killing-Yano tensors, objects responsible for hidden symmetries. We also demonstrate how starting with a principal CKY tensor (that is a closed non-degenerate conformal Killing-Yano 2-form) in 4D flat spacetime one can"generate"4D Kerr-NUT-(A)dS solution and its hidden symmetries. After this we consider higher-dimensional Kerr-NUT-(A)dS metrics and demonstrate that they possess a principal CKY tensor which allows one to generate the whole tower of Killing-Yano and Killing tensors. These symmetries imply complete integrability of geodesic equations and complete separation of variables for the Hamilton-Jacobi, Klein-Gordon, and Dirac equations in the general Kerr-NUT-(A)dS metrics.


A. Symmetries
In modern theoretical physics one can hardly overestimate the role of symmetries. They comprise the most fundamental laws of nature, they allow us to classify solutions, in their presence complicated physical problems become tractable. The value of symmetries is especially high in nonlinear theories, such as general relativity. * Electronic address: frolov@phys.ualberta.ca, kubiznak@phys.ualberta.ca In curved spacetime continuous symmetries (isometries) are generated by Killing vector fields. Such symmetries have clear geometrical meaning. Let us assume that in a given manifold we have a 1-parameter family of diffeomorphisms generated by a vector field ξ.
Such a vector field determines the dragging of tensors by the diffeomorphism transformation.
If a tensor field T is invariant with respect to this dragging, that is its Lie derivative along ξ vanishes, L ξ T = 0, we have a symmetry. A vector field which generates transformations preserving the metric is called a Killing vector field, and the corresponding diffeomorphisman isometry. For each of the Killing vector fields there exists a conserved quantity. For example, for a particle geodesic motion this conserved quantity is a projection of the particle momentum on the Killing vector.
Besides isometries the spacetime may also possess hidden symmetries, generated by either symmetric or antisymmetric tensor fields. Such symmetries are not directly related to the metric invariance under diffeomorphism transformations. They represent the genuine symmetries of the phase space rather than the configuration space. For example, symmetric Killing tensors give rise to conserved quantities of higher order in particle momenta, and underline the separability of scalar field equations. Less known but even more fundamental are antisymmetric Killing-Yano tensors which are related to the separability of field equations with spin, the existence of quantum symmetry operators, and the presence of conserved charges.

B. Miraculous properties of the Kerr geometry
To illustrate the role of hidden symmetries in general relativity let us recapitulate the "miraculous" properties [1] of the Kerr geometry. This astrophysically important metric was obtained in 1963 by Kerr [2] as a special solution which can be presented in the Kerr-Schild form [3] g ab = η ab + 2Hl a l b , where η is a flat metric and l is a null vector, in both metrics g and η. 1 The Kerr solution is stationary and axially symmetric, and it belongs to the metrics of the special algebraic type D of Petrov's classification [6].
Although the Killing vector fields ∂ t and ∂ φ are not enough to provide a sufficient number of integrals of motion 2 in 1968 Carter [7,8] demonstrated that both-the Hamilton-Jacobi and scalar field equations-can be separated. This proved, apart from other things, that there exists an additional integral of motion, 'mysterious' Carter's constant, which makes the particle geodesic motion completely integrable. In 1970, Walker and Penrose [9] pointed out that Carter's constant is quadratic in particle momenta and its existence is directly connected with the symmetric Killing tensor [10] K ab = K (ab) , K (ab;c) = 0 .
During the several following years it was discovered that it is not only the Klein-Gordon equation which allows the separation of variables in the Kerr geometry. In 1972, Teukolsky decoupled the equations for electromagnetic and gravitational perturbations, and separated variables in the resulting master equations [11]. One year later the massless neutrino equation by Teukolsky and Unruh [12,13], and in 1976 the massive Dirac equation by Chandrasekhar and Page [14,15] were separated.
Meanwhile a new breakthrough was achieved in the field of hidden symmetries when in 1973 Penrose and Floyd [16] discovered that the Killing tensor for the Kerr metric can be written in the form where the antisymmetric tensor f is the Killing-Yano (KY) tensor [17] f ab = f [ab] , f a(b;c) = 0 .
1 If the ansatz (1) is inserted into the Einstein equations, one effectively reduces the problem to a linear one (see, e.g., [4]). This gives a powerful tool for the study of special solutions of the Einstein equations. This method works in higher dimensions as well. For example, the Kerr-Schild ansatz was used by Myers and Perry to obtain their higher-dimensional black hole solutions [5]. 2 For example, for a particle motion these isometries generate the conserved energy and azimuthal component of the angular momentum, which together with the conservation of p 2 gives only 3 integrals of motion. For separability of the Hamilton-Jacobi equation in the Kerr spacetime the fourth integral of motion is required.
A Killing-Yano tensor is in many aspects more fundamental than a Killing tensor. Namely, its 'square' is always Killing tensor, but in a general case, the opposite is not true [18].
Many of the remarkable properties of the Kerr spacetime are consequences of the existence of the Killing-Yano tensor. In particular, in 1974 Collinson demonstrated that the integrability conditions for a non-degenerate Killing-Yano tensor imply that the spacetime is necessary of the Petrov type D [19]. 3 In 1975, Hughston and Sommers showed that in the Kerr geometry the Killing-Yano tensor f generates both of its isometries [22]. Namely, the Killing vectors ξ and η, generating the time translation and the rotation, can be written as follows: This in fact means that all the symmetries necessary for complete integrability of geodesic motion are 'derivable' from the existence of this Killing-Yano tensor.
In 1977, Carter demonstrated [23] that given an isometry ξ and/or a Killing tensor K one can construct the operatorsξ which commute with the scalar Laplacian 4 Under additional conditions, satisfied by the Kerr geometry, these operators commute also between themselves and provide therefore good quantum numbers for scalar fields. In 1979 Carter and McLenaghan found that an operator commutes with the Dirac operator γ a ∇ a [25]. This gives a new quantum number for the spinor wavefunction and explains why separation of the Dirac equation can be achieved. 3 All the vacuum type D solutions were obtained by Kinnersley [20]. Demianski and Francaviglia showed that in the absence of acceleration these solutions admit Killing and Killing-Yano tensors [21]. 4 In fact, the operatorK defined by (6) commutes with provided that the background metric satisfies the vacuum Einstein or source-free Einstein-Maxwell equations. In more general spacetimes, however, a quantum anomaly proportional to a contraction of K with the Ricci tensor may appear. Such anomaly is not present if an additional condition (3) is satisfied [24]. Similar symmetry operators for other equations with spin, including electromagnetic and gravitational perturbations, were constructed later [26].
In 1983, Marck solved equations for the parallel transport of an orthonormal frame along geodesics in the Kerr spacetime and used this result for the study of tidal forces [27]. For this construction he used a simple fact that the vector is parallel-propagated along a geodesic p.
In 1987, Carter [28] pointed out that the Killing-Yano tensor itself is derivable from a Such a form b is usually called a KY potential. It satisfies the Maxwell equations and can be interpreted as a 4-potential of an electromagnetic field with the source current proportional to the primary Killing vector field ∂ t , cf. eq. (5). In the paper [29], the equations for an equilibrium configuration of a cosmic string near the Kerr black hole were separated.
In 1993, Gibbons et al. demonstrated that due to the presence of Killing-Yano tensor the classical spinning particles in this background possess enhanced worldline supersymmetry [30]. Conserved quantities in the Kerr geometry generated by f were discussed in 2006 by Jezierski and Lukasik [31].
To conclude this section we mention that many of the above statements and results, which we have formulated for the Kerr geometry, are in fact more general. Their validity can be extended to more general spacetimes, or even to an arbitrary number of spacetime dimensions. For example, the whole Carter's class of solutions [32] admits a KY tensor and possesses many of the discussed properties. 5 General results on Killing-Yano tensors and algebraic properties were gathered by Hall [34]. A relationship among the existence of Killing tensors, Killing-Yano tensors, and separability structures for the Hamilton-Jacobi equation in arbitrary number of spacetime dimensions was discussed in [21,35]. 5 A general form of a line element in four dimensions admitting a Killing-Yano tensor was obtained by Dietz and Rüdiger [33].

C. Higher-dimensional black holes
Recently, a lot of interest focuses on higher-dimensional (D > 4) black hole spacetimes.
In the widely discussed models with large extra dimensions it is assumed that one or more additional spatial dimensions are present. In such models one expects mini black hole production in the high energy particle collisions [36]. These metrics were obtained in special coordinates which are the natural higherdimensional generalization of the Carter's 4D canonical coordinates [32]. So far this metric remains the most general black hole type solution of the Einstein equations with the cosmological constant (with horizon of the spherical topology) which is known analytically. 6 For 6 Besides the brane-world scenarios, these black holes find their applications in the ADS/CFT correspondence. In the BPS limit odd dimensional metrics lead to the Sasaki-Einstein metrics [45,46] whereas even dimensional metrics lead to the Calabi-Yau spaces [47]. There have been also several attempts to a recent extended review on higher-dimensional black holes see [51].
In connection with these black holes a natural question arises: To what extent are the remarkable properties of four-dimensional black holes carried by their higher-dimensional analogues? And in particular, do these spacetimes possess hidden symmetries?
The hidden symmetries of higher-dimensional rotating black holes were first discovered for the 5D Myers-Perry metrics [52,53]. It was demonstrated that both, the Hamilton-Jacobi and scalar field equations, allow the separation of variables and the corresponding Killing tensor was obtained. This, for example, allows one to obtain a cross-section for the capture of particles and light by 5D black holes [54]. Later it was shown that 5D results can be extended to arbitrary number of dimensions, provided that rotation parameters of the MP metric can be divided into two classes, and within each of the classes these parameters are equal one to another. Similar results were found in the presence of the cosmological constant and NUT parameters [43,55]. It was also demonstrated that a stationary string configuration in the 5D Myers-Perry spacetime is completely integrable [56].

D. Recent developments
Recently, a new breakthrough in the study of higher-dimensional rotating black holes was achieved. It turned out that the properties of even the most general known higherdimensional Kerr-NUT-(A)dS black holes [45] are, in many aspects, similar to the properties of their 4-dimensional 'cousins'. These follow from the existence of a special closed conformal Killing-Yano (CKY) tensor, which is called principal.
The principal CKY tensor was first discovered for the Myers-Perry metrics [57], and soon after that for the completely general Kerr-NUT-(A)dS spacetimes [58]. Starting with this tensor, one can generate the whole tower of Killing-Yano and Killing tensors [59] which are responsible for complete integrability of geodesic motion in these spacetimes [60,61].
Such integrability was independently proved by separating the Hamilton-Jacobi and Klein-Gordon equations [62]. These results are very promising and might suggest that also the equations with spin can be decoupled and separated. The separation of the Dirac equation was already demonstrated [63]. Also worth mentioning is a recent work on separability of generalize these solutions. For example, to find a similar solution of the Einstein-Maxwell equations either in an analytical form [48] or numerically [49]. See also [50].
vector and tensor fields in D = 5 Myers-Perry spacetimes with equal angular momenta [64], and proved complete integrability of stationary string configurations in general Kerr-NUT-(A)dS spacetimes [65]. For a brief review of these results see [66].
In this paper, we discuss the hidden symmetries of rotating black holes. The following section contains basic definitions. In Section III, starting from a 4D flat space and choosing a special principal CKY tensor, we 'derive' the 4D Kerr-NUT-(A)dS spacetime and its hidden symmetries. In Section IV we prove the central theorem concerning the properties of closed CKY tensors and introduce the tower of Killing and Killing-Yano tensors in higher-dimensional spacetimes. We also construct the canonical basis associated with the principal CKY tensor. In Section V we apply these results to higher-dimensional Kerr-NUT-(A)dS solutions. Section VI contains discussion of the separability problem. Possible future developments are discussed in Section VII.

A. Definitions
Let us consider a D-dimensional spacetime with a metric g. In order to simultaneously cover both cases of odd and even dimensions we write D = 2n + ε, where ε = 0 (ε = 1) for the even (odd) dimensional case. A spacetime possesses an isometry generated by the Killing vector field ξ if this vector obeys the Killing equation For a geodesic motion of a particle in such curved spacetime the quantity p a ξ a , where p is the momentum of the particle, remains constant along the particle's trajectory. Similarly, for a null geodesic, p a ξ a is conserved provided ξ is a conformal Killing vector obeying the equation There exist two natural (symmetric and antisymmetric) generalizations of a (conformal) Killing vector.
A symmetric (rank-p) conformal Killing tensor [9] K obeys the equations As in the case of a conformal Killing vector, the tensorK is determined by tracing both sides of equation (13). IfK vanishes, the tensor K is called a Killing tensor [10]. In a presence of the Killing tensor K the conserved quantity for a geodesic motion is For null geodesics this quantity is conserved not only for a Killing tensor, but also for a conformal Killing tensor.
A conformal Killing-Yano (CKY) tensor [24,67] h is an antisymmetric tensor which obeys By tracing both sides of this equation one obtains the following expression forh In the case whenh = 0 one has a Killing-Yano (KY) tensor [17]. For the KY tensor h the is parallel-propagated along the geodesic p.
Let us mention two additional important properties: having a KY tensor h the quantity is an associated Killing tensor. Here c p is an arbitrary constant, which is often taken to be one. For a different convenient choice see Section IV. For a CKY tensor h of rank-2 the vector obeys the following equation [68] ξ (0) Thus, in an Einstein space, that is when R ab = Λg ab , ξ (0) is the Killing vector.

B. Killing-Yano equations in terms of differential forms
The CKY tensors are forms and operations with them are greatly simplified if one uses the 'language' of differential forms.
Let us remind some of the relations we shall use later. If α p and β q are p-and q-forms, respectively, the external derivative d of their exterior product ∧ obeys a relation A Hodge dual * α p of the p-form α p is (D − p)-form defined as where e a 1 ...a D is a totally antisymmetric tensor. The co-derivative δ is defined as follows: One also has * * α p = ǫ p α p .
If {e a } is a basis of vectors, then dual basis of 1-forms ω a is defined by the relations ω a (e b ) = δ a b . We denote η ab = g(e a , e b ) and by η ab the inverse matrix. The operations with the indices enumerating the basic vectors and forms are performed by using these matrices.
In particular, e a = η ab e b , and so on. We denote a covariant derivative along the vector e a by ∇ a = ∇ ea . One has In tensor notations the 'hook' operator (inner derivative) along a vector X, applied to a p-form α p , corresponds to a contraction It satisfies the properties For a given vector X one defines X ♭ as a corresponding 1-form with the components (X ♭ ) a = g ab X b . In particular, one has η ab (e b ) ♭ = ω a . We refer to [69,70] where these and many other useful relations can be found.
Definition (16) of the CKY tensor h (which is a p-form) is equivalent to the following equation [70]: That is, a CKY tensor is a form for which the covariant derivative splits into the exterior and divergence parts. 7 Using the relation it is easy to show that under the Hodge duality the exterior part transforms into the divergence part and vice versa. In particular, (29) implies The Hodge dual * h of a CKY tensor h is again a CKY tensor.
Two special subclasses of CKY tensors are of particular interest: (a) Killing-Yano tensors with zero divergence part δh = 0 and (b) closed CKY tensors with vanishing exterior part dh = 0. Under the Hodge duality these subclasses transform into each other.
For a closed CKY tensor there exists locally a (KY) potential b, which is a (p − 1)-form, The Hodge dual of such a tensor h, is a Killing-Yano tensor, cf. eq. (10).

III. 4D KERR-NUT-(A)DS SPACETIME AND ITS HIDDEN SYMMETRIES
Before we proceed to higher-dimensional rotating black holes and their hidden symmetries it is instructive to illustrate the basic ideas on the well known 4D case. As we shall see later, a key object of the theory in higher dimensions is a principal CKY tensor. We start discussing this object and its properties in 4D flat spacetime and demonstrate how it generates other objects (Killing-Yano and Killing tensors) responsible for the hidden symmetries. We also show how this principal CKY tensor allows one easily to 'generate' the 4D Kerr-NUT-(A)dS metric starting from the flat one-written in the canonical coordinates determined by this tensor. 8 We also demonstrate the separation of variables in the 4D Kerr-NUT-(A)dS spacetime in the canonical coordinates. It should be emphasized, that this section plays the role of an introduction for newcomers to the field which should illuminate the main ideas of more complicated higher-dimensional theory.

A. Principal conformal Killing-Yano tensor
Consider a four-dimensional flat spacetime with the metric The principal CKY tensor h is the rank-2 closed CKY tensor. Therefore, there exists a 1-form potential b so that h = db. Let us consider the following ansatz: Our choice of this special form for the potential b will become clear later, when it will be shown that this is a flat spacetime limit of the potential for the principal CKY tensor in It can be easily shown that is a closed conformal Killing-Yano tensor. It means that its dual 2-form f = * h is the 8 For an alternative 'derivation' of the Kerr-NUT-(A)dS spacetime see, e.g., [71] or [72]. 9 In D dimensions the maximum number of (linear independent) Killing-Yano tensors of a given rank-p is This reflects the fact that, similar to Killing vectors, Killing-Yano tensors are completely determined by Let us put, for a moment, a = 0. Then the KY tensor f ab has only spatial components f ik , and the Killing tensor (19), associated with it, reads Here ξ (k) i are the spatial rotational Killing vectors. Therefore, the Killing tensor K can be written as a sum of products of Killing vectors, and thus it is reducible. Parallel-propagated vector (18) has the meaning of the conserved angular momentum. 10 The conserved quantity is the square of the total angular momentum.
For a = 0 the conserved quantity is also reducible. Here E = −p T and p Z are the conserved energy and the momentum in the Z-direction, respectively.

B. 'Derivation' of the 4D Kerr-NUT-(A)dS metric
Consider a general case with a = 0. We first introduce the ellipsoidal coordinates 11 the values of their components and the values of their (completely antisymmetric) first derivatives at a given point. Flat space has the maximum number of independent Killing-Yano tensors of each rank. Any KY tensor there can be written as a linear combination of 'translational' KY tensors (which are a simple wedge product of translational Killing vectors) and 'rotational' KY tensors (which are a wedge product of translations with a spacetime rotation, completely antisymmetrized) [73]. In particular case of D = 4 we have 10 rank-2 KY tensors (6 translational and 4 rotational). 10 In general, for a simple spacelike (f ab f ab > 0) Killing-Yano tensor f , there exists a close analogy between the angular momentum of classical mechanics and the vector L a = f ab p b [33]. 11 In this step, we associate constant a with 'rotation' parameter. and rewrite the metric, the potential, the principal CKY tensor, and the KY tensor as dS 2 = − dT 2 + (r 2 + a 2 ) sin 2 θdφ 2 + (r 2 + a 2 cos 2 θ) dr 2 Second, we introduce the new coordinates in which the metric takes the 'algebraic' form where The hidden symmetries are In the potential b the term proportional to a 2 is constant and may be omitted. We remind that (46)- (47) is just a metric of the flat space written in special coordinates.
Let us consider now the metric (46) without imposing conditions (47) on functions ∆ r and ∆ y , but assuming that they are functions of r and y, respectively. Then substituting this ansatz into the Einstein equations one finds that these equations are satisfied provided the following relation is valid: The most general solution of this equation is In other words, a simple replacement of functions (47) by more general polynomials (51) generates a non-trivial solution of the Einstein equations from a flat one. This solution is the Kerr-NUT-(A)dS metric written in the canonical form [32]. It obeys the Einstein equations with the cosmological constant (see (49)). M stands for mass, and parameters a and N are connected with rotation and NUT parameter [74].
A remarkable fact is that in canonical coordinates, with arbitrary ∆ r (r) and ∆ y (y), the objects b, h, and f (48) are again the potential, the principal CKY tensor, and the KY tensor for the metric (46) [58,75]. In particular, these relations give a principal CKY tensor, where K ab = f ac f c b , is well defined and conserved. In the absence of the cosmological constant and NUT parameter, that is for the Kerr black hole, this quantity can be presented in the form (42) in the asymptotic region, where the spacetime is practically flat. The angular momentum and other quantities which enter (42) must be then understood as the corresponding asymptotically conserved quantities. Since the energy E and the angular momentum along the axis of symmetry L Z are conserved exactly in any stationary axisymmetric spacetime they can be excluded from (42) and the asymptotically conserved quantity can be written as follows [76]: For a scattering of particles in the Kerr metric, the presence of an exact integral of motion connected with the Carter's constant implies that the quantity Q calculated for the incoming from infinity particle must be the same as Q calculated at the infinity for the outgoing particle. An interesting question is the following: suppose that such a conservation law is established for any scattering of particles by a localized object, can one conclude that the metric of this object possesses a hidden symmetry?

C. Symmetric form of the metric
Let us perform the 'Wick' rotation in radial coordinate r. This transforms the metric (46) and its hidden symmetries into a symmetric form [45]. After transformation the metric and the KY objects take the form This form of the Kerr-NUT-(A)dS spacetime and of the potential (the principal CKY tensor) allows a natural generalization to higher dimensions [45,58]. 12

D. Principal conformal Killing-Yano tensor and canonical coordinates
We demonstrate now that the coordinates (t, x, y, ψ) used in (54)-(58) have a deep invariant meaning. Let us define then one has 12 It is obvious from the derivation that this symmetric form of the metric and of its hidden symmetries is an analytical continuation of the real physical quantities (46), (48), (51). The signature of the metric for this continuation depends on the domain of coordinates x and y and the signature of ∆ x and ∆ y . For example, for x > y and ∆ x > 0, ∆ y < 0 it is of the Euclidean signature. The transition to the physical space is given by (53). In higher dimensions it is very convenient to work with a generalization of this symmetric form.
The condition det(∆) = 0 which determines the eigenvalues H of the operator H 13 is equivalent to the following equation: Hence the eigenvalues of H are Simple calculations using (43) give Thus the coordinates x and y in (54) Moreover, is the secondary Killing vector, cf. eq (5). In coordinates (45) one has It means that the coordinates t and ψ are the affine parameters along the primary and secondary Killing vectors ξ (0) and ξ (ψ) , determined by the tensor h. It can be checked that the same is true for the Kerr-NUT-AdS metric (54), (55) with the principal CKY tensor h given by (57). This underlines the exceptional role of this tensor. Remarkably, the existence of a similar object in the higher-dimensional Kerr-NUT-(A)dS spacetime generates besides the tower of hidden symmetries also all the isometries of this spacetime, in a way exactly analogous to four dimensions (see Section V). It also determines the canonical coordinates for these metrics [59,77,78]. 13 The operator H is the conformal Killing tensor. It is related to K as

E. Separation of variables
The last subject we would like to discuss in this brief review of properties of the 4D Kerr-

NUT-(A)dS metrics is the separation of variables for the Hamilton-Jacobi and Klein-Gordon
equations.
Let us first consider the Klein-Gordon equation in a spacetime (54) with ∆ x and ∆ y arbitrary functions of x and y, respectively. The separation of variables of equation (68) in the canonical coordinates (τ, x, y, ψ) means that Φ can be decomposed into modes Indeed, substituting this expression in the Klein-Gordon equation (68) one obtains Here the prime stands for the derivative of function with respect to its single argument. Similarly, the Hamilton-Jacobi equation for geodesic motion in a generalized metric (54) allows a separation of variables and S can be written in the form S = µ 2 λ + ετ + mψ + S x (x) + S y (y) .
The functions S x and S y obey the equations Presented 'derivation' of 4D Kerr-NUT-(A)dS metric and its hidden symmetries can be naturally generalized into higher dimensions. We shall not do this here, but simply mention that the corresponding expressions for the flat spacetime metric in the canonical coordinates and for the potential b can be easily obtained by taking the flat space limit of the formulae (90) and (94) (see Section V).

A. Important property of closed CKY tensors
The following result [59] plays a central role in the construction of the hidden symmetry objects in higher-dimensional spacetimes.
Theorem. Let h (1) and h (2) be two closed CKY tensors. Then their external product is also a closed CKY tensor.
We shall prove this Theorem in two steps. The fact that h is closed is trivial, cf. eq. (22). Let us first show that for a p-form α p obeying the equation one has Indeed, we find Here we have used eq. (25), and relations (27), (28).
The second step in the proof of the Theorem is to show that if α p and β q are two closed CKY tensors then Really, one has where Combining (79) with (76) we arrive at the statement of the theorem.

B. Principal CKY tensor and towers of hidden symmetries
Let us consider now a special case which is important for applications. Namely, we assume that a spacetime allows a 2-form h which is a closed CKY tensor, h = db. We also assume that h is non-degenerate, that is its (matrix) rank is 2n. We call this object a principal CKY tensor [59].
According to the Theorem of the previous section, the principal CKY tensor generates a set (tower) of new closed CKY tensors h (j) is a (2j)-form, and in particular h (1) = h. Since h is non-degenerate, one has a set of n non-vanishing closed CKY tensors. In an even dimensional spacetime h (n) is proportional to the totally antisymmetric tensor whereas it is dual to a Killing vector in odd dimensions.
In both cases such CKY tensor is trivial and can be excluded from the tower of hidden symmetries. The CKY tensors can be generated from the potentials b (j) (cf. eq. (32)) Each (2j)-form h (j) determines a (D − 2j)-form of the Killing-Yano tensor In their turn, these tensors give rise to the Killing tensors K (j) A choice of the coefficient in the definition (84) is adjusted to Section V, cf. eq. (19). It is also convenient to include the metric g, which is a trivial Killing tensor, as an element K (0) of the tower of the Killing tensors. The total number of irreducible elements of this extended tower is n. 14

C. Canonical basis and canonical coordinates
Similar to 4D case, let us consider the eigenvalue problem for the conformal Killing tensor It is easy to show that in the Euclidean domain its eigenvalues x 2 , are real and non-negative. Using a modified Gram-Schmidt procedure it is possible to show that there exists such an orthonormal basis in which the operator h has the following structure: where Λ i are matrices of the form and I i are unit matrices. Such a basis is known as the Darboux basis (see, e.g., [79]). Its elements are unit eigenvectors of the problem (85). and eμ span a 2-dimensional plane of eigenvectors of (85) with the same eigenvalue x µ . We denote by ω µ and ωμ ≡ ω n+µ (and ω 0 if ε = 1) the dual basis of 1-forms. The metric g and the principal CKY tensor h in this basis take the form was found by Chen, Lü, and Pope [45]. In an (analytically continued) symmetric form it is written as (88) where Here Metric coefficients X µ are functions of x µ only, and for the Kerr-NUT-(A)dS solution take the form Time is denoted by ψ 0 , azimuthal coordinates by ψ k , k = 1, . . . , m, and x µ , µ = 1, . . . , n, (For more details see [45].) Similar to the 4D case, the signature of the symmetric form of the metric depends on the domain of x µ 's and the signatures of X µ 's.
Hamamoto, Houri, Oota, and Yasui [80] derived explicit formulas for the curvature and demonstrated that in all dimensions this metric obeys the Einstein equations, The limit of flat spacetime is recovered when c n = 0 and all of the parameters b µ are zero (equal one to each other) in the even (odd) dimensional case. The metric belongs to the class of special algebraic type D solutions [80] of the higher-dimensional algebraic classification [81]. It may be understood as a higher-dimensional generalization of the 4-dimensional Ker-NUT-(A)dS solution obtained by Carter [8]. Moreover, the coordinates (x µ , ψ k ) used in the metric are the higher-dimensional analogue of the canonical coordinates [8,32].
It was shown in [57,58] that this spacetime possesses a principal CKY tensor h which has the form (89) and its potential b, h = db, The tower of Killing tensors (84) associated with this principal CKY tensor is [59] In the Kerr-NUT-(A)dS spacetime besides this tower of Killing tensors also all the isometries follow from the existence of the principal CKY tensor h [59]. The primary Killing vector ξ (0) is given by (20) and reads This vector plays a special role. After the analytical continuation to the physical space it is the only one timelike in the black hole exterior. Besides the primary Killing vector, this 15 In fact, this potential generates a principal CKY tensor for a general form of the metric (88),(90), and (91) with arbitrary functions X µ (x µ ).
spacetime has n − 1 secondary Killing vectors ξ (j) , In odd dimensions the last Killing vector is given by the n-th Killing-Yano tensor f (n) , which in the Kerr-NUT-(A)dS spacetime turns out to be The total number of these Killing vectors is n + ε. For a geodesic motion they give n + ε linear in momentum integrals of motion, a p a . The extended tower of the Killing tensors K (j) (j = 0, . . . n − 1) gives n additional integrals of motion, which are quadratic in the momentum, κ j = K (j) ab p a p b . Thus the total number of conserved quantities for a geodesic motion is 2n + ε, that is it coincides with the number of spacetime dimensions D.
It is possible to show that these integrals of motion are independent and in involution, so that the geodesic motion in the Kerr-NUT-(A)dS spacetime is completely integrable [60,61]. The components of the particle momentum p can be written as functions of the integrals of motion as follows [59]: Constants σ µ which denote the choice of signs are independent of one another.
It should be emphasized that, similar to 4D case, the coordinates (x µ , ψ k ) in the metric The existence of a principal CKY tensor imposes non-trivial restrictions on the geometry of the spacetime. Namely, the following result was proved in [77,78]. Let h be a principal CKY tensor and ξ (0) be its primary Killing vector. Then if Let us finally mention that recently it was shown [82] that the following operators (cf. eq. (6)):ξ

VI. HIDDEN SYMMETRIES AND SEPARATION OF VARIABLES
The massive scalar field equation in the Kerr-NUT-(A)dS metric allows a complete separation of variables [62]. Namely, the solution can be decomposed into modes Substitution of (104) into equation (103) results in the following second order ordinary differential equations for functions R µ (x µ ) Here Here κ 0 = −µ 2 and for ε = 1 we put κ n = Ψ 2 n /c. The parameters κ k (k = 1, . . . n + ε − 1) are separation constants. Using (102) one hasK (0) = − . Since all the operators (101)-(102) commute with one another, their common eigenvalues can be used to specify the modes. It is possible to show [82] that the eigenvectors of these commuting operators are the modes (104) and one hasξ Similar to the case discussed in 4D, in the symmetric form of the metric (88), (90), (91) all the equations (105) 'look the same'. However, after the transformation to the physical space the equation for R n plays the role of an equation for radial modes, whereas the other equations present the eigenvalue problem. For a discussion of special sub-cases of these equations see, e.g, [83] and reference therein.
The Hamilton-Jacobi equation for geodesic motion in the Kerr-NUT-(A)dS spacetime also allows a complete separation of variables [62] The functions S µ obey the first order ordinary differential equations where the functions V µ and W µ are defined in (106).
Recently it was shown that the massive Dirac equation in the Kerr-NUT-(A)dS spacetime also allows the separation of variables [63]. It was also proved that the Nambu-Goto equations for a stationary test string in the Kerr-NUT-(A)dS background are completely integrable [65] .

VII. CONCLUSIONS
In this review we discussed recent developments of the theory of higher-dimensional black holes. We focused mainly on the problem of hidden symmetries and separation of variables.
(For more general discussion of the modern status of the theory of higher-dimensional black holes see a recent review [51].) In our presentation, we started with a description of known results concerning four-dimensional black holes. We found this important since many of Petrov type D. A natural conjecture in higher dimensions is that any Einstein space which possesses the principal CKY tensor is of the special algebraic type D of higher-dimensional classification [81].
Does this analogy go further? We focused on black holes, but in the higher-dimensional gravity there exists a variety of other black objects such as black rings or black saturns. Do these spacetimes also possess hidden symmetries? Such symmetries would be very helpful for the study of the stability of these 'exotic' objects. However, for example black rings are of the algebraic type I i [86] and thus their symmetry properties might be quite different from the properties of higher-dimensional black holes. In particular, if the above conjecture is correct they do not allow a principle CKY tensor and so nor a corresponding tower of hidden symmetries.
The results on the separability till now obtained can be used for the study of the particle and light propagation in higher-dimensional rotating black hole spacetimes. They allow one to calculate the contribution of the scalar and Dirac fields to the bulk Hawking radiation of higher-dimensional rotating black holes, without any restrictions on black hole parameters.
An important open question is a separability problem for the gravitational perturbations in higher-dimensional black hole spacetimes. A certain progress in this direction was achieved recently (see, e.g., [84], [64]). These results are very important for the study of the stability of such black holes and different aspect of the Hawking radiation produced by them. Another important direction of research is the study of the quasinormal modes in higher dimensions.
Most of the results obtained in these directions (see, e.g., [85] and references therein) assumed for the study of the stability of higher-dimensional black hole solutions. One might hope that it will not take too long time before this and other interesting open questions connected with the existence of hidden symmetries in higher-dimensional black holes will find their answers.