Cubic interactions of higher-spin gauge fields in (A)dS d are studied in the metric-like approach. Making use of the traceless and transverse constraints together with the ambient-space formalism, all consistent parity-invariant cubic vertices are obtained for d ≥ 4 in a closed form pointing out the key role of their flat-space counterparts.
IntroductionUnderstanding the systematics of higher-spin (HS) gauge theories 1 has been attracting an increasing attention in recent years, and finding a consistent Lagrangian that describes their interactions is one of the main problems in the subject. Vasiliev's equations [10,11] provide at present the only known fully non-linear consistent description, at least at the classical level, of an infinite number of HS gauge fields of all spins. 2 However the nature of their couplings still leaves interesting questions to be answered. The generalization of the lower-spin gauge interactions of Yang-Mills and Gravity to HS is associated with a non-linear deformation of the Abelian HS gauge symmetries of the free theory [19,20] 3 and can be studied perturbatively by means of the Noether procedure. This actually rests on enforcing gauge invariance of the full Lagrangian order by order in the number of fields, and has been considered by mainly two different perspectives: frame-like or metric-like formalisms.Important progress on HS cubic interactions in the frame-like approach was obtained by Fradkin and Vasiliev (FV) [31,32] who extended the gravitational minimal coupling to s 1 −s 2 −s 3 HS couplings. Their construction of cubic couplings is consistent in (Anti) de Sitter ((A)dS) backgrounds, and one of its essential features is the presence of inverse powers of the cosmological constant. Very recently, these interaction vertices in AdS 4 were generalized to AdS d [33] , that were conjectured to cover all vertices that can be constructed in terms of connection one-forms and curvature two-forms of symmetric HS gauge fields. In fact, the goal of the present paper is the same as that of [33], and it would be in principle interesting to explore the relation of our results with those of [33]. This comparison is although non-trivial since the two constructions use very different mathematical devices, and we will only discuss in the conclusion how the FV structure of the vertices is recovered in our approach. See [34][35][36][37] for other recent developments in the frame-like approach to the cubic-interaction problem.On the other hand, the flat-space cubic vertices of HS gauge fields in the metric-like formalism were investigated first by Berends, Burgers and van Dam [38,39], and then by many other authors. 4 Notably, the consistent vertices were classified by Metsaev [40][41][42] in terms of the number of derivatives within the lightcone approach. Despite various efforts made along the years by a number of authors, only recently has it been possible to arrive at a covariant description of all bosonic flat-space cubic interactions by Manvelyan, from a field theoretical perspective. At the...