Worldline Higher Spin Gravity

We propose a worldline formulation of higher-spin gravity (HSG) in $\mathrm{AdS}_4$, based on a simple twistor action. Taken at face value, the model describes only the free propagation of massless higher-spin fields. The central observation of this work is that the model admits a natural double-line interpretation, which supplies a geometric prescription for gluing worldlines at interaction vertices, in close parallel with the joining of strings in string theory. Building on this picture, we construct $\mathrm{AdS}$-covariant vertex operators for all massless higher-spin fields, show that they satisfy the Bargmann-Wigner equations, and use them to compute the n-point correlation functions of type-A and type-B HSG as worldline path integrals of these vertex operators. In the boundary limit these correlators reproduce the higher-spin current correlators of free boson and free fermion vector models. We further discuss the embedding of the worldline theory into Poisson sigma model, where the doubled-line structure acquires a geometric origin as the two edges of an open string worldsheet, together with several consequences of this enlarged framework -- fractional branes, loop expansion, unoriented projection, and the prospect of a worldsheet formulation of HSG.