A discrete basis for celestial holography

Abstract: Celestial holography provides a reformulation of scattering amplitudes in four dimensional asymptotically flat spacetimes in terms of conformal correlators of operators on the two dimensional celestial sphere in a basis of boost eigenstates. A basis of {massless particle} states has previously been identified in terms of conformal primary wavefunctions labeled by a boost weight $\Delta = 1 + i\lambda$ with $\lambda \in \mathbb{R}$. Here we show that a {\it discrete} orthogonal and complete basis exists for $\Delta \in \mathbb{Z}$. This new basis consists of a tower of discrete memory and Goldstone observables, which are conjugate to each other and allow to reconstruct gravitational signals belonging to the Schwartz space. We show how generalized dressed states involving the whole tower of Goldstone operators can be constructed and evaluate the higher spin Goldstone 2-point functions. Finally, we recast the tower of higher spin charges providing a representation of the $w_{1+\infty}$ loop algebra (in the same helicity sector) in terms of the new discrete basis.