Continuous spin particles, on-shell

Continuous spin particles (CSPs) arise naturally in the classification of unitarity representations of the Poincaré group, but have long been dismissed because of their unintuitive nature. Recent progress has revived CSPs, dismantling old prejudices. In particular, CSPs are massless states for which helicity is not a good quantum number and can take any integer (bosonic CSPs) or half-integer (fermionic CSPs) values, which mix under boosts. Among other counter-intuitive properties and contrary to the effective-field-theory intuition, CSPs could be interpreted as a long-distance deformation of ordinary, fixed-helicity, massless states. In this talk, I will show how to bootstrap from physical principles (such as Lorentz invariance, little-group covariance, unitarity) the S-matrix elements involving CSPs as internal and/or external states.