On the Theory of Continuous-Spin Particles: Wavefunctions and Soft-Factor Scattering Amplitudes

The most general massless particles allowed by Poincare-invariance are "continuous-spin" particles (CSPs) characterized by a scale \rho, which at \rho=0 reduce to familiar helicity particles. Though known long-range forces are adequately modeled using helicity particles, it is not known whether CSPs can also mediate long-range forces or what consequences such forces might have. We present sharp evidence for consistent interactions of CSPs with matter: new CSP equations of motion, wavefunctions, and covariant radiation amplitudes. In a companion paper, we use these results to resolve old puzzles concerning CSP thermodynamics and exhibit a striking correspondence limit where CSP amplitudes approach helicity-0, 1 or 2 amplitudes.