Universal bound on the strong coupling scale of a gravitationally coupled massive spin-2 particle

摘要

We find a model-independent upper bound on the strong coupling scale for a massive spin-2 particle coupled to Einstein gravity. Our approach is to directly construct tree-level scattering amplitudes for these degrees of freedom and use them to find the maximum scale of perturbative unitarity violation. The highest scale is Λ3 = (m~2M_P)~1/3, which is saturated by ghost-free bigravity. The strong coupling scale can be further raised to M_P if the kinetic term for one particle has the wrong sign, which uniquely gives the amplitudes of quadratic curvature gravity. We also discuss the generalization to massive higher-spin particles coupled to gravity.