IAS Amplitudes Group Meeting

Abstract: Recent developments in massive on-shell scattering amplitudes in 4D have opened the door to understanding particle physics efficiently from an on-shell point-of-view, including on-shell notions of minimal coupling and black holes as elementary particles. Important special structures, like the $x$-factor, only appear when the masses of the particles look like they come from compactifying a 5D theory. We develop the groundwork for an on-shell spinor-helicity formalism for scattering amplitudes in 5 space-time dimensions and present a systematic treatment of on-shell 5D amplitudes of massless particles. We discover special structures of 5D massless amplitudes and connect these to massive 4D amplitudes. These structures indicate that massless higher spin particles cannot be minimally-coupled even at the level of kinematics, giving evidence that minimal coupling for black holes are very different in $D>4$.
In this talk, we will review minimal coupling and why it’s a hard problem at higher spin. We will exhibit multiple ways that D=4 special, using both local lagrangians and on-shell physics. It becomes clear that it is prudent to think of minimal coupling and non-minimal coupling in terms of invariant on-shell physics. We will have discussion on the applications of 5D amplitudes and the issue with higher dimensional black holes and generic minimal coupling in stringy scattering amplitudes.