Physics Group Meeting

We will discuss the issue of UV completion in the context of weakly coupled theories. For concreteness, we will ask: is it possible to complete a low-energy 4-particle scattering amplitude into one which is well-behaved at high-energies, with only poles in s,t,u? The physical requirements of causality and unitarity are incredibly
constraining, and in theories which have a 3-particle amplitude (and so 4-particle amplitudes with IR poles), we'll see that no UV improvement is possible with a finite number of new massive particles; instead an infinite tower of particles and higher spins are required. We will then discuss a sharply posed conjecture, that UV complete
amplitudes with these properties are uniquely the ones given by string theory. We'll especially highlight the crucial role of unitarity, in the form of certain positivity properties of residues at the poles, and the miraculous way in which string theory amplitudes satisfy them, reflecting the real "teeth" of the no-ghost theorem. Finally we will discuss amplitudes for a number of other theories, which are UV completed in a way that looks "stringy"; some of which don't seem to correspond to any worldsheet description.