Worldsheet description of high-energy string amplitudes

The high-energy behaviour of string scattering amplitudes is 
expected to be governed by a tensionless string theory. In the 
tensionless limit, the worldsheet becomes null, and the residual 
Virasoro symmetry is replaced by 2D conformal Carrollian symmetry, which 
is isomorphic to 3D BMS symmetry. Quantization of the tensionless string 
admits three distinct choices of vacuum, leading to different quantum 
theories. In this talk, I will focus on the induced vacuum, which is the 
smooth tensionless limit of the highest weight vacuum of tensile string 
theory. I will discuss the construction of tree-level scattering 
amplitudes in the tensionless theory and show how they reproduce the 
characteristic high-energy behaviour of string amplitudes. If time 
permits, I will also comment on the corresponding amplitudes in the 
highest-weight representation of tensionless string theory.