Johan Bijnens, Karol Kampf, Mattias Sjö
Higher-order tree-level amplitudes in the nonlinear sigma model


We present a generalisation of the flavour-ordering method applied to the chiral nonlinear sigma model with any number of flavours. We use an extended Lagrangian with terms containing any number of derivatives, organised in a power-counting hierarchy. The method allows diagrammatic computations at tree-level with any number of legs at any order in the power-counting. Using an automated implementation of the method, we calculate amplitudes ranging from 12 legs at leading order, O(p^2), to 6 legs at next-to-next-to-next-to-leading order, O(p^8). In addition to this, we generalise several properties of amplitudes in the nonlinear sigma model to higher orders. These include the double soft limit and the uniqueness of stripped amplitudes.

LU TP 19-46