Abstract

Six-dimensional theories provide a unification of four-dimensional theories via dimensional reduction and access to some of the novel features arising from M-theory. Ambitwistor strings directly generate S-matrices for massless theories in terms of formulae that localize on the solutions to the scattering equations; algebraic equations that determine n points on the Riemann sphere from n massless momenta. These are sufficient to provide compact formulae for tree-level S-matrices for bosonic theories. This talk introduces their extension to the polarized scattering equations that arise from twistorial versions of ambitwistor-strings. These lead to simple explicit formulae for superamplitudes in 6d for gauge, gravity and brane theories and the framework extends to 10 and 11 dimensions. These satisfy a double copy principle that leads to analogous formulae for theories containing gerbes, but their physical interpretation remaions controversial. This is based on joint work with Yvonne Geyer, arxiv:1812.05548 and 1901.00134 and with Giulia Albonico also 2001.05928.