Double soft theorems in gauge and string theories

We investigate the tree-level S-matrix in gauge theories and open superstring theory with several soft particles. We show that scattering amplitudes with two or three soft gluons of non-identical helicities behave universally in the limit, with multi-soft factors which are not the product of individual soft gluon factors. The results are obtained from the BCFW recursion relations in four dimensions, and further extended to arbitrary dimensions using the CHY formula. We also find new soft theorems for double soft limits of scalars and fermions in N = 4 $$ \mathcal{N}=4 $$ and pure N = 2 $$ \mathcal{N}=2 $$ SYM. Finally, we show that the double-soft-scalar theorems can be extended to open superstring theory without receiving any α ′ corrections