Extremal couplings and gluon scattering in M-theory
We consider M-theory on the backgrounds AdS$_4\times S^7/\mathbb{Z}_{N_f}$ and AdS$_7\times S^4/\mathbb{Z}_2$, which have fixed point locii AdS$_{d+1}\times S^3$ for $d=3,6$. These theories are holographically dual to certain CFTs in $d=3,6$ with eight supercharges. We compute the bulk cubic couplings between graviton KK modes and gluon KK modes living on the fixed points of these theories, which are generically extremal. We use these couplings to compute the graviton exchange term that appears in the strong coupling expansion of holographic correlators of gluon KK modes $\langle 22pp\rangle$ in these theories, and check that it matches the expected flat space limit. We express the answer in terms of a new reduced correlator solution to the superconformal Ward identities, which we derive for all CFTs with eight supercharges in $3\leq d\leq6$.