ternal momenta of the amplitude head to tail [31]. In fact, this was the first hint of the
amplitude/Wilson loop duality. For the M5-brane theory, we would expect the analogous
calculation to involve the minimal volume of an M2-brane whose boundary is a 2d surface
which somehow encodes a null polygon. Remarkably, by relaxing a constraint of the Alday-
Maldacena solution, we obtain a solution which minimizes the volume of a 2-brane in AdS
and the boundary of this solution is a pillow-shaped surface whose seams correspond to a
null polygon.
4
We evaluate the on-shell action for the 2-brane solution and find a structure
very similar to the 6d 1-loop amplitude, which suggests an amplitude/Wilson surface dual-
ity and the existence of an all-loop BDS-like formula for the 4-point amplitude. Unlike the
on-shell action of the Alday-Maldacena string solution, the on-shell action of the 2-brane
has a prefactor that depends on the dimensional regularization parameter = (6 − d)/2
and diverges like 1/ as → 0. This additional divergence is consistent with previous holo-
graphic calculations of Wilson surfaces [65, 66], and can be associated with the conformal
anomaly of Wilson surfaces [67–70].
5
This note is organized as follows. In 2 we explore the consequences of dual conformal
symmetry in six dimensions. In particular, we find that dual conformal symmetry fixes the
integrand of the 1-loop 4-point amplitude, which we integrate in d = 6−2 dimensions using
the Mellin-Barnes technique. We also initiate the study of 2-loop 4-point amplitudes. In
section 3, we find a solution describing a minimal volume 2-brane in AdS whose boundary is
a 2d surface encoding a null polygon. We then compute the on-shell action for this solution
and find that it has a very similar structure to the 1-loop 4-point amplitude we computed in
section 2, suggesting the existence of an amplitude/Wilson surface duality and an all-loop
formula for the 4-point amplitude analogous to the BDS formula of N = 4 SYM. Finally,
in section 4 we present our conclusions and describe future directions. Appendix A gives
more details about 2-loop 4-point integrands and appendix B provides more details about
the calculation of the on-shell action of the 2-brane in AdS.
2 6d dual conformal amplitudes
Let us consider a hypothetical 6d theory for which a planar limit can be defined and
whose loop integrands are rational. We shall study the consequences of dual conformal
symmetry for such a theory, primarily focusing on the 4-point case. We shall also assume
that in the planar limit it is possible to define color-ordered amplitudes which are cyclically
symmetric (the definition of color-ordering in gauge theories can be found in many reviews,
see for example [73]). In N = 4 SYM and the ABJM theory, it is possible to encode the
four-point amplitudes with all possible asymptotic states into a single quantity known as a
superamplitude. When this quantity is divided by the tree-level four-point superamplitude,
the resulting function does not depend on asymptotic states and is determined to all orders
by dual conformal symmetry in the planar limit. Moreover, it can be computed at strong
4
Note that since the original null polygon of [31] is embedded within this more general surface, it is
parameterized by the same two parameters corresponding to the Mandelstam variables s and t in the
amplitude picture.
5
Also see [71, 72] for related discussion of Wilson surface operators in the M5-brane theory.
– 4 –