JHEP06(2017)093
would factor over the state-sum of unitarity cuts, proved critical for gathering information
about the spectacular ultraviolet behavior of the m ax i mal l y supersymmetric supergravity
theory through four-lo ops [
33–37]. In the process of such explorations, the field of scatter-
ing amplitudes acquired a new set of insights relevant to the double-c opy story when Bern,
Johansson, and one of the current authors (BCJ) observed [
5] that there was a very di r e c t
path to gravity-theory predictions, where the double-copy construction could be made man-
ifest graph by graph. This has particular value at the multiloop integrand level [
7], where
integrand labeling ambiguities can create obstacles for realizing generalized-gau ge-i nvariant
double-copy relationships, outside of certain kinematic limits like unitarity cuts.
The BCJ double-copy appr oach r elies critically on the realization [
5] that gauge-theor y
predictions (and t he i r supersymmetric partners) admit a color-kinematic duality satisfying
representation (color-dual representations are ones where graph by gr aph color-weights
and kinematic weights obey the same generic algebraic properties). The existence of such
color-dual representations resulted in the discovery of new relations between color-ordered
amplitudes known now as the BCJ relations. With such a dual representation, color-factors
could be consistently replaced
6
by kinematic weights, recycling a small set of kinemat i c
predictions to describe a wide variety of theories [
7]. Additionally, color-dual kinematic
weights could be solved for in terms of color -or de r ed amplitudes, thus allowing for the
generation of gener al i ze d KLT relations.
While a Lagrangian understanding of the organizing pri nc i pl e is only available in the
four-dimensional self-dual case [39], many theories, includi n g the NLSM [40], in a variety
of spacetime dimensions, admit the duali ty betwee n color and kinematics, and associated
double-copy construction [
41–51]. This new perspec ti ve on field-theor y predict i ons has
proven critical in developing aspects of our understanding of non-planar scatt er i ng ampli-
tudes over the last decade, both formally as well as through practical reach i n computation.
Jacobi relations drastically constrain the independent information relevant to a given scat-
tering calculation. For instance, the closely related double-copy constructions of multi l oop
gravity amplitudes [
7] has allowed many explicit c al c ul at i ons that can probe the possible
onset of ultraviolet divergences in supergravity theories [
52–55].
String theory continues to provide key insights probing the color-kinematics dual-
ity and its associated representation of the double-copy construction: from the powerful
proof [
11, 12] of the (n−3)!-basis of Yang-Mills tree amplitudes as the low-energy limit of
the related string monodromy relations,
7
to the elegant construction of explicit loc al tree-
level numerators [
57–59], to the construction of string-inspir e d BCJ numerators [60–62] at
loop level. The fact that the BCJ-du al i ty also applies to the NLSM [
40], c an now be ap-
preciated either as a consequence of the BCJ relations satisfied [
3] by Z-theory as in (1.3) ,
or as a require me nt for the NL SM to be able to par ti c i pat e in Z-theory ’ s construction of
the open superstring. Following the recent result of Du and Fu [
19] who present an elegant
closed-form construction of l ocal col or - k i nem ati c s satisfying numerators in the NLSM, we
will discuss it s applicability to all orders in α
′
.
6
Established to some finite multiplicity at tree-level in [
5], but later pr oven via BCFW and the KLT
relations in [
38].
7
See [
56] for a recent higher-loo p generalization.
– 5 –