
Having a higher k version of the scattering equations, one can immediately generalize
the Cachazo-He-Yuan (CHY) formula for biadjoint amplitudes [7]. Such amplitudes can
then be expanded in terms of higher k Feynman diagrams as defined by facets of the tropical
Grassmanian [5]. Even though a direct quantum field theory derivation of such generalized
Feynman diagrams is not yet known, it is important to mention that the known physically
relevant case of k = 2 implies via the duality X(2, n) ∼ X(n − 2, n) that k = n − 2 is also
physically relevant. It is therefore very natural to complete the spectrum of possibilities
between the two ends, i.e., for 2 < k < n − 2. At the very least one can expect that a good
understanding of such cases would deepen that of the k = 2 and k = n − 2 cases.
Given two orderings α and β one defines [5]
m
(3)
n
(α|β) :=
1
vol(SL(3, C))
Z
n
Y
a=1
dx
a
dy
a
n
Y
0
a=1
δ
∂S
∂x
a
δ
∂S
∂y
a
PT
(3)
(α)PT
(3)
(β) (1.4)
where k = 3 Parke-Taylor functions [8] are naturally generalized to
PT
(3)
(1, 2, . . . , n) :=
1
|123| |234| · · · |n12|
. (1.5)
CHY integrals are localized to the solutions of the scattering equations [4]. For k = 2 it
is known that the number of solutions is (n − 3)!. For k = 3 it is known that there are 2
and 26 solutions for n = 5 and n = 6 respectively [5]. Also in [5] two lower bounds for the
number of solutions for n = 7 were obtained using soft limit arguments. The stronger of
the two is 1152. In this note we prove that the number of solutions is 1272. This number
is consistent with a computation done by T. Lam using uniform matriods over finite fields
which also reproduces all other known results [9].
In [5] the authors provided evidence that CHY integrals defined on X(k, n) have a deep
and surprising connection to the tropical Grassmannian Trop G(k, n). The connection for
k = 2 is fairly direct. It maps all possible 2
n−1
− n − 1 propagators to all possible vertices
(or rays) of Trop G(2, n). In addition, there is also a bijection between every one of the
(2n − 5)!! tree-level Feynman diagrams in a cubic scalar theory and facets of Trop G(2, n).
In [5] the case of n = 6 and k = 3 was studied in detail. In this work we extend the
same analysis to n = 7, k = 3 biadjoint amplitudes and Trop G(3, 7). Luckily, the structure
of Trop G(3, 7) has been carefully studied by Herrmann, Jensen, Joswig, and Sturmfels [10]
and we make use of their results to carry out all our Feynman diagram computations which
lead to the explicit form of all 360 × 360 biadjoint amplitudes m
(3)
7
(α|β).
The strategy for determining the number of solutions to the X(3, 7) scattering equa-
tions is the following. We start with the dual version, i.e., the X(4, 7) scattering equations
near a soft limit. The strict soft limit gives equations on X(4, 6) w X(2, 6) and produces
6 solutions. Each solution is then used to find a polynomial of degree 192 from the equa-
tions for the soft particle. This produces 6 × 192 = 1152 seeds of solutions from which
a numerical search for the true solutions can be launched. In the kinematics we use this
search returned 1162 solutions which we obtained with more than 2000 digits of precision.
Having the explicit form of all 360 × 360 biadjoint amplitudes m
(3)
7
(α|β) and their
corresponding approximation m
(3):approx
7
(α|β) from using (1.4) with 1162 solutions, we
– 2 –