JHEP09(2016)155
contractions of covariant derivatives and Riemann tensors. The tensor structure of each
operator as well as its coefficient furnished by multiple zeta values (MZVs)
ζ
n
1
,n
2
,...,n
r
≡
∞
X
0<k
1
<k
2
<...<k
r
1
k
n
1
1
k
n
2
2
. . . k
n
r
r
(1.2)
can be derived by expanding string-theory graviton amplitudes in α
′
. MZVs can be conjec-
turally categorized according to their transcendental weight n
1
+n
2
+. . .+n
r
and constitute
a fruitful domain of common interest between high-energy physics and number theory. In
fact, for type-II theories, the transcendental weight for each coefficient matches the order
of α
′
. This property will be referred to as uniform transcendentality, and it al so exists
for open strings in the type-I the ory. The type-I effective action is now an expansi on in
non-abelian field-strength operators tr(D
n
F
m
). In th i s light, uniform transcendentality for
closed strings is inherited from open strings through the Kawai, Lewellen and Tye (KLT)
relations [
7].
In this letter, we conjecture that the leading transcendental coefficient at each order
in the α
′
-expansion of t ree-level ampli t u de s is u niv er s al among a ll perturbative open- and
closed-string theories. We have explicitly verified this up to the seven-p oi nt level, and the
conjectural all-multiplicity extension is further investigated in a companion paper [
8]. This
also implies that at finite α
′
, pert ur bat i ve closed string amplitudes contain a universal piece
that cor r es pond to the UV c ompl e t i on of tree-level Einstein-Hilbert graviton amplitudes.
This is given by t he type-II theories. For Heterotic and Bosonic closed string theories,
this i s augmented by s ep arat e terms that correspond to UV completions of amplitudes
generated by higher dime ns i onal operators such as R
2
, R
2
φ and R
3
, where φ can be the
dilaton or the Tachyon. This remarkable property can be best understood by inspecting
the world-sheet correlator of the open-string amplitud es .
It was shown in [
9] that the n-poi nt tree amplitude of the open supe r st r i ng can be cast
into an (n−3)! basi s of disk i ntegrals, e ach augmented with Yang-Mills tree amplitudes
of different color-orderings. These basis integrals exhibit uniform transcendentality upon
α
′
-expansion, see e.g. [
10] for a proof. We claim that bosonic open-string disk integral can
be cast into the very same basis where — in contrast to the superstring — the augmented
function depends on α
′
. More precisely, this augmented function contains apart from the
Yang-Mills tree amplitude, additional rational functions that contain Tachyon poles. Thus
in their low-energy limit α
′
→ 0, the α
′
-corrections of the kinematic functions excl usi vely
involve rational numbers upon Taylor-expansion, i.e. they do not carr y any transcendental
weight. This implies that the resulting α
′
-expansion of the bosonic string amplitude will
have the same leading transcendental pieces as found for the superst r i ng.
The same property can be ex t e nde d to closed strings by utilizing the KLT-relations [
7],
which assemble closed-string tree amplitudes fr om products of two open-string tr ee s. The
accompanying sin-functions with α
′
-dependent argu me nts do not alter the uniform tran-
scendentality of the type-II t he ory. Different double-copies of open bosonic strings and su-
perstrings give rise to three different closed-string theories — bosonic, heterotic and type-II
superstrings. Their tree amplitudes are governed by a universal basis of (n−3)! × (n−3)!
– 2 –