Loops in Anti–de Sitter Space
Igor Bertan
*
and Ivo Sachs
†
Arnold Sommerfeld Center for Theoretical Physics, Ludwig Maximilian University of Munich,
Theresienstraße 37, D-80333 München, Germany
(Received 3 May 2018; published 4 September 2018)
We obtain analytic results fo r the four-point amplitude, at one loop, of an interacting scalar field theory in
four-dimensional, Euclidean anti–de Sitter space without exerting any conformal field theory knowledge.
For the two-point function, we provide analytic expressions up to two loops. In add ition, we argue that the
critical exponents of correlation functions near the conformal boundary of anti–de Sitter space provide the
necessary data for the renormalization conditions, thus replacing the usual on-shell condition.
DOI: 10.1103/PhysRevLett.121.101601
Introduction.—Over the past sixty years, there has been
tremendous progress in the calculation of scattering ampli-
tudes in quantum field theory—in particular, concerning
higher loop amplitudes in Yang-Mills theory and (super)
gravity.
At the same time, we have very few analytic results for
loop amplitudes in curved space-times. More precisely,
while their short-distance expansion and, in particular, the
structure of counterterms they give rise to is rather well
known [1,2], we know little about their dependence on
coordinates beyond that (e.g. Ref. [3] and references
therein). Even in de Sitter (dS) or anti–de Sitter (AdS)
space, which are maximally symmetric, admitting the same
number of isometries as Minkowski space, little is known
about such amplitudes; see Refs. [4–10] for recent progress.
The reason for this is that, while in Minkowski space, the
momentum representation leads to a hierarchy of elementary
integrals; in dS or AdS, this is not the case, and the coordinate
representation generally leads to integral expressions that
are more manageable in (A)dS. Still, except for some
special cases, we lack the technical tools for performing
the integrations completely.
In this Letter we report on some progress considering the
simplest interacting renormalizable field theory. Concretely,
we compute the two- and four-point functions for λϕ
4
theory
[11] to the second order in the coupling λ on the Poincar´e
patch of Euclidean AdS
4
by explicitly evaluating the
corresponding one- and two-loop integrals in coordinate
representation. Working on AdS, we avoid complications
that arise from IR effects on dS, for instance [3]. It turns out
that even this simplified setting is beyond reach for external
legs at generic points in AdS, but for insertions on the
conformal boundary we are able to get explicit expressions.
Being able to go beyond the short-distance expansion,
we encounter an interesting complication concerning the
renormalization conditions: For distances that are small
compared to the curvature scale, the problem reduces to
that in flat space, and the physical masses provide the right
boundary conditions for the renormalized propagator, for
instance. At scales of the order of the curvature radius,
however, there is no meaningful definition of the mass of
a scalar field, and one needs to identify a reasonable
renormalization condition. In the present case, we will
find that the critical exponents of correlation functions at
the conformal boundary of AdS provide just that. Indeed,
the bulk amplitudes on AdS with external legs inserted
at the boundary define a crossing symmetric point corre-
lation function of some hitherto unknown primary operator
of a conformal field theory (CFT) on the conformal
boundary by construction, and therefore a consistent
CFT. Of course, we do not know what is the microscopic
realization of this CFT, nor do we need it. What matters is
that a primary operator has a well-defined dimension which
is given by the critical exponent of its correlation function
near the conformal boundary. This is what replaces the
physical mass at large distances (see also Ref. [12]).
Concretely, let us consider a scalar field with classical
action [13]
S ¼ −
Z
dμ
x
1
2
ð∂ϕÞ
2
þ
m
2
2
ϕ
2
þ
λ
4!
ϕ
4
ð1Þ
on the Poincar´e patch of hyperbolic space of radius 1=a
with the metric
ds
2
¼
1
a
2
z
2
ðdz
2
þ dx
i
2
Þ; ðz; x
i
Þ ∈ ðR
≥0
; R
3
Þ: ð2Þ
There are two admissible boundary conditions for the
classical scalar field corresponding to the asymptotic
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3
.
PHYSICAL REVIEW LETTERS 121, 101601 (2018)
0031-9007=18=121(10)=101601(6) 101601-1 Published by the American Physical Society