Dynamics of Finite-Temperature Conformal Field Theories
from Operator Product Expansion Inversion Formulas
Anastasios C. Petkou
1,2
and Andreas Stergiou
2
1
Institute of Theoretical Physics, Aristotle University of Thessaloniki, Thessaloniki 54124, Greece
2
Theoretical Physics Department, CERN, 1211 Geneva 23, Switzerland
(Received 26 June 2018; published 17 August 2018)
We apply the operator product expansion inversion formula to thermal two-point functions of bosonic
and fermionic conformal field theories in general odd dimensions. This allows us to analyze in detail the
operator spectrum of these theories. We find that nontrivial thermal conformal field theories arise when the
thermal mass satisfies an algebraic transcendental equation that ensures the absence of an infinite set of
operators from the spectrum. The solutions of these gap equations for general odd dimensions are in general
complex numbers and follow a particular pattern. We argue that this pattern unveils the large-N vacuum
structure of the corresponding theories at zero temperature.
DOI: 10.1103/PhysRevLett.121.071602
Introduction.—The description of critical systems in
nontrivial backgrounds requires data not present in the
plane geometry. Perhaps the simplest example is that of
conformal field theories (CFTs) on S
1
β
× R
d−1
, with β the
radius of the circle, that describe finite-size or finite-
temperature critical systems. In such a case, the two-point
function of a scalar operator ϕðxÞ will in principle depend
on the one-point functions of all operators that appear in its
operator product expansion (OPE) with itself, since the
latter can be nonzero. In particular, for an operator OðxÞ
with dimension Δ
O
we schematically have hOðxÞi
S
1
β
×R
d−1
∝
b
O
=β
Δ
O
, where b
O
is a dimensionless parameter.
In d ¼ 2 the plane is conformally related to the cylinder
and, although one-point functions of conformal primaries
vanish on the latter, there exist operators such as the energy-
momentum tensor which transform anomalously under a
conformal map. This fixes their one-point functions on the
cylinder, and therefore the CFT data on R
2
determine the
finite-size or finite-temperature corrections to correlation
functions on S
1
β
× R [1,2].
For d>2 there is no conformal transformation between
R
d
and S
1
β
× R
d−1
, and generically one needs to find other
ways to determine the additional data b
O
. A first step in this
direction was described in Ref. [3], where the leading
anisotropic finite-size corrections to the two-point function
of scalars in R
d
were connected to the ratio of the thermal
free-energy density of the system and the normalization C
T
of the energy-momentum tensor two-point function. An
extension of these ideas to the nontrivial 3D OðNÞ vector
model was performed in Refs. [4,5], where the relevance of
the planar OPE to the description of the finite-size or finite-
temperature CFTs was demonstrated. In the latter works the
crucial point was that parameters such as b
O
were inde-
pendently determined by the gap equation of the vector
model. In particular, once the bosonic thermal mass was
determined, all one-point functions could be evaluated and
hence the full finite-temperature two-point function could
be reconstructed.
In more recent developments, the improved understand-
ing of CFTs on R
d
using numerical and analytic bootstrap
methods (see Ref. [6] for a recent review) calls for an
extension of these advances to finite-size or finite-temper-
ature critical systems. In this context an interesting work
has recently appeared [7], whose main result is a Lorentzian
inversion formula for the thermal two-point function of a
scalar ϕðxÞ with dimension Δ
ϕ
. Using the OPE one can
show that the Euclidean position-space [8] thermal two-
point function takes the generic form
hϕðxÞϕð0Þi
β
≡ gðr; cos θÞ¼
X
O
s
a
O
s
r
β
Δ
O
s
C
ν
s
ðcos θÞ
r
2Δ
ϕ
;
ð1Þ
where x
μ
¼ðτ; xÞ are coordinates on S
1
β
× R
d−1
with
period τ ∼ τ þ β, r ¼jxj, and θ ∈ ½0; π is a polar angle
when R
d−1
is written in spherical coordinates. C
ν
s
ðcos θÞ are
Gegenbauer polynomials with ν ¼ d=2 − 1. The sum in
Eq. (1) runs over all operators O
s
in the OPE ϕ × ϕ with
spin s and dimension Δ
O
s
. The coefficients a
O
s
are given
by (following the conventions of Ref. [7])
Published by the American Physical Society under the terms of
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3
.
PHYSICAL REVIEW LETTERS 121, 071602 (2018)
0031-9007=18=121(7)=071602(6) 071602-1 Published by the American Physical Society