interaction
R
d
d
x φ O. We will see that also in this case there is a Higgs-like mechanism at
work, albeit of a different kind, which exists only in AdS.
In order to study this problem, we find it useful to start considering two CFT single-
trace operators (O
1
, O
2
) of dimension (∆
1
, ∆
2
) with ∆
1
+ ∆
2
< d, and thereafter take the
decoupling limit ∆
1
→
d
2
− 1. The relevant double-trace deformation
Z
d
d
x f O
1
O
2
, (1.1)
leads to an IR fixed point where (O
1
, O
2
) are replaced by two operators (
˜
O
1
,
˜
O
2
) of di-
mension (d − ∆
1
, d − ∆
2
), respectively. In the limit ∆
1
→
d
2
− 1, many terms in the low
energy limit of the correlators become analytical in the momentum, and can be removed
by appropriate counterterms. Focusing on the physical part of the correlators, we will find
that in this case multiplets recombine. In particular,
˜
O
1
∝
˜
O
2
, the IR dimensions being
related as
∆
IR
1
= ∆
IR
2
+ 2 , (1.2)
with ∆
IR
2
= d − ∆
2
.
In the bulk the interaction (1.1) gets mapped into a non scale-invariant boundary
condition for the scalar fields (Φ
1
, Φ
2
) dual to (O
1
, O
2
) [4, 5] (see also [6–10]). These bulk
scalars are free at leading order in 1/N expansion. The presence of the coupling f implies
that the boundary modes of Φ
1
and Φ
2
get mixed. For O
1
and O
2
above the unitarity
bound, the holographic analysis is standard, and the results agree with the field theory
analysis. The limit ∆
1
→
d
2
− 1 should instead be treated with some care. One needs to
rescale the field Φ
1
, otherwise the normalization of the two-point correlator of O
1
would
vanish. Doing so, one sees that the on-shell action for Φ
1
reduces to the action of a free
scalar field living on the boundary of AdS, i.e. a singleton [11–15]. In the IR limit of the
holographic RG-flow triggered by (1.1), the singleton gets identified with a boundary mode
of Φ
2
corresponding to the VEV of the dual operator, i.e. the singleton becomes a long
multiplet by eating-up the degrees of freedom of the bulk scalar.
The rest of the paper is organized as follows. In section 2 we perform the large-N
field theory analysis, and show that recombination takes place in the limit ∆
1
→
d
2
−1. In
section 3 we review the singleton limit in the bulk, and derive the holographic dual of the
multiplet recombination flow. We conclude with some comments on relations to previous
work and possible future directions. The appendix contains the calculation of the variation
of the quantity
˜
F [16] induced by the flow (1.1), which shows that δ
˜
F =
˜
F
UV
−
˜
F
IR
> 0,
in agreement with the generalized F-theorem advocated in [16, 17].
2 Large-N multiplet recombination: field theory
Consider a free scalar φ coupled to a large-N CFT through the interaction
Z
d
d
xfφ O , (2.1)
where O is a single-trace primary operator of dimension ∆ <
d
2
+ 1, so that the deforma-
tion (2.1) is relevant and triggers an RG-flow.
– 2 –