brane. Therefore, the trace of the boson mass matrix that one reads off from the bulk will
always be equal to the trace of the square of the fermion mass matrix.
This, in turn, indicates that in the presence of fermion masses, the stringy operator is
not dual to the sum of the squares of the boson masses, but to the difference between it and
the sum of the squares of the fermion masses. Mass deformations of the N = 4 theory where
the supertrace of the square of the masses is zero can therefore be described holographically
by asymptotically-AdS supergravity solutions [7–16]. However, to describe theories where
this supertrace is nonzero, one has to turn on “stringy” non-normalizable modes that
correspond to dimension-(g
s
N)
1/4
operators, which will destroy the AdS asymptotics.
To see this we begin by considering the backreaction of the three-form field strengths
corresponding to fermion mass deformations on the metric, the dilaton and the four-form
potential, which has been done explicitly for several particular choices of masses [4, 17].
This backreaction can give several terms that modify the action of a probe D3 brane,
giving rise to a Coulomb-branch potential that is quadratic in the fermion masses and that
transforms either in the 1 or in the 20
0
of SO(6). Furthermore, one can independently
turn on non-normalizable modes in the 20
0
of SO(6) that correspond to deforming the
Lagrangian with traceless boson bilinears, and that can also give rise to a Coulomb-branch
potential. Since all these terms behave asymptotically as r
−2
and transform in the same
SO(6) representation, disentangling the contributions of the non-normalizable modes from
the terms coming from the backreaction of the three-forms can be quite nontrivial. For
example, in equation (62) in [1], the Coulomb-branch potential appears to contain both
contributions in the 1 and in the 20
0
of SO(6) coming from the backreaction of the fermion
mass tensor T
ijk
, and to have no non-normalizable contribution.
We will show that the backreaction of the modes dual to the fermion masses can only
source terms in the D3 brane Coulomb-branch potential that are singlets under SO(6), and
hence the Coulomb-branch potential terms that transform in the 20
0
of SO(6) can only
come from non-normalizable L = 2 (traceless) modes that one has to turn on separately
from the fermion masses. Since the singlet term in the Coulomb-branch potential is the
supergravity incarnation of the trace of the boson mass matrix, our result implies that in
the bulk this boson mass trace is completely determined by the fermion masses: the sum
of the squares of the boson masses will always be equal to the sum of the squares of the
fermion masses.
Our calculation establishes that asymptotically-AdS
5
solutions can only be dual to
theories in which the sum of the squares of the boson masses is the same as the sum of the
squares of the fermion masses. Theories where these quantities are not equal cannot by
described holographically by such solutions.
From a field theory perspective this interpretation is very natural: the solutions that
are asymptotically AdS
5
can only be dual to field theories that have a UV conformal fixed
point, and therefore their masses and coupling constants should not run logarithmically in
the UV (their beta-functions should be zero). At one loop this cannot happen unless the
sum of the squares of the boson masses is equal to the sum of the squares of the fermion
masses [18], which reduces the degree of divergence in the corresponding Feynman diagram
– 3 –