Combined with a gauge field on C, the configuration in (2.1) forms a solution to
Hitchin’s equations and describes the Higgs branch of the mirror of the S
1
reduction of
our 4D theories of interest (the reduction of the 5D theory on C). Therefore, it describes
the Coulomb branch of the direct S
1
reduction and also, via the base of the corresponding
fibration, the Coulomb branch of the 4D theory itself. For example, the Seiberg-Witten
curve of the 4D theory may be read off from the spectral curve [23, 24]
det(x − Φ
z
) = 0 . (2.2)
In order for the description of the moduli space to not jump discontinuously as a
function of the parameters residing in the T
i
, a sufficient condition on the T
i
is that they
are regular
9
semisimple (see [26] and references therein for a discussion in a closely related
context). In particular, this statement means that the T
i
can be brought to the form
of diagonal matrices with non-degenerate eigenvalues. These singularities give rise to 4D
theories with Coulomb branch operators of non-integer scaling dimensions and generalize
the theories described in [27, 28].
10
The above class of theories, while very broad, is (modulo some caveats we will discuss)
not closed under the natural SCFT operation of conformal gauging [30] or under the RG
flow. In fact, these SCFTs form a part of a much broader but still relatively poorly
understood class of theories called the “type III” theories [23] (these theories are expected
to exhibit various interesting phenomena; e.g., see [22, 31–33]).
To define the type III SCFTs, we relax the condition of regularity of the T
i
. In this
case, the requirement of smoothness away from the origin of the moduli space implies
that [26]
L
−1
⊆ L
0
⊆ · · · ⊆ L
`−2
, (2.3)
where the L
a
are the Levi subalgebras associated with the T
i
.
11
This restriction can be
conveniently described in terms of certain Young diagrams [23]
T
i
↔ Y
i
= [n
i,1
, n
i,2
, · · · , n
i,k
i
] , n
i,a
≥ n
i,a+1
∈ Z
>0
,
k
i
X
a=1
n
i,a
= N , (2.4)
where the columns of height n
i,a
represent the eigenvalue degeneracies of the T
i
. The
condition (2.3) amounts to the statement that Young diagram i and Young diagram i − 1
are related by taking some number of columns (possibly zero) in diagram i and decomposing
each of them into columns in diagram i − 1.
In this picture, the T
−1
matrix has a special status: it contains mass parameters (or,
equivalently, vevs for the corresponding background vector multiplets) of the theory. By
N = 2 SUSY, such mass parameters correspond to elements of the Cartan subalgebra
9
Note that the puncture of C is still irregular!
10
Just as in the case of regular singularities, irregular singularities may be enriched by the presence of
certain co-dimension one symmetry defects. Such a construction can lead to 4D SCFTs if there is also a
regular singularity present [29].
11
Specifically, L
a
is defined as the centralizer (in A
N−1
) of the T
i
with a ≤ i ≤ ` − 2. Note that the
conditions in (2.3) are necessary but not sufficient to have a sensible SCFT.
– 3 –