
subspaces, which are preserved by the action of the bosonic elements and are swapped by
the action of the fermionic elements. In general, we have the parity reversal functor Π
which redefines all bosonic states to be fermionic and all fermionic ones to be bosonic. For
example, ΠV
NS;+
j;∆
∼
=
V
NS;−
j;∆
.
As usual, Neveu-Schwarz Verma modules have unique maximal proper submodules and
we shall denote their irreducible quotients by L
NS;±
j;∆
. Both V
NS;±
j;∆
and L
NS;±
j;∆
are modules
over the universal N = 2 algebra of central charge c (which is left implicit). A Neveu-
Schwarz highest-weight vector is a simultaneous eigenvector of J
0
, L
N=2
0
and 1 that is
annihilated by every mode of positive index. We shall say that a singular vector of a given
module is a highest-weight vector that does not generate the entire module. For example, if
v denotes the generating highest-weight vector of V
NS;+
0;0
, then G
+
−1/2
v and G
−
−1/2
v are both
singular vectors. Quotienting by the sum of the submodules that they generate results in
the vacuum module of the universal N = 2 algebra.
In the Ramond sector, one defines Verma modules by choosing a triangular decomposi-
tion such that G
+
0
is an annihilation operator and G
−
0
is a creation operator. For j, ∆ ∈ C,
let C
R;±
j;∆
be the one-dimensional module of even (+) or odd (−) parity over the Ramond
subalgebra (of central charge c) spanned by 1, J
0
, L
N=2
0
, G
+
0
and the positive index modes,
where every mode acts as zero except J
0
, L
N=2
0
and 1 which act as multiplication by j, ∆
and 1, respectively. Inducing then gives the N = 2 Ramond Verma module V
R;±
j;∆
. It, and
its irreducible quotient L
R;±
j;∆
, are (Z
2
-twisted) modules over the universal N = 2 algebra of
central charge c.
A Ramond highest-weight vector is then a simultaneous eigenvector of J
0
, L
N=2
0
and
1 that is annihilated by all the modes of positive index and G
+
0
, while a Ramond singular
vector is a Ramond highest-weight vector that does not generate the entire module. Let v
be a Ramond highest-weight vector of charge (J
0
-eigenvalue) j and conformal dimension
(L
N=2
0
-eigenvalue) ∆. It satisfies
G
+
0
G
−
0
v = 2
∆ −
c
24
v. (2.5)
When ∆ =
c
24
, G
−
0
v is thus a singular vector, so L
R;±
j;∆
has a one-dimensional space of ground
states spanned by v. When ∆ 6=
c
24
, G
−
0
v is not singular and L
R;±
j;∆
has a two-dimensional
space of ground states spanned by v and G
−
0
v. Note that these states have charges j and
j − 1, respectively; their common conformal dimension is ∆.
It is always useful to consider families of modules that are related by twisting by
an automorphism ω. As we want to distinguish between the elements of the module M
and those of the resulting twisted module, we let ω
∗
denote an (arbitrary) vector space
isomorphism from M to the twisted module, hereafter denoted by ω
∗
(M), equipping the
latter with the following algebra action:
x · ω
∗
(m) = ω
∗
(ω
−1
(x) · m), for all modes x and m ∈ M. (2.6)
This action promotes ω
∗
to an invertible (and therefore structure-preserving) functor on
an appropriate module category. The categories of interest here are the Neveu-Schwarz
and Ramond weight modules over either the universal or minimal model N = 2 algebra,
– 7 –