T.A. Ivanova et al. / Nuclear Physics B 882 (2014) 205–218 207
Tw
R
6
=Q
6
∼
=
R
6
×CP
3
. (1.3)
On the other hand, Sämann and Wolf have shown [6] that holomorphic line bundles over Tw(R
6
)
trivial on all CP
3
x
→ Tw(R
6
) correspond to pure-gauge Maxwell potentials on R
6
,i.e.the
twistor transform fails for the metric twistor space Tw(R
6
). This was partially cured in [15]
where it was shown that instantons on the six-sphere S
6
= R
6
∪{∞}correspond to complex
vector bundles over the reduced twistor space
ˆ
Z = G
2
/U(2)→Tw(S
6
) with flat partial connec-
tions, where
Tw
S
6
=Spin(7)/U(3) (1.4)
is a compactification of the twistor space (1.3). For the definition of the instanton equations in di-
mensions higher than four and for some instanton solutions see e.g. [16–23]. Hence, constructing
instanton configurations in six dimensions is a task more complicated than one might expect.
Instanton equations on the six-sphere S
6
are not quite standard since S
6
is a nearly Kähler
space with a nonintegrable almost complex structure. In fact, instantons on S
6
are connections on
pseudo-holomorphic bundles satisfying the Donaldson–Uhlenbeck–Yau (DUY) equations [17].
Hence, for checking the power of the twistor approach it is worthwhile to consider a Kähler
6-manifold. We choose the complex projective space CP
3
which can be considered as yet another
compactification of R
6
∼
=
C
3
.
On CP
3
the DUY equations are the standard Hermitian Yang–Mills (HYM) equations [17].
They are SU(3) invariant but not invariant under the SO(6) Lorentz-type rotations of orthonormal
frames. Therefore, one should describe them with reduced twistor spaces. The DUY equa-
tions are well defined on six-dimensional Kähler manifolds X (as well as on nearly Kähler
spaces [24–26]), and their solutions are natural connections A on holomorphic vector bundles
E → X [17]. As reduced twistor spaces of CP
3
one can consider
SU(4)/U(2) ×U(1) =:Z → CP
3
∼
=
SU(4)/U(3) (1.5)
or
Sp(2)/U(1) ×U(1) =:Z
→CP
3
∼
=
Sp(2)/Sp(1)×U(1) (1.6)
which both are complex submanifolds of Tw(CP
3
), with fibres CP
2
and CP
1
, respectively.
We will show that bundles (E , A) over CP
3
with HYM connections A are pulled back to holo-
morphic vector bundles (
˜
E,
˜
A) over the reduced twistor spaces (1.5) or (1.6), depending on the
choice for CP
3
, being trivial along the fibres of the fibrations (1.5) or (1.6), with a Hermi-
tian Yang–Mills connection
˜
A on
˜
E. Thus, contrary to the four-dimensional case, the twistor
transform in six dimensions does not parametrize instantons by unconstrained holomorphic data
on the twistor space, since the corresponding holomorphic bundles over Z and Z
have to be
polystable. In other words, in four dimensions the twistor transform establishes a correspondence
between solutions of the instanton equations in d = 4 and solutions of holomorphic Chern–
Simons theory on d = 6 twistor space, but in six dimensions the twistor transform establishes a
correspondence between solutions of the instanton (HYM) equations in d = 6 and solutions of
the HYM equations on the twistor space. The latter does not facilitate solving the d =6 instanton
equations. This is the outcome of our study of instantons in six dimensions.
The structure of the remainder of this paper is as follows. In Section 2 we
portray the space
CP
3
as a homogeneous space SU(4)/U(3) and Sp(2)/Sp(1) × U(1), with Kähler structures
in both cases and allowing for the introduction of a quasi-Kähler structure in the second case.
In Section 3 we describe the geometry of the twistor spaces Z and Z
for SU(4)/U(3) and