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C.-S. Chu / Nuclear Physics B 882 (2014) 289–302 291
Evidence that this self-duality equation describes the physics of multiple M5-branes was
provided in [1], and further in [21–23].In[21,22], non-abelian self-dual string solutions were
constructed and a precise agreement [22] of the field theory results and the supergravity descrip-
tions [24] was found. Moreover, it was found that the constant c is fixed by quantization condition
of the self-dual strings solution of the theory. This is satisfying as otherwise c would be a free
dimensionless constant in the theory and hence contradicts with what we know about M5-branes
in flat space. In [23], non-abelian wave configurations which are supported by Yang–Mills in-
stanton were constructed and they were found to match up nicely with the description of M-wave
on the worldvolume of M5-branes system.
One thing interesting about the self-dual string solutions constructed in [21,22] is that the
auxiliary gauge field is
always given by a magnetic monopole which gives rise to the charge of
the self-dual string. This was shown to be case for the original Perry–Schwarz self-dual string and
the Wu–Yang self-dual string [21], as well as for the generalized Wu–Yang self-dual string [22],
with the corresponding monopole configurations given by the Dirac monopole, the Wu–Yang
monopole and the generalized Wu–Yang monopole. It is natural to ask if this connection with
monopole is a general feature of the non-abelian self-dual string. In this paper, we show that this
is indeed the case. This result is potentially interesting as, given this rather explicit connection
between BPS monopole and self-dual string, one may be able to provide a Nahm like construction
for non-abelian self-dual string, which has been speculated and analyzed by other authors [25].
In the next section, we provide a general formalism for the construction of non-abelian self-
dual string starting from an 1/2
-BPS monopole solution in four dimensions. In Section 3,we
show that one can recover the previously constructed self-dual string solutions with this new for-
malism. We also construct, for the ’t Hooft–Polyakov monopole [26,27], a singular solution that
describes two finitely separated M5-branes meeting midway in between. The paper is concluded
with some further discussions in Section 4.
2. A general construction of self-dual strings in terms of BPS monopoles
In this section, we give a general construction for self-dual strings solutions to the non-abelian
self-duality equations (1), (2). We will be interested in static configurations with the self-dual
string being infinite long straight line, say, in the x
4
-direction. As a result, physical properties of
the system are independent of x
0
and x
4
.
Let us consider an ansatz with the following non-vanishing components of the B-field:
B
ij
,B
04
:=−φ. (6)
The non-vanishing components of H are
H
ij k
=D
[i
B
jk]
,H
5ij
=∂
5
B
ij
, (7)
H
04i
=−D
i
φ, H
045
=−∂
5
φ, (8)
and the self-duality equation (1) reads
∂
5
φ =−
1
2
ij k
D
i
B
jk
, (9)
D
k
φ =
1
2
kij
∂
5
B
ij
. (10)
We remark that for U(1) gauge group, the self-duality equations (9), (10) are precisely the
same as the BPS equations of Howe–Lambert–West [28] for the (2, 0) M5-branes theory in