Eur. Phys. J. C (2018) 78:668 Page 5 of 24 668
simplicity. In the following, we will continue to do so for
the left currents and we will only write the superscript R for
the right currents if necessary. The corresponding conserved
charges are
Q
L
=
dξ
1
∂
0
ff
−1
, Q
R
=
dξ
1
f
−1
∂
0
f. (3.9)
3.2 The dressing method
Let F = SL(n, C) and suppose that we already know a solu-
tion f – the seed solution – of the equation of motion (3.5).
The dressing transformation allows us to construct a new
solution f
from the seed solution f . In principle, we can
solve the auxiliary system (3.6) with the condition Ψ(0) = f
and find Ψ(λ). The dressing transformation involves con-
structing a new solution Ψ
(λ) of the auxiliary system (3.6)
of the form
Ψ
(λ) = χ(λ)Ψ (λ). (3.10)
The n ×n matrix χ(λ) is called the dressing factor. It can be
shown [27] that the general form of χ is
χ(λ) = I +
i
Q
i
λ − λ
i
,χ(λ)
−1
= I +
i
R
i
λ − μ
i
.
(3.11)
It turns out that at the level of the F = SL(n, C) NLSM,
the poles can be selected at arbitrary positions on the com-
plex plane and we are left with the problem of specifying
the appropriate residues. There are two conditions that the
residues must satisfy, which are adequate for their specifi-
cation. The first one is the demand that χ(λ)χ(λ)
−1
= I .
Taking the residues of this equation at the positions of the
poles λ
i
and μ
i
provides a set of algebraic equations for Q
i
and R
i
. Notice that one has to be careful when a pole of χ(λ)
coincides with a pole of χ(λ)
−1
, since in this case the prod-
uct χ(λ)χ(λ)
−1
will have a second order pole, which has to
be considered separately.
The solution Ψ
(λ) of the auxiliary system gives rise to
a new solution f
= Ψ
(0) of the NLSM. It follows that f
and Ψ
(λ) must satisfy Eq. (3.6), namely,
J
±
= (1 ± λ)∂
±
Ψ
(λ)(Ψ
(λ))
−1
. (3.12)
Using (3.10) this reduces to
J
±
= (1 ± λ)∂
±
χχ
−1
+ χ J
±
χ
−1
=−(1 ± λ)χ∂
±
χ
−1
+ χ J
±
χ
−1
.
(3.13)
Taking the residues of the previous equations at the positions
of the poles λ
i
and μ
j
, yields two more relations for the
unknown matrices Q
i
and R
i
, which are first order differen-
tial equations for the latter. These, combined with the set of
algebraic equations derived from the residues of the equation
χ(λ)χ(λ)
−1
= I , are sufficient for the specification of the
residues Q
i
and R
i
. More details are provided in appendix
A and in [27].
We now turn to the effect of the dressing transformation
on the sigma model charge. The latter gets altered by
ΔQ
L
=
dξ
1
(J
0
− J
0
)
=
1
2
dξ
1
(J
+
− J
−
− J
+
+ J
−
). (3.14)
We notice that the left hand side of (3.13) is independent of
λ. In the limit |λ|→∞(3.13) reduces to
J
±
=±∂
±
j
Q
j
+ J
±
(3.15)
Using (3.15) we arrive at the equation
ΔQ
L
=
j
dξ
1
∂
1
Q
j
, (3.16)
which relates the charges of the seed and dressed solutions.
3.3 Involutions
As it has already been mentioned, the previous results refer to
the SL(n, C) NLSM. For our purposes f must take values in
some symmetric space F/G, where F, G are Lie groups and
G ⊂ F. This can be achieved by constraining appropriately
the field f to take values in the coset F/G with the help of an
involution. An involution is a bijective mapping σ : F → F
with the properties
σ
2
= 1, (3.17)
and
σ(f
1
f
2
) = σ(f
1
)σ ( f
2
), (3.18)
where f
1
, f
2
∈ F. Furthermore, we demand that the involu-
tion σ obeys
σ(g) = g, ∀g ∈ G. (3.19)
On the Lie algebra side, the mapping σ is just a linear operator
acting on the vector space f, having the property σ
2
= 1.
Since σ
2
= 1, σ has eigenvalues ±1 and thus the vector
space f can be decomposed as follows
f = g ⊕ p, (3.20)
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