C. Klimˇcík / Nuclear Physics B 900 (2015) 259–272 261
We finish by two more remarks:
4) The statement of Result
3 could be in principle reconstructed by composing together several
facts established already in [9,40,46], however, we consider as an independent result the way
how we obtain it directly and naturally from the formalism of the E-models.
5) The Poisson structure {., .}
0
+ε{., .}
1
is the (symplectic version of) the current algebra built
on a one-parameter family D
ε
of the Drinfeld doubles of the Lie algebra G ≡ Lie(G). The
Hamiltonian H is a quadratic expression in the currents and it is completely determined by
the Hamiltonian of the principal chiral model because it does not depend on ε.
2. Introduction
A problem how to deform an integrable non-linear σ -model on group manifold in a way pre-
serving the integrability was formulated some forty years ago and it turned out to be a difficult
one. Several integrable deformations of the principal chiral model have been found in the eight-
ies and the nineties for the simplest case of the group SU(2) [4,7,13,14] bu
t for long decades no
examples were constructed for higher dimensional groups. Some effort (see e.g. [37]) has been
made to determine a complete system of conditions which a target geometry on a general Lie
group must fulfill in order to guarantee inte
grability, however, attempts to find solutions of this
complicated highly overdetermined system of conditions essentially failed for other groups than
SU(2). This situation lasted until 2008 when, in [31], the present author established the integra-
bility of the so-called η-deformed (or, equivalently, Yang–Baxter) σ -model [30] for an
y simple
compact Lie group target G.
The inte
grable η-deformation of the principal chiral model described in [31] was generalized
to the context of integrable coset and supercoset targets in [9] and [10], respectively. In par-
ticular, the result [10] has triggered an important activity in the field because of its relevance
in the AdS/CFT story [1–3,5,8,12,17,19,22,20,23,33,35,36,42,44,45]. In a short period of fe
w
years, several new integrable deformations of the integrable nonlinear σ -models were obtained,
some of them multi-parametric [6,11,16,21,32,34,40]. In the present paper, we shall concentrate
mainly on the integrable deformation of the WZW model proposed in [40]. It is now called the
“λ-deformation”, it belongs to a class of σ -models introduced in [43] and, similarly as in the η
case, it wa
s later generalized to the integrable supercoset targets [15].
Three papers [46,18] and [41] ha
ve recently discussed the issue of possible structural relations
between the integrable η- and λ-deformations and all of them emphasized the relevance of the
concept of the Poisson–Lie T-duality [24–26,39] in this context. In particular, Vicedo [46] studied
extensively the case of the λ-model on a non-compact simple Lie group admitting the so-called
split Y
ang–Baxter operator on its Lie algebra and pointed out the existence of the Poisson–Lie
T-dual theory
1
resembling the variant of the η-model with real poles of the so-called twist func-
tions (the poles of the twist function of the original η-model [30,31] are complex conjugated). On
the other hand, Hoare and Tseytlin [18] and Sfetsos, Siampos and Thompson [41] have sticked to
the compact case and showed that the λ-deformation on the SU(2) tar
get is related by an appro-
priate analytic continuation to the Poisson–Lie T-dual of the η-deformation. The principal goal
of the present paper is to generalize this result of [18,41] to any target G.
1
A second-order action of this dual theory is not explicitly given in [46] because of the problems with the factorizability
of the underlying Drinfeld double. In this respect, the formula (14) of the present paper includes also the case of the
non-factorizable doubles and its usefulness for the further development of the results of [46] looks very probable.