Physics Letters B 772 (2017) 725–730
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Physics Letters B
www.elsevier.com/locate/physletb
Yang–Baxter σ -model with WZNW term as E-model
Ctirad Klim
ˇ
cík
Institut de Mathématiques de Luminy, Aix Marseille Université, CNRS, Centrale Marseille I2M, UMR 7373, 13453 Marseille, France
a r t i c l e i n f o a b s t r a c t
Article history:
Received
3 July 2017
Accepted
24 July 2017
Available
online 26 July 2017
Editor:
N. Lambert
It turns out that many integrable σ -models on group manifolds belong to the class of the so-called
E-models which are relevant in the context of the Poisson–Lie T-duality. We show that this is the case
also for the Yang–Baxter σ -model with WZNW term introduced by Delduc, Magro and Vicedo in [5].
© 2017 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license
(http://creativecommons.org/licenses/by/4.0/). Funded by SCOAP
3
.
1. Introduction
Integrability and T-dualisability are both rare properties of non-
linear
σ -models and it is not known what necessary conditions
must be imposed on a target space metric G and on closed 3-form
field H in such a way that the corresponding σ -model enjoys
either one or another. It turns out, however, that those two prop-
erties
share something in common, in the sense that the majority
of integrable sigma models [1,13–15] recently constructed fall in
the class of the so-called E-models [2], which are the building-
blocks
for the construction of T-dualisable models [3,4]. It is not
yet clear why it is so, but we find that trying to put all inte-
grable
σ -models under the common roof of the E-models is a
nice guiding principle which may eventually lead to a deeper un-
derstanding
of this phenomenon. In particular, the purpose of the
present note is to give the E -model interpretation of one of a
few integrable σ -models for which this was not yet done, namely
the Yang–Baxter σ -model with WZNW term. This model was in-
troduced
in [5] (see also [6] for previous work) and it is a two
parameter
1
deformation of the principal chiral model defined by
the action
S[g]=−
K
4
dτ
g
−1
∂
−
g,(1 + η
2
+ AR + η
2
R
2
)g
−1
∂
+
g
+
k
d
−1
(
g
−1
dg, [g
−1
∂
σ
g, g
−1
dg])
. (1.1)
E-mail address: ctirad.klimcik@univ-amu.fr.
1
Since the constant K is just the overall normalisation constant and the relation
(1.2) is supposed to hold, the action (1.1) is indeed a two-parameter deformation of
the principal chiral model; the latter is recovered for the particular values η = A =
k = 0.
Here (., .) is the Killing–Cartan form on the Lie algebra G of a sim-
ple
compact Lie group G, R : G → G is the so-called Yang–Baxter
operator (see Section 3 for its definition), τ and σ stand respec-
tively
for the world-sheet time and space variables (the time and
space integrals are notationally separated as
and
expressing
2
the angular character of the variable σ ), g ≡ g(σ , τ ) is a dynami-
cal
field configuration with values in G, we set also ∂
±
:= ∂
τ
± ∂
σ
and, finally, the parameters of the model are constrained by the
relation
A = η
1 −
k
2
1 +η
2
. (1.2)
While Delduc, Magro and Vicedo showed in [5] that the model
(1.1) is integrable only if the relation (1.2) holds, we show in
the present paper that (1.1) has the structure of the E -model
also only in the case when the constraint (1.2) is imposed.
This fact illustrates again the mysterious relation between the
E-model formalism and the integrability which motivates our
work.
The
plan of this note is as follows: We first review how the
concept of the E-model is built up on that of the Drinfeld dou-
ble,
then we introduce a particular Drinfeld double and show that
the E -model which corresponds to it is just the Delduc, Magro and
Vicedo model (1.1). We also express the Lax pair of the integrable
model (1.1) in the formalism of the E-model therefore our paper
can be viewed also as an alternative demonstration of the integra-
bility
of the model (1.1).
2
Note also our perhaps non-standard way of writing of the Wess–Zumino term
which is however very useful for “travelling” between the first order and the second
order description of the σ -model dynamics. Mathematically speaking, the object
(
g
−1
dg, [g
−1
dg, g
−1
∂
σ
g]) should be viewed as a two-form on the infinite dimen-
sional
loop group manifold LG.
http://dx.doi.org/10.1016/j.physletb.2017.07.051
0370-2693/
© 2017 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Funded by
SCOAP
3
.