
Eur. Phys. J. C (2019) 79 :770 Page 3 of 11 770
T-dual action is obtained by substituting the expressions for
the gauge fields obtained from these equations of motion,
into the gauge fixed action. The T-dual action reads [10]
S =
κ
2
2
dξ
2
∂
+
y
μ
Θ
μν
−
∂
−
y
ν
, (9)
The dual background field composition equals
Π
μν
±
=
κ
2
Θ
μν
∓
=−
G
−1
E
Π
∓
G
−1
μν
,
(G
E
)
μν
= (G − 4BG
−1
B)
μν
, (10)
where G
E
is the effective metric. The T-dual metric is its
inverse
G
μν
= (G
−1
E
)
μν
, (11)
and a T-dual Kalb–Ramond field is
B
μν
=
κ
2
θ
μν
, (12)
where θ
μν
=−
2
κ
(G
−1
E
BG
−1
)
μν
is the noncommutativity
parameter.
Because of the choice (8), the composition of the T-dual
background fields is also block diagonal
Θ
μν
±
=
Θ
ab
±
0
0 Θ
ij
±
, (13)
given in terms of the inverse of the initial metric and the
effective metric
(G
−1
)
μν
=
(G
−1
)
ab
0
0 (G
−1
)
ij
,
(G
−1
E
)
μν
=
(G
−1
E
)
ab
0
0 (G
−1
E
)
ij
, (14)
by
Θ
ab
±
=−
2
κ
G
−1
E
ac
Π
±cd
(G
−1
)
db
= θ
ab
∓
1
κ
G
−1
E
ab
,
Θ
ij
±
=−
2
κ
G
−1
E
ik
Π
±kl
(G
−1
)
lj
= θ
ij
∓
1
κ
G
−1
E
ij
,
(15)
where (G
E
)
ab
= G
ab
− 4B
ac
(G
−1
)
cd
B
db
and (G
E
)
ij
=
G
ij
−4B
ik
(G
−1
)
kl
B
lj
. The components of the non-commu-
tativity parameter are
θ
ab
=−
2
κ
G
−1
E
ac
B
cd
(G
−1
)
db
,
θ
ij
=−
2
κ
G
−1
E
ik
B
kl
(G
−1
)
lj
. (16)
The coordinates of the initial and the T-dual theory are con-
nected by T-duality coordinate transformation laws, which
read
∂
±
x
μ
∼
=
−κΘ
μν
±
∂
±
y
ν
,
∂
±
y
μ
∼
=
−2Π
∓μν
∂
±
x
ν
. (17)
The T-dual boundary conditions are
γ
(0)μ
δy
μ
π
0
= 0, (18)
where
γ
(0)μ
=
κ
2
2
Θ
μν
−
∂
−
y
ν
+ Θ
μν
+
∂
+
y
ν
. (19)
The T-dual theory (9) is equivalent to an open string theory (2)
with chosen boundary conditions (7), if the T-dual boundary
conditions are fulfilled in a Neumann way for coordinates y
i
and in a Dirichlet way for y
a
Neumann:
γ
(0)i
∂Σ
= 0,
N
γ
i
0
≡
γ
(0)i
=
κ
2
2
Θ
ij
−
∂
−
y
j
+ Θ
ij
+
∂
+
y
j
,
Dirichlet: κ ˙y
a
∂Σ
= 0,
D
γ
0
a
≡ κ ˙y
a
. (20)
This is because of the T-duality transformation law (17),
which gives
− κ ˙x
μ
∼
=
γ
(0)μ
(y), γ
(0)
μ
(x)
∼
=
−κ ˙y
μ
, (21)
and consequently
D
γ
i
0
≡ κ ˙x
i
∼
=
−
γ
(0)i
≡−
N
γ
i
0
,
N
γ
0
a
≡ γ
(0)
a
∼
=
−κ ˙y
a
=−
D
γ
0
a
. (22)
So, performing T-dualization one changes the type of the
boundary conditions which the coordinates in i and a direc-
tions satisfy.
3 Dirac consistency procedure applied to the boundary
conditions
The coordinates of the initial and T-dual open string satisfy
the appropriate set of the boundary conditions (7) and (20),
obtained from the actions (2) and (9). In this section, we are
going to treat them as constraints and we will apply the Dirac
consistency procedure. In order to implement the procedure,
let us find the canonical form of the boundary conditions, and
express them in terms of the currents building the energy-
momentum tensors, and consequently the hamiltonians.
The momenta conjugated to the coordinates of the initial
and T-dual theories (2) and (9)are
π
μ
=−2κ B
μν
x
ν
+ κ G
μν
˙x
ν
,
π
μ
=−κ
2
θ
μν
y
ν
+ κ
G
−1
E
μν
˙y
ν
=−2κ
B
μν
y
ν
+ κ
G
μν
˙y
ν
. (23)
The energy-momentum tensor components for the initial
theory can be expressed in terms of currents
j
±μ
= π
μ
+ 2κΠ
±μν
x
ν
, (24)
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