JHEP10(2018)081
Here we are using the flat metric g
µν
= diag(1, −1, −1) and the real triplet of scalar fields
φ
φ
φ = (φ
1
, φ
2
, φ
3
) is constrained to the surface of a sphere of unit radius: φ
φ
φ · φ
φ
φ = 1. The
coupling constants κ
0
, κ
2
and κ
4
are some real positive parameters. The Lagrangian of
the planar Skyrme model (
2.2) also includes a potential term V , which, in the absence of
other fields stabilizing the solitons. Here we consider the most common choice of the O(3)
symmetry breaking potential
V = 1 − φ
3
. (2.3)
The topological restriction on the field φ
φ
φ is that on the spati al boundary φ
φ
φ
∞
= (0, 0, 1).
This allows a one- point compactific ati on of the domain space R
2
to S
2
and the field of the
finite energy soliton solutions of the model, so called baby Skyrmi ons , is a map φ
φ
φ : S
2
7→ S
2
which belongs to an equivalence class characterized by the homotopy group π
2
(S
2
) = Z.
The corresponding topological invariant is
Q = −
1
4π
Z
φ
φ
φ ·(∂
1
φ
φ
φ ×∂
2
φ
φ
φ) d
2
x (2.4)
The Lagrangian for fermions coupled to the b aby Skyrmions is give n by
L
f
=
¯
Ψ
i
/
∂ − gτ
τ
τ ·φ
φ
φ −m
Ψ, (2.5)
Note that the fermion field Ψ is a spin and an isospin spinor, the isospin matr i ce s are
defined as τ
τ
τ = I ⊗σ
σ
σ, whereas the spin matrices are ˆγ
µ
= γ
µ
⊗I. Here I is two-dimensional
identity matrix,
/
∂ = ˆγ
µ
∂
µ
and σ
σ
σ are the us ual Pauli matrices. The coupling constant
g and the fermions mass m are parameters of the model. In order to satisfy the usual
anticommutation relations of the Clifford algebra, the two-dimensional gamma-matrices
are defined as follows: γ
1
= −iσ
1
, γ
2
= −iσ
2
and γ
3
= σ
3
.
There are five parameter s of the coupled fermion-Skyrmion system (
2.1), κ
0
, κ
2
, κ
4
, g
and m. The dimensions of the parameters an d the fields are
κ
0
:
L
−3
, κ
2
:
L
−1
, κ
4
:
L
1
, g :
L
−1
m :
L
−1
, φ
φ
φ :
L
0
, Ψ :
L
−1
.
Thus, an appropr i ate rescaling of the action by an overall const ant, and rescaling of the
length scale and the fermion field
r →
r
κ
4
κ
2
r, Ψ →
r
κ
2
κ
4
Ψ, κ
0
→
κ
2
2
κ
4
κ
0
, g →
r
κ
2
κ
4
g, m →
r
κ
2
κ
4
m , (2.6)
effectively reduces the number of independent parameters to four,
√
κ
2
κ
4
, κ
0
, m and g.
Hereafter we fix
√
κ
2
κ
4
= 1 without loss of generality.
Considering the model (
2.1) we do not impose the usual assumption that the Skyrmion
is a fixed static background field [
12, 33–35]. We restrict our considerat i on to the rota-
tionally invar i ant stationary configuration of topological degree Q = 1, thus Ψ = ψe
−iεt
and we suppose that the Sk y rm i on field φ
φ
φ is static. Then the resc al e d Hamiltonian of the
coupled system can be written as :
H =
Z
d
2
x ψ
†
Hψ +
√
κ
2
κ
4
Z
d
2
x
1
2
(∂
i
φ
φ
φ)
2
+
1
4
(∂
i
φ
φ
φ ×∂
j
φ
φ
φ)
2
+ κ
0
V
, (2.7)
– 3 –