Physics Letters B 769 (2017) 362–367
Contents lists available at ScienceDirect
Physics Letters B
www.elsevier.com/locate/physletb
BPS submodels of the Skyrme model
C. Adam
a,∗
, J. Sanchez-Guillen
a
, A. Wereszczynski
b
a
Departamento de Física de Partículas, Universidad de Santiago de Compostela and Instituto Galego de Física de Altas Enerxias (IGFAE), E-15782 Santiago de
Compostela, Spain
b
Institute of Physics, Jagiellonian University, Lojasiewicza 11, Kraków, Poland
a r t i c l e i n f o a b s t r a c t
Article history:
Received
23 March 2017
Accepted
3 April 2017
Available
online 5 April 2017
Editor:
A. Ringwald
We show that the standard Skyrme model without pion mass term can be expressed as a sum of two BPS
submodels, i.e., of two models whose static field equations, independently, can be reduced to first order
equations. Further, these first order (BPS) equations have nontrivial solutions, at least locally. These two
submodels, however, cannot have common solutions. Our findings also shed some light on the rational
map approximation. Finally, we consider certain generalisations of the BPS submodels.
© 2017 The Authors. 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
The Skyrme model [1] is a nonlinear field theory supporting
topological soliton solutions (“Skyrmions”), which finds its main
application as a low-energy effective theory for QCD [2]. In the
analysis of Skyrme models (and related theories supporting topo-
logical
solitons), two important concepts are topological energy
bounds and the related notion of Bogomolnyi equations [3,4]. In-
deed,
sometimes it is possible to reduce the static field equations
to first-order equations (“Bogomolnyi equations”) such that the
corresponding solutions saturate the bound. Within the set of gen-
eralised
Skyrme models (but all based on the same Skyrme field
U ∈ SU(2)), two cases of BPS Skyrme models satisfying a Bogomol-
nyi
equation are known. The first one is the “BPS Skyrme model”
consisting of a term sextic in first derivatives and a potential [5]. In
this model, Bogomolnyi solutions exist for arbitrary topological de-
gree
(“baryon number”) B. The second one consists of the Skyrme
term (quartic in first derivatives) and a particular potential [6].
In this second case, however, only the |B| = 1solutions (and, of
course, the trivial vacuum solution) saturate the bound and obey
the corresponding Bogomolnyi equation. Higher B configurations
are unbound in this model and turn into lightly bound Skyrmions
once further terms are added with sufficiently small coupling con-
stants
[7–9]. Both these BPS models are genuine Skyrme models
in the sense that they can be found within the set of generalised
Skyrme model by an appropriate choice of coupling constants.
In
the present paper we want to consider a slightly different
type of BPS submodels. We will find that the standard Skyrme
*
Corresponding author.
E-mail
address: adam@fpaxp1.usc.es (C. Adam).
model (consisting of the sigma model term and the Skyrme term),
too, may be written as a sum of two BPS submodels, where each
of the two submodels separately leads to a Bogomolnyi equation
and nontrivial solutions (at least, locally). These submodels, how-
ever,
are not Skyrme models on their own, i.e., it is not possible to
get just one of these submodels by a choice of coupling constants
within the set of generalised Skyrme models. In other words, the
two submodels are multiplied by the same coupling constants, and
eliminating one automatically eliminates the other. Further, the
two submodels have no common solutions (except for the trivial
vacuum solution). This must, of course, be expected, because it
is known that, although the standard Skyrme model has a topo-
logical
energy bound (the Skyrme–Faddeev bound [1,10]) and a
corresponding Bogomolnyi equation, this first-order equation is too
restrictive and only allows for the trivial solution. Despite these
impediments, nevertheless, the existence of further BPS submod-
els
is of interest and sheds new light on several issues within the
Skyrme model.
The
generalised Skyrme model we want to consider consists of
four terms. The sigma model (Dirichlet) term
L
2
=
1
2
Tr
(L
μ
L
μ
), (1.1)
the Skyrme (quartic) term
L
4
=
1
16
Tr
([L
μ
, L
ν
]
2
), (1.2)
and the sextic term, which is just the baryonic current squared
term
L
6
=λ
2
π
4
B
μ
B
μ
, B
μ
=
1
24π
2
μνρσ
Tr L
ν
L
ρ
L
σ
, (1.3)
http://dx.doi.org/10.1016/j.physletb.2017.04.003
0370-2693/
© 2017 The Authors. 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
.