Physics Letters B 791 (2019) 287–292
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Physics Letters B
www.elsevier.com/locate/physletb
On the topological charge of SO(2) gauged Skyrmions in 2 + 1and
3 + 1dimensions
Francisco Navarro-Lérida
a,∗
, Eugen Radu
b,c
, D.H. Tchrakian
c,d
a
Dept. de Física Teórica and IPARCOS, Ciencias Físicas, Universidad Complutense de Madrid, E-28040 Madrid, Spain
b
Center for Research and Development in Mathematics and Applications, (CIDMA), Campus de Santiago, 3810-183 Aveiro, Portugal
c
School of Theoretical Physics, Dublin Institute for Advanced Studies, 10 Burlington Road, Dublin 4, Ireland
d
Department of Computer Science, NUI Maynooth, Maynooth, Ireland
a r t i c l e i n f o a b s t r a c t
Article history:
Received
11 December 2018
Received
in revised form 5 February 2019
Accepted
11 February 2019
Available
online 13 February 2019
Editor:
N. Lambert
Keywords:
Solitons
Skyrmions
Topological
charge
Instantons
The question of the dependence of the topological charge q of a gauged Skyrmion, on the gauge field, is
studied quantitatively. Two examples, both gauged with SO(2) are studied and contrasted: i) The O (3)
model in 2 + 1 dimensions, and ii) The O (4) model in 3 + 1 dimensions. In case i), where the (usual)
Chern–Simons (CS) term is present, the value of q changes sign, going through zero. This evolution is
tracked by a parameter characterising the solutions in the given theory. In case ii), in which dimensions
no CS density is available, the evolution of q is not observed.
© 2019 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
The topological charge q of gauged Skyrmions presents pecu-
liarities
that are absent in gauged Higgs solitons. While in the
latter case this quantity always equals the winding number of the
Higgs field, in the case of Skyrmions the value of q in general may
depart from the winding number, or the “baryon number”.
The
winding number of a Skyrme system is a topological
charge [1], which provides the static energy with a lower bound.
What we refer to as the topological charge of a gauged Skyrme
system, is merely the number which gives the lower bound on the
static energy. This contrasts with (gauged) Higgs systems where
the topological charge encoded in the Higgs (and gauge) field, also
supplies the energy lower bound [2,3]. While in Higgs models
this topological charge is the magnetic monopole charge [4], by
contrast the magnetic flux of gauged Skyrme models in space di-
mensions
D ≥ 3 vanishes [5], except in D = 2,
1
and that only in
the presence of Chern–Simons dynamics.
The
topological charge for the SO(2) gauged O(4) (Skyrme)
model in 3 + 1dimensions is defined in Ref. [7] and in [8,9], and,
for the SO(2) gauged O (3) (planar Skyrme) model in 2 + 1dimen-
*
Corresponding author.
E-mail
address: fnavarro@fis.ucm.es (F. Navarro-Lérida).
1
See for example [6].
sions in Ref. [10]. The topological charges for the SO(D) gauged
O (D + 1) sigma model on R
D
(D = 2, 3, 4) were defined in [11],
and the soliton of the SO(3) gauged O (4) sigma model on R
3
was first reported in [5]. More detailed studies of the latter soli-
tons
were given in [12], while solitons of the SO(4) gauged O (5)
sigma model on R
4
were reported in [13]. The most comprehen-
sive
and recent definitions of the generic topological charges of
gauged Skyrmions are given in Appendix B of Ref. [14].
With
the exception of the work in Ref. [7], where attention was
focused on the decay of the baryon number,
2
the works of [8–11,
5,13]are
concerned with the construction of topologically stable
solitons. Here, we are concerne d with the SO(2) gauged O (3) and
O (4) sigma models in 2 + 1and 3 + 1 dimensions, respectively.
But unlike in the works of Refs. [10,8,9], our focus here will be the
evolution of the topological charge, e.g., its possible decay.
3
In odd dimensional spacetimes, where the Chern–Simons (CS)
term is defined, one finds that the effect of this CS dynamics re-
sults
in the mass/energy of the static solitons both to increase
and to decrease with increasing global charge (electric charge and
2
Well known works in this direction are, e.g., [16,17], employ instead SU(2)
gauging that does not result in topologically stable solitons.
3
SO(2) gauged Skyrmions in 2 + 1dimensions have been studied extensively
(see Refs. [10,6,18–20]and more recently in [21,22]). The SO(2) gauged Skyrmions
in 3 + 1dimensions have been quantitatively studied mainly in Refs. [8,9](see also
the recent analytical results in Ref. [23]).
https://doi.org/10.1016/j.physletb.2019.02.011
0370-2693/
© 2019 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
.