Physics Letters B 772 (2017) 621–627
Contents lists available at ScienceDirect
Physics Letters B
www.elsevier.com/locate/physletb
Exact self-dual skyrmions
L.A. Ferreira
a
, Ya. Shnir
b,c,d
a
Instituto de Física de São Carlos; IFSC/USP; Universidade de São Paulo, USP; Caixa Postal 369, CEP 13560-970, São Carlos-SP, Brazil
b
BLTP, JINR, Dubna 141980, Moscow Region, Russia
c
Department of Theoretical Physics and Astrophysics, BSU, Minsk 220004, Belarus
d
Department of Theoretical Physics, Tomsk State Pedagogical University, Russia
a r t i c l e i n f o a b s t r a c t
Article history:
Received
13 June 2017
Received
in revised form 29 June 2017
Accepted
17 July 2017
Available
online 21 July 2017
Editor: M.
Cveti
ˇ
c
We introduce a Skyrme type model with the target space being the sphere S
3
and with an action
possessing, as usual, quadratic and quartic terms in field derivatives. The novel character of the model
is that the strength of the couplings of those two terms are allowed to depend upon the space–time
coordinates. The model should therefore be interpreted as an effective theory, such that those couplings
correspond in fact to low energy expectation values of fields belonging to a more fundamental theory
at high energies. The theory possesses a self-dual sector that saturates the Bogomolny bound leading
to an energy depending linearly on the topological charge. The self-duality equations are conformally
invariant in three space dimensions leading to a toroidal ansatz and exact self-dual Skyrmion solutions.
Those solutions are labelled by two integers and, despite their toroidal character, the energy density is
spherically symmetric when those integers are equal and oblate or prolate otherwise.
© 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
Self-dual field configurations possess very nice physical and
mathematical properties, and they are important in the study of
non-linear aspects of field theories possessing topological soli-
tons.
The best known examples are the instanton solutions of the
Yang-Mills theory in four dimensional Euclidean space [1] and
the self-dual Bogomol’nyi–Prasad–Sommerfield (BPS) monopoles in
the 3 + 1 dimensional Yang–Mills–Higgs theory [2,3]. The self-
dual
solitons satisfy first order differential equations which yields
the absolute minimum of the energy, and by construction they
are also solutions of the full dynamical system of the field equa-
tions.
Another feature of the self-dual field configurations is that
the corresponding topological solitons always saturate the topo-
logical
bound, their static energy (or the Euclidean action in the
case of the Yang–Mills instantons) depends linearly on the topolog-
ical
charge. Moreover, there are very elegant mathematical meth-
ods
of construction of various multi-soliton configurations in these
models, the Nahm equation [4] and the algebraic Atiyah–Hitchin–
Drinfeld–Manin
scheme [5].
However
the usual Skyrme model [6,7], which can be suggested
as an effective low-energy theory of pions, do not support self-dual
E-mail address: shnir@maths.tcd.ie (Ya. Shnir).
equations [8], the mass of the soliton solutions for this model, the
Skyrmions, is always above the topological lower bound in a given
topological sector [9]. As a consequence, there is no exact math-
ematical
scheme of construction of multi-soliton solutions of the
Skyrme model, the only way to obtain these solutions in any topo-
logical
sector, is to implement various numerical methods, some of
them are rather sophisticated, they usually need a large amount of
computational power.
Recently
some modification of the Skyrme model was proposed
to construct the soliton solutions which satisfy the first-order
Bogomol’nyi-type equation [10–12]. In the first case the conven-
tional
Skyrme model was drastically changed via replacement of
the usual sigma model term and the quartic Skyrme term with a
term sextic in first derivatives and a potential [10,11]. In the sec-
ond
case the usual Skyrme model is coupled to the infinite tower
of vector mesons [12]. These self-dual models are directly related
to the usual Skyrme model since they can be considered as sub-
models
of a general model of that type. Further, it was shown very
recently that the standard Skyrme model without the potential
term can be expressed as a sum of two BPS submodels with differ-
ent
solutions [13]. The corresponding submodels, however, are not
directly related to the generalized Skyrme model of any type.
Another
modification of the Skyrme model, which supports
self-dual soltions and has an exact BPS bound, was suggested in
[14]. Similar to the usual Skyrme model, or its generalizations,
http://dx.doi.org/10.1016/j.physletb.2017.07.040
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
.