Physics Letters B 776 (2018) 236–241
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Physics Letters B
www.elsevier.com/locate/physletb
Hairy AdS black holes with a toroidal horizon in 4D Einstein-nonlinear
σ -model system
Marco Astorino
a
, Fabrizio Canfora
b
, Alex Giacomini
c,∗
, Marcello Ortaggio
d,c
a
Universidad Adolfo Ibanez, Viña del Mar, Chile
b
Centro de Estudios Científicos (CECS), Casilla 1469, Valdivia, Chile
c
Instituto de Ciencias Físicas y Matemáticas, Universidad Austral de Chile, Edificio Emilio Pugin, cuarto piso, Campus Isla Teja, Valdivia, Chile
d
Institute of Mathematics of the Czech Academy of Sciences, Žitná 25, 115 67 Prague 1, Czech Republic
a r t i c l e i n f o a b s t r a c t
Article history:
Received
19 October 2017
Received
in revised form 16 November 2017
Accepted
21 November 2017
Available
online 24 November 2017
Editor:
M. Cveti
ˇ
c
An exact hairy asymptotically locally AdS black hole solution with a flat horizon in the Einstein-nonlinear
sigma model system in (3+1) dimensions is constructed. The ansatz for the nonlinear SU(2) field is
regular everywhere and depends explicitly on Killing coordinates, but in such a way that its energy–
momentum
tensor is compatible with a metric with Killing fields. The solution is characterized by a
discrete parameter which has neither topological nor Noether charge associated with it and therefore
represents a hair. A U (1) gauge field interacting with Einstein gravity can also be included. The
thermodynamics is analyzed. Interestingly, the hairy black hole is always thermodynamically favoured
with
respect to the corresponding black hole with vanishing Pionic field.
© 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 nonlinear sigma model is a useful theoretical tool with
applications ranging from quantum field theory to statistical me-
chanics
systems like quantum magnetism, the quantum Hall effect,
meson interactions, super fluid
3
He, and string theory [1]. The
most relevant application of the SU(2) non-linear sigma model
in particle physics is the description of the dynamics of Pions at
low energy in 3+1 dimensions (see for instance [2]; for a detailed
review [3]). Consequently, the analysis of the coupling of the non-
linear
sigma model to General Relativity is extremely important
both from the theoretical and from the phenomenological point
of view. On the other hand, due to the complexity of the field
equations (which usually reduce to a non-linear system of coupled
PDEs), the Einstein-nonlinear sigma model system has been ana-
lyzed
mostly relying on numerical analyses (classic references are
[4–8]).
However,
the recent generalization of the boson star ansatz to
SU(2)-valued scalar fields (introduced in [9–19] and [20–22]) al-
lows
to construct also non-trivial gravitating soliton solutions (see
in particular [10] and [16]) as well as to analyse explicitly ther-
*
Corresponding author.
E-mail
addresses: marco.astorino@gmail.com (M. Astorino), canfora@cecs.cl
(F. Canfora),
alexgiacomini@uach.cl (A. Giacomini), ortaggio@math.cas.cz
(M. Ortaggio).
modynamic properties of solitons (see [22]). These are necessary
ingredients to build Pionic black holes with non-trivial thermody-
namics.
Following
the strategy devised in those references, we will con-
struct
a class of analytic black hole solutions of the Einstein--
nonlinear
sigma model system with intriguing geometrical prop-
erties.
This family of black holes possesses a flat horizon
1
and a
discrete hairy parameter. The thermodynamics can be analysed ex-
plicitly.
The first law is satisfied and an interesting feature of these
black holes is that the hairy solution has always less free energy
than the corresponding vacuum solution: thus, the present analysis
suggests that the coupling of Pions with the gravitational field can
act as a sort of catalysis for the Pions themselves. This family of
black holes with flat horizons can also be generalized to the case in
which there is a U(1) gauge field coupled to Einstein gravity. The
interesting thermodynamical features of these black holes remain
in this case as well. This is qualitatively similar (in a phenomeno-
logical
setting such as the Einstein–Pions system) to the recent
findings of [23] in which a family of asymptotically flat black holes
with non-Abelian hair has been presented which are thermody-
namically
favoured over the Reissner–Nordström solution.
1
Black holes with planar horizons have recently attracted a lot of attention due
to their applications in holography, see, e.g., [24–26].
https://doi.org/10.1016/j.physletb.2017.11.051
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
.