Sigma Models |
In General
* Motivation: Non-linear
models are useful in treating spontaneous symmetry breaking, where the absence
of an invariant ground state is described in terms of constraints on the fields,
equivalent to non-linear submanifolds of vector spaces on which the group acts.
* History: The name
σ-model comes from the original theory, which described QCD
phenomenology, and contained a pion triplet field and a scalar, the σ
particle; It was a harmonic map with target space S3
and fields ΦA, with the constraint
∑k \(\pi^k \pi^k\)
+ σ2 = f 2
= constant; Notice that, with the constraint, the values of the fields
do not form a vector space, but they have a Riemannian structure; Later the
name has been extended to other kinds of theories, other kinds of harmonic maps.
$ Def: A σ-model
is a theory of a spacetime scalar field χ, described by the action
S = \({1\over4}\)∫ tr(Ja Ja) dv = \({1\over4}\)∫ hAB ∂aΦA ∂aΦB dv ,
where the metric hAB is defined by
ds2 = hAB
dΦA dΦB
= tr[(dχ) \(\chi^{-1}\)]n,
n being the dimension of spacetime, and Ja:=
(∂aχ) \(\chi^{-1}\).
* Field equations: They are of the form
Ja, a = 0 .
* Coupled to gravity: The action and the equations of motion become
S = ∫ (R − \({1\over4}\)tr Ja Ja) dv and Rab = \({1\over2}\)tr(Ja Jb) .
Examples, Special Types > s.a. canonical quantum theory;
solution methods for the einstein equation.
* On a symmetric space G/H:
A non-linear σ-model on G/H is an example of a harmonic map between
two Riemannian spaces, as can be seen from the action; Non-linear σ-models
on symmetric spaces appear naturally
- In the study of n-dimensional
solutions of Einstein's equation with a set of k commuting Killing vector fields;
The theory reduces to (d−k)-dimensional gravity coupled to a
SL(k+1, \(\mathbb R\))/SO(j, k−j+1) σ-model
on the space of orbits of the Killing vector fields, where j is the number of
timelike Killing vector fields.
- In dimensionally reduced supergravity.
@ Topological: Witten CMP(88).
@ Poisson-sigma-models:
Schaller & Strobl MPLA(94),
LNP(94)gq,
ht/94,
LNP(96)ht/95 [intro];
Bandos & Kummer IJMPA(99)ht/97;
Hirshfeld & Schwarzweller ht/00-proc;
Batalin & Marnelius PLB(01) [generalized];
Cattaneo m.QA/07 [and deformation quantization];
Bonechi et al JHEP(12)-a1110.
@ Sigma-models and gravity: Bizoń & Wasserman PRD(00)gq [self-similar spherical];
Clément a0812-conf [higher-dimensional, and supergravity];
> s.a. massive gravity.
@ Supersymmetric σ-models: Barnes PLB(99)ht;
Imbiriba ht/99;
Higashijima & Nitta ht/00-conf [as gauge theories];
Albertsson et al NPB(04) [with boundaries].
@ Other types: Fujii & Suzuki LMP(98) [(2+1)-dimensional, conserved currents];
Percacci & Zanusso PRD(10)-a0910 [with higher-derivative terms].
Specific Features and Effects
> s.a. duality; Ricci Flow
[renormalization-group flow]; solitons.
@ Critical phenomena: Liebling et al JMP(00)mp/99 [singular/non-singular solutions].
@ Global identity: Mazur PLA(84) [generalization of Green identities].
@ As deformed topological field theories: Fendley PRL(99)ht.
@ Related topics: Bastianelli et al PLB(00) [dimensional regularization];
> s.a. critical collapse;
lie algebras [algebroids]; phase transitions.
References
@ Original model: Gell-Mann & Levy NC(60).
@ General: Coleman, Wess & Zumino PR(69);
Salam & Strathdee PR(69),
PR(69);
Duff & Isham PRD(77);
Lindström a1803-conf [uses, rev];
Ang & Prakash a1810
[solitonic sectors, and ordered phases of systems with spontaneously broken symmetries].
@ Textbooks and reviews: Percacci 86.
@ Quantization: Isham & Klauder JMP(90);
Nguyen JMP-a1408 [2D, perturbative].
@ Renormalization:
Bonneau ht/99-in [and BRS symmetry];
Ferrari JHEP(05)ht [flat connection structure];
Ferrari & Quadri IJTP(06)ht/05 [weak power-counting theorem],
JHEP(06)ht/05 [4D, two-loop];
Codello & Percacci PLB(09)-a0810 [fixed points].
> Online resources:
see Wikipedia page.
main page
– abbreviations
– journals – comments
– other sites – acknowledgements
send feedback and suggestions to bombelli at olemiss.edu – modified 31 oct 2018