1,545 research outputs found
The Gross-Neveu Model from String Theory
We study an intersecting D-brane model which at low energies describes
(1+1)-dimensional chiral fermions localized at defects on a stack of N_c
D4-branes. Fermions at different defects interact via exchange of massless
(4+1)-dimensional fields. At weak coupling this interaction gives rise to the
Gross-Neveu (GN) model and can be studied using field theoretic techniques. At
strong coupling one can describe the system in terms of probe branes
propagating in a curved background in string theory. The chiral symmetry is
dynamically broken at zero temperature and is restored above a critical
temperature T_c which depends on the coupling. The phase transition at T_c is
first order at strong coupling and second order at weak coupling.Comment: 32 pages, harvmac (b
On 3d N=8 Lorentzian BLG theory as a scaling limit of 3d superconformal N=6 ABJM theory
We elaborate on the suggestion made in arXiv:0806.3498 that the 3d \N=8
superconformal SU(N) Chern-Simons-matter theory of Lorentzian
Bagger-Lambert-Gustavson type (L-BLG) can be obtained by a scaling limit
(involving sending the level k to infinity and redefining the fields) from the
\N=6 superconformal U(N)xU(N) Chern-Simons-matter theory of Aharony, Bergman,
Jafferis and Maldacena (ABJM). We show that to implement such a limit in a
consistent way one is to extend the ABJM theory by an abelian "ghost"
multiplet. The corresponding limit at the 3-algebra level also requires
extending the non-antisymmetric Bagger-Lambert 3-algebra underlying the ABJM
theory by a negative-norm generator. We draw analogy with similar scaling
limits discussed previously for bosonic Chern-Simons theory and comment on some
implications of this relation between the ABJM and L-BLG theories.Comment: 16 pages; published version - reference added, minor correction
Chiral Symmetry Breaking from Intersecting D-Branes
We study a class of intersecting D-brane models in which fermions localized
at different intersections interact via exchange of bulk fields. In some cases
these interactions lead to dynamical symmetry breaking and generate a mass for
the fermions. We analyze the conditions under which this happens as one varies
the dimensions of the branes and of the intersections.Comment: 29 pages, harvmac (b
Holographic U(1)_A and String Creation
We analyze the resolution of the U(1)_A problem in the Sakai-Sugimoto
holographic dual of large N_c QCD at finite temperature. It has been shown that
in the confining phase the axial symmetry is broken at order 1/N_c, in
agreement with the ideas of Witten and Veneziano. We show that in the
deconfined phase the axial symmetry remains unbroken to all orders in 1/N_c. In
this case the breaking is due to instantons which are described by spacelike
D0-branes, in agreement with 'tHooft's resolution. The holographic dual of the
symmetry breaking fermion condensate is a state of spacelike strings between
the D0-brane and the flavor D8-branes, which result from a spacelike version of
the string creation effect. In the intermediate phase of deconfinement with
broken chiral symmetry the instanton gas approximation is possibly regulated in
the IR, which would imply an eta' mass-squared of order exp(-N_c).Comment: 18 pages, 19 figures, minor change
Quark mass and condensate in HQCD
We extend the Sakai-Sugimoto holographic model of QCD (HQCD) by including the
scalar bi-fundamental "tachyon" field in the 8-brane-anti-8-brane probe theory.
We show that this field is responsible both for the spontaneous breaking of the
chiral symmetry, and for the generation of (current algebra) quark masses, from
the point of view of the bulk theory. As a by-product we show how this leads to
the Gell-Mann- Oakes-Renner relation for the pion mass.Comment: 23 pages, 7 figures; v2: corrected typos in eqs. (4.3), (4.4), (4.5),
(4.9) and (4.11), and corrected figures 3, 4, 5 and 6; v3: section 5.3 on the
pion mass rewritten in a clearer way, version published in JHE
Holographic QCD and Pion Mass
To realize massive pions, we study variations of the holographic model of
massless QCD using the D4/D8/anti-D8 brane configuration proposed by Sakai and
Sugimoto. We propose deformations which break the chiral symmetry explicitly
and compute the mass of the pions and vector mesons. The observed value of the
pion mass can be obtained. We also argue a chiral perturbation corresponding to
our deformation.Comment: 23pages, minor changes, references adde
Stringy NJL and Gross-Neveu models at finite density and temperature
Nonlocal stringy versions of the Nambu-Jona-Lasinio and Gross-Neveu models
arise in a certain limit of holographic QCD. We analyze the phase structure at
finite density and temperature at strong coupling in terms of probe branes in
the gravity dual. Comparison with the phase structure of the local field theory
models shows qualitative agreement with some aspects, and disagreement with
others. Finally, we explain how to construct the Landau potentials for these
models by taking the probe branes off-shell.Comment: 32 pages, uses JHEP3.cls; v2, references added, version to be
submitted to JHE
On extending actions of groups
Problems of dense and closed extension of actions of compact transformation
groups are solved. The method developed in the paper is applied to problems of
extension of equivariant maps and of construction of equivariant
compactifications
- …