743 research outputs found
Skyrmions in Orientifold and Adjoint QCD
This is a review of recent developments regarding the Skyrmion sector of
higher representation QCD. Ordinary QCD is a SU(n) gauge theory with n_f Dirac
quarks in the fundamental representation. Changing the representation of quarks
leads to different and interesting theories, which are not as well studied as
ordinary QCD. In order to be able to have a consistent asymptotically free
large n limit, we must limit ourselves to three cases: two-index representation
(symmetric or anti-symmetric) and adjoint representation. Skyrmions of the
low-energy effective Lagrangian shall be the main subject of this review. There
are puzzling aspects, both in orientifold and adjoint QCD, regarding the
identification of the Skyrmion and its quantum stability, that have not yet
been understood. We shall explain these problems and the solution we proposed
for them. The first part is dedicated to the two-index representation. Here the
challenge is to identify the correct particle in the spectrum that has to be
identified with the Skyrmion. It turns out not to be the simplest baryon (as in
ordinary QCD) but a baryonic state with higher charge, precisely composed by
n(n\pm 1)/2 quarks. Although not the simplest among the baryons, it is the one
that minimizes the mass per unit of baryonic charge and thus is the most stable
among them. The second part is devoted to the quarks in the adjoint
representation. The task here assume a different perspective. We do not have a
baryon charge, like in ordinary QCD. An important role is now played by a
massive fermion that must be considered in the low-energy effective Lagrangian.
Through this fermion, the Skyrmion acquires an anomalous fermionic number
(-1)^F and, as a consequence, an odd relationship between the latter and its
spin/statistic. This implies a Z_2 stability of the Skyrmion.Comment: 38 pages; 15 figures. v2: ref adde
Baryons and Skyrmions in QCD with Quarks in Higher Representations
We study the baryonic sector of QCD with quarks in the two index symmetric or
antisymmetric representation. The minimal gauge invariant state that carries
baryon number cannot be identified with the Skyrmion of the low energy chiral
effective Lagrangian. Mass, statistics and baryon number do not match. We
carefully investigate the properties of the minimal baryon in the large N limit
and we find that it is unstable under formation of bound states with higher
baryonic number. These states match exactly with the properties of the Skyrmion
of the effective Lagrangian.Comment: 23 pages, 13 figures. v2: minor changes. v3: corrected a mistake and
some typos. v4: modifyed the part about the stability of the Skyrmio
Monopoles Can be Confined by 0, 1 or 2 Vortices
There are three types of monopole in gauge theories with fundamental matter
and N=2 supersymmetry broken by a superpotential. There are unconfined
0-monopoles and also 1 and 2-monopoles confined respectively by one or two
vortices transforming under distinct components of the unbroken gauge group. If
a Fayet-Iliopoulos term is added then there are only 2-monopoles. Monopoles
transform in the bifundamental representation of two components of the unbroken
gauge symmetry, and if two monopoles share a component they may form a
boundstate. Selection rules for this process are found, for example vortex
number is preserved modulo 2. We find the tensions of the vortices, which are
in general distinct, and also the conditions under which vortices are mutually
BPS. Results are derived in field theory and also in MQCD, and in quiver
theories a T-dual picture may be used in which monopoles are classified by
quiver diagrams with two colors of vertices.Comment: 46 pages, 13 figures, V2: Comment on non-BPS correction added; 1
figure adde
From the Sakai-Sugimoto Model to the Generalized Skyrme Model
We derive the generalized Skyrme model as a low-energy effective model of the
Sakai-Sugimoto model. The novelty with the past is the presence of the sextic
term equal to the topological charge squared. This term appears when the
meson, and the tower of states on top of it, are integrated out. We
claim that, in the small 't Hooft coupling limit, the instanton is well
described by a Skyrmion arising from the low energy effective Lagrangian of the
Sakai-Sugimoto model. The sextic term plays a dominant role in this limit.
Moreover, when a pion mass term is added we recover the BPS Skyrme model in the
small 't Hooft coupling limit.Comment: 17 pages, 6 figures. v2: minor correction
NonAbelian Vortices, Large Winding Limits and Aharonov-Bohm Effects
Remarkable simplification arises from considering vortex equations in the
large winding limit. This was recently used in [1] to display all sorts of
vortex zeromodes, the orientational, translational, fermionic as well as
semi-local, and to relate them to the apparently distinct phenomena of the
Nielsen-Olesen-Ambjorn magnetic instabilities. Here we extend these analyses to
more general types of BPS nonAbelian vortices, taking as a prototype a system
with gauged U(1) x SU(N) x SU(N) symmetry where the VEV of charged scalar
fields in the bifundamental representation breaks the symmetry to SU(N)_{l+r} .
The presence of the massless SU(N)_{l+r} gauge fields in 4D bulk introduces all
sorts of non-local, topological phenomena such as the nonAbelian Aharonov-Bohm
effects, which in the theory with global SU(N)_r group (g_r=0) are washed away
by the strongly fluctuating orientational zeromodes in the worldsheet. Physics
changes qualitatively at the moment the right gauge coupling constant g_r is
turned on.Comment: 31 pages, 4 figure
A Coincidence Problem: How to Flow from N=2 SQCD to N=1 SQCD
We discuss, and propose a solution for, a still unresolved problem regarding
the breaking from super-QCD to super-QCD. A mass term W=\mu \Tr
\Phi^2 / 2 for the adjoint field, which classically does the required breaking
perfectly, quantum mechanically leads to a relevant operator that, in the
infrared, makes the theory flow away from pure SQCD. To avoid this
problem, we first need to extend the theory from \SU (n_c) to \U (n_c). We
then look for the quantum generalization of the condition ,
that is, the coincidence between a root of the derivative of the superpotential
and the mass of the quarks. There are of such points
in the moduli space. We suggest that with an opportune choice of
superpotential, that selects one of these coincidence vacua in the moduli
space, it is possible to flow from SQCD to SQCD. Various
arguments support this claim. In particular, we shall determine the exact
location in the moduli space of these coincidence vacua and the precise
factorization of the SW curve.Comment: 45 pp. v2: typos corrected. v3,v4: other minor correction
Large N, Z_N Strings and Bag Models
We study Z_N strings in nonabelian gauge theories, when they can be
considered as domain walls compactified on a cylinder and stabilized by the
flux inside. To make the wall vortex approximation reliable, we must take the
't Hooft large N limit. Our construction has many points in common with the
phenomenological bag models of hadrons.Comment: 24 pages, 5 figures. v2: Corrected a 1/N factor in the large N QCD
sectio
On Some Universal Features of the Holographic Quantum Complexity of Bulk Singularities
We perform a comparative study of the time dependence of the holographic
quantum complexity of some space like singular bulk gravitational backgrounds.
This is done by considering the two available notions of complexity, one that
relates it to the maximal spatial volume and the other that relates it to the
classical action of the Wheeler-de Witt patch. We calculate and compare the
leading and the next to leading terms and find some universal features. The
complexity decreases towards the singularity for both definitions, for all
types of singularities studied. In addition the leading terms have the same
quantitative behavior for both definitions in restricted number of cases and
the behaviour itself is different for different singular backgrounds. The
quantitative details of the next to leading terms, such as their specific form
of time dependence, are found not to be universal. They vary between the
different cases and between the different bulk definitions of complexity. We
also address some technical points inherent to the calculation.Comment: 24 pages, 6 figures. v2: minor correction
- …