438 research outputs found
SUSY Relics in One-Flavor QCD from a New 1/N Expansion
We suggest a new large-N_c limit for multi flavor QCD. Since fundamental and
two-index antisymmetric representations are equivalent in SU(3), we have the
option to define SU(N_c) QCD keeping quarks in the latter. We can then define a
new 1/N_c expansion (at fixed number of flavors N_f) that shares appealing
properties with the topological (fixed N_f/N_c) expansion while being more
suitable for theoretical analysis. In particular, for N_f=1, our large-N_c
limit gives a theory that we recently proved to be equivalent, in the bosonic
sector, to N=1 supersymmetric gluodynamics. Using known properties of the
latter, we derive several qualitative and semi-quantitative predictions for
N_f=1 massless QCD which can be easily tested in lattice simulations. Finally,
we comment on possible applications for pure SU(3) Yang-Mills theory and real
QCD.Comment: 4 pages, RevTex. v2: note added at the end of the paper, ref. added.
To appear in Phys.Rev.Let
Screening in Supersymmetric Gauge Theories in Two Dimensions
We show that the string tension in N=1 two-dimensional super Yang-Mills
theory vanishes independently of the representation of the quark anti-quark
external source. We argue that this result persists in SQCD_2 and in
two-dimensional gauge theories with extended supersymmetry or in chiral
invariant models with at least one massless dynamical fermion. We also compute
the string tension for the massive Schwinger model, as a demonstration of the
method of the calculation.Comment: 8 pages, Latex. Minor changes. Accepted to Phys.Lett.
From Super-Yang-Mills Theory to QCD: Planar Equivalence and its Implications
We review and extend our recent work on the planar (large N) equivalence
between gauge theories with varying degree of supersymmetry. The main emphasis
is made on the planar equivalence between N=1 gluodynamics (super-Yang-Mills
theory) and a non-supersymmetric "orientifold field theory." We outline an
"orientifold" large N expansion, analyze its possible phenomenological
consequences in one-flavor massless QCD, and make a first attempt at extending
the correspondence to three massless flavors. An analytic calculation of the
quark condensate in one-flavor QCD starting from the gluino condensate in N=1
gluodynamics is thoroughly discussed. We also comment on a planar equivalence
involving N=2 supersymmetry, on "chiral rings" in non-supersymmetric theories,
and on the origin of planar equivalence from an underlying, non-tachyonic
type-0 string theory. Finally, possible further directions of investigation,
such as the gauge/gravity correspondence in large-N orientifold field theory,
are briefly discussed.Comment: 106 pages, LaTex. 15 figures. v2:minor changes, refs. added. To be
published in the Ian Kogan Memorial Collection "From Fields to Strings:
Circumnavigating Theoretical Physics," World Scientific, 200
QCD-like Theories on R_3\times S_1: a Smooth Journey from Small to Large r(S_1) with Double-Trace Deformations
We consider QCD-like theories with one massless fermion in various
representations of the gauge group SU. The theories are formulated on
. In the decompactification limit of large all these
theories are characterized by confinement, mass gap and spontaneous breaking of
a (discrete) chiral symmetry (SB). At small , in order to
stabilize the vacua of these theories at a center-symmetric point, we suggest
to perform a double trace deformation. With these deformation, the theories at
hand are at weak coupling at small and yet exhibit basic features of
the large- limit: confinement and SB. We calculate the string
tension, mass gap, bifermion condensates and dependence. The
double-trace deformation becomes dynamically irrelevant at large .
Despite the fact that at small confinement is Abelian, while it is
expected to be non-Abelian at large , we argue that small and
large- physics are continuously connected. If so, one can use
small- laboratory to extract lessons about QCD and QCD-like theories on
.Comment: 49 pages, 2 figure
Exact Results in Non-Supersymmetric Large N Orientifold Field Theories
We consider non-supersymmetric large N orientifold field theories.
Specifically, we discuss a gauge theory with a Dirac fermion in the
anti-symmetric tensor representation. We argue that, at large N and in a large
part of its bosonic sector, this theory is non-perturbatively equivalent to N=1
SYM, so that exact results established in the latter (parent) theory also hold
in the daughter orientifold theory. In particular, the non-supersymmetric
theory has an exactly calculable bifermion condensate, exactly degenerate
parity doublets, and a vanishing cosmological constant (all this to leading
order in 1/N).Comment: 18 pages, Latex. 3 figures. v2,v3: minor changes, refs. added, to
appear in Nucl.Phys.
The spectrum of states with one current acting on the adjoint vacuum of massless QCD2
We consider a ``one current'' state, which is obtained by the application of
a color current on the ``adjoint'' vacuum. This is done in , with the
underlying quarks in the fundamental representation. The quarks are taken to be
massless, in which case the theory on the light-front can be ``currentized'',
namely, formulated in terms of currents only. The adjoint vacuum is shown to be
the application of a current derivative, at zero momentum, on the singlet
vacuum. We apply the operator on these states and find that in
general they are not eigenstates of apart from the large limit.
Problems with infra-red regularizations are pointed out. We discuss the
fermionic structure of these states.Comment: 18 pages, no figures. v2: minor corrections. v3: added some
clarifications and remarks, mainly on the contribution of zero modes. Typos
corrected, references added. To appear in Nuclear Physics
The string tension in massive
We compute the string tension in massive . It is shown that the string
tension vanishes when the mass of the dynamical quark is zero, with no
dependence on the representations of the dynamical or of the external charges.
When a small mass () is added, a tension appears and we calculate its
value as a function of the representations.Comment: 10 pages, Late
The spectrum of multi-flavor QCD_2 and the non-Abelian Schwinger equation
Massless is dominated by classical configurations in the large
limit. We use this observation to study the theory by finding solutions to
equations of motion, which are the non-Abelian generalization of the Schwinger
equation. We find that the spectrum consists of massive mesons with which correspond to Abelian solutions. We generalize previously
discovered non-Abelian solutions and discuss their interpretation. We prove a
no-go theorem ruling out the existence of soliton solutions. Thus the
semi-classical approximation shows no baryons in the case of massless quarks, a
result derived before in the strong-coupling limit only.Comment: 17 pages, Latex. 1 figur
Degeneracy Between the Regge Slope of Mesons and Baryons from Supersymmetry
We consider the degeneracy between the Regge slope of mesons and baryons in
QCD. We argue that within the "orientifold large-N approximation"
asymptotically massive mesons and baryons become supersymmetric partners and
hence degenerate. To this end, we generalize QCD by a SU(N) theory with a quark
in the two-index antisymmetric representation. We show that in this framework
the meson is represented by an oriented bosonic QCD-string and the baryon is
represented by an un-oriented fermionic QCD-string. At large-N, due to an
equivalence with super Yang-Mills, the tensions of the bosonic and the
fermionic strings coincide. Our description of mesons and baryons as oriented
and un-oriented bosonic and fermionic QCD-strings is in full agreement with the
spectra of open strings in the dual type 0' string theory.Comment: v2: extended version. Appendices and references adde
Non-Perturbative Planar Equivalence and the Absence of Closed String Tachyons
We consider 'orbifold' and 'orientifold' field theories from the dual closed
string theory side. We argue that a necessary condition for planar equivalence
to hold is the absence of a closed string tachyonic mode in the dual
non-supersymmetric string. We analyze several gauge theories on R3xS1. In the
specific case of U(N) theories with symmetric/anti-symmetric fermions
('orientifold field theories') the relevant closed string theory is
tachyon-free at large compactification radius (due to winding modes), but it
develops a tachyonic mode below a critical radius. Our finding is with
agreement with field theory expectations of a phase transition from a C-parity
violating phase to a C-parity preserving phase as the compactification radius
increases. In the case of U(N)xU(N) theories with bi-fundamental matter
('orbifold field theories') a tachyon is always present in the string spectrum,
at any compactification radius. We conclude that on R4 planar equivalence holds
for 'orientfiold field theories', but fails for 'orbifold field theories'
daughters of N=4 SYM and suggest the same for daughters of N=1 SYM. We also
discuss examples of SO/Sp gauge theories with symmetric/anti-symmetric
fermions. In this case planar equivalence holds at any compactification radius
-in agreement with the absence of tachyons in the string dual.Comment: 14 pages, Latex. 3 eps figures. v2: ref. added. v3: clarifying
sentences added in the abstract and at the end of section 4. version accepted
to JHE
- …