4,563 research outputs found
The Master Field of QCD and the 'T Hooft Equation
We rewrite the action for in the light cone gauge only in terms of a
bilocal mesonic field. In this formalism the expansion can be done in a
straightforward way by a saddle point technique that determines the master
field to be identified with the vacuum expectation value of the bilocal field.
Finally we show that the equation of motion for the fluctuations around the
master field is identical with the 't Hooft meson equation.Comment: 7 pages, Latex, NORDITA-93-4
N=2 four-dimensional gauge theories from fractional branes
This is a pedagogical and extended version of the results published in Refs.
[1,2] and presented by the authors in various talks during the last year. We
discuss the type II D-branes (both regular and fractional) of the orbifold
R^{1,5}*R^4/Z_2, we determine their corresponding supergravity solution and
show how this can be used to study the properties of N=2 super Yang-Mills.
Supergravity is able to reproduce the perturbative moduli space of the gauge
theory, while it does not encode the non-perturbative corrections. The short
distance region of space-time, which corresponds to the infrared region of the
gauge theory, is excised by an enhancon mechanism, and more states should be
included in the low energy effective action in order to enter inside the
enhancon and recover the instanton corrections. (To be published on a Memorial
Volume commemorating Michael Marinov)Comment: 44 pages, AMS-LaTeX, no figure
New twist field couplings from the partition function for multiply wrapped D-branes
We consider toroidal compactifications of bosonic string theory with
particular regard to the phases (cocycles) necessary for a consistent
definition of the vertex operators, the boundary states and the T-duality
rules. We use these ingredients to compute the planar multi-loop partition
function describing the interaction among magnetized or intersecting D-branes,
also in presence of open string moduli. It turns out that unitarity in the open
string channel crucially depends on the presence of the cocycles. We then focus
on the 2-loop case and study the degeneration limit where this partition
function is directly related to the tree-level 3-point correlators between
twist fields. These correlators represent the main ingredient in the
computation of Yukawa couplings and other terms in the effective action for
D-brane phenomenological models. By factorizing the 2-loop partition function
we are able to compute the 3-point couplings for abelian twist fields on
generic non-factorized tori, thus generalizing previous expressions valid for
the 2-torus.Comment: 36 pages, 1 figure; v2: typos corrected, proof in the Appendix
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More Anomalies from Fractional Branes
In this note we show how the anomalies of both pure and matter coupled N=1,2
supersymmetric gauge theories describing the low energy dynamics of fractional
branes on orbifolds can be derived from supergravity.Comment: 11 pages, latex; v2: minor typos fixe
Fractional Branes and N=1 Gauge Theories
We discuss fractional D3-branes on the orbifold C^3/Z_2*Z_2. We study the
open and the closed string spectrum on this orbifold. The corresponding N=1
theory on the brane has, generically, a U(N_1)*U(N_2)*U(N_3)*U(N_4) gauge group
with matter in the bifundamental. In particular, when only one type of brane is
present, one obtains pure N=1 Yang-Mills. We study the coupling of the branes
to the bulk fields and present the corresponding supergravity solution, valid
at large distances. By using a probe analysis, we are able to obtain the
Wilsonian beta-function for those gauge theories that possess some chiral
multiplet. Although, due to the lack of moduli, the probe technique is not
directly applicable to the case of pure N=1 Yang-Mills, we point out that the
same formula gives the correct result also for this case.Comment: 21 pages, AMS-LaTeX, v2: references added and typos correcte
Gauge/Gravity Correspondence from Open/Closed String Duality
We compute the annulus diagram corresponding to the interaction of a
fractional D3 brane with a gauge field on its world-volume and a stack of N
fractional D3 branes on the orbifolds C^2 /Z_2 and C^3/Z_2 x Z_2. We show that
its logarithmic divergence can be equivalently understood as due either to
massless open string states circulating in the loop or to massless closed
string states exchanged between two boundary states. This follows from the fact
that, under open/closed string duality, massless states in the open and closed
string channels are matched into each other without mixing with massive states.
This explains why the perturbative properties of many gauge theories living on
the worldvolume of less supersymmetric and nonconformal branes have been
recently obtained from their corresponding supergravity solution.Comment: LaTeX, 28 page
Anomalies and Tadpoles in Open/Closed String Duality
We discuss the role played by the divergences appearing in the interaction
between a fractional D3 brane dressed with an SU(N) gauge field and a stack of
N fractional D3 branes on the orbifolds C^2/Z_2 and C^3/(Z_2 x Z_2). In
particular we show that the logarithmic divergences in the closed string
channel, interpreted as due to twisted massless tadpoles, are mapped, under
open/closed string duality, in the logarithmic ones in the open string channel,
due to the massless states circulating in the annulus diagram and corresponding
to the one-loop divergences that one finds in the gauge theory living in the
world volume of the brane. This result provides a quantitative evidence of why
the chiral and scale anomalies of the supersymmetric and non conformal gauge
theories supported by the world volume of the branes can be inferred from
supergravity calculations.Comment: LaTeX, 8 pages. Contribution to the proceedings of the workshop of
the RTN Network "The quantum structure of space-time and the geometric nature
of fundamental interactions", Copenhagen, September 200
Closed string exchanges on in a background B-field
In an earlier work it was shown that the IR singularities arising in the
nonplanar one loop two point function of a noncommutative gauge
theory can be reproduced exactly from the massless closed string exchanges. The
noncommutative gauge theory is realised on a fractional brane localised
at the fixed point of the orbifold. In this paper we identify the
contributions from each of the closed string modes. The sum of these adds upto
the nonplanar two-point function.Comment: 27 page
Brane-Inspired Orientifold Field Theories
In this paper we consider the gauge theory living on the world-volume of a
stack of N D3-branes of Type 0B/\Omega' I_6(-1)^{F_{L}} and of its orbifolds
C^2/Z_2 and C^3/(Z_2 x Z_2). The gauge theories obtained in the three cases are
a brane realization of ``orientifold field theories'' having the bosonic sector
common with N=4,2,1 super Yang-Mills respectively. In these non-supersymmetric
theories, we investigate the possibility of keeping the gauge/gravity
correspondence that has revealed itself so successful in the case of
supersymmetric theories. In the open string framework we compute the
coefficient of the gauge kinetic term showing that the perturbative behaviour
of the orientifold field theory can be obtained from the closed string channel
in the large N limit, where the theory exhibits Bose-Fermi degeneracy.Comment: 27 pages, LaTe
Is a classical description of stable non-BPS D-branes possible?
We study the classical geometry produced by a stack of stable (i.e. tachyon
free) non-BPS D-branes present in K3 compactifications of type II string
theory. This classical representation is derived by solving the equations of
motion describing the low-energy dynamics of the supergravity fields which
couple to the non-BPS state. Differently from what expected, this configuration
displays a singular behaviour: the space-time geometry has a repulson-like
singularity. This fact suggests that the simplest setting, namely a set of
coinciding non-interacting D-branes, is not acceptable. We finally discuss the
possible existence of other acceptable configurations corresponding to more
complicated bound states of these non-BPS branes.Comment: Latex file: 41 pages, 5 figures; some minor corrections and
references adde
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