Compactified NCOS and duality

We study four-dimensional U(1) on a non-commutative T^2 with rational Theta. This theory has dual descriptions as ordinary SYM or as NCOS. We identify a set of massive non-interacting KK states in the SYM theory and track them through the various dualities. They appear as stretched strings in the non-commutative U(1) providing another example of the IR/UV mixing in non-commutative field theories. In the NCOS these states appear as D-strings with winding and momentum. They form an unconventional type of 1/4 BPS state with the 3-brane. To obtain a consistent picture of S-duality for compactified theories it is essential to keep track of both the NS and the RR B-fields.Comment: 16 pages, references correcte

Thick domain walls and singular spaces

We discuss thick domain walls interpolating between spaces with naked singularities and give arguments based on the $AdS$/CFT correspondence why such singularities may be physically meaningful. Our examples include thick domain walls with Minkowski, de Sitter, and anti-de Sitter geometries on the four-dimensional slice. For flat domain walls we can solve the equivalent quantum mechanics problem exactly, which provides the spectrum of graviton states. In one of the examples we discuss, the continuum states have a mass gap. We compare the graviton spectra with expectations from the $AdS$/CFT correspondence and find qualitative agreement. We also discuss unitary boundary conditions and show that they project out all continuum states.Comment: 21 pages, v2: references corrected, v3: minor corrections in sect.

Mirror symmetry for N=1 QED in three dimensions

We construct three-dimensional N=1 QED with N_f flavors using branes of type IIB string theory. This theory has a mirror, which can be realized using the S-dual brane configuration. As in examples with more supersymmetry, the Higgs branch of the original theory gets mapped into the Coulomb branch of the mirror. We use parity invariance to argue that these branches cannot be lifted by quantum corrections.Comment: 10 pages, Latex, 1 figure, reference adde

Heterotic Little String Theories and Holography

It has been conjectured that Little String Theories in six dimensions are holographic to critical string theory in a linear dilaton background. We test this conjecture for theories arising on the worldvolume of heterotic fivebranes. We compute the spectrum of chiral primaries in these theories and compare with results following from Type I-heterotic duality and the AdS/CFT correspondence. We also construct holographic duals for heterotic fivebranes near orbifold singularities. Finally we find several new Little String Theories which have Spin(32)/Z_2 or E_8 \times E_8 global symmetry but do not have a simple interpretation either in heterotic or M-theory.Comment: 27 pages, LaTe

A Simple Volcano Potential with an Analytic, Zero-Energy, Ground State

We describe a simple volcano potential, which is supersymmetric and has an analytic, zero-energy, ground state. (The KK modes are also analytic.) It is an interior harmonic oscillator potential properly matched to an exterior angular momentum-like tail. Special cases are given to elucidate the physics, which may be intuitively useful in studies of higher-dimensional gravity.Comment: 8 pages, 3 figure

Order 1/m_b^3 corrections to B\to X_c\ell\bar\nu decay and their implication for the measurement of \bar\Lambda and \lambda_1

We compute the order 1/m_b^3 nonperturbative contributions to the inclusive differential B\to X_c\ell\bar\nu decay rate. They are parametrized by the expectation values of two local and four nonlocal dimension-six operators. We use our results to estimate part of the theoretical uncertainties in the extraction of matrix elements \bar\Lambda and \lambda_1 from the lepton spectrum in the inclusive semileptonic B decay and find them to be very large. We also compute the 1/m_b^3 corrections to the moments of the hadronic invariant mass spectrum in this decay, and combine them with the extracted values of \bar\Lambda and \lambda_1 to put an upper bound on the branching fraction Br(B\to D^{**}\ell\bar\nu).Comment: 21 pages, revtex, 2 postscript figures. Section on the nonrelativistic quark model estimates is removed; some points of the computation are clarified; results unchange

Aspects of Heavy Quark Physics

We discuss the use of two QCD based effective field theories in determining parameters of the standard model. Heavy Quark Effective theory provides a framework for studying the weak decays of the B meson. We calculate the B → Xclv decay rate to second nonvanishing order in the HQET expansion and compare the theoretical prediction to experimental data. This allows us to determine the b and c pole quark masses and the weak mixing angle Vcb. Nonrelativistic QCD is an effective field theory in which bound states of two heavy quarks can be analyzed conveniently. We use this approach to compute the leading relativistic corrections to the decay of S-wave quarkonia. These corrections can be expressed in terms of the leading order result and the quark pole mass. Including color octet contributions and the first relativistic corrections, we extract the value of αs from low energy data. We also show that the color octet contributions to the decay rates are not negligible.</p