Assuming Regge trajectories in holographic QCD: from OPE to Chiral Perturbation Theory

The soft wall model in holographic QCD has Regge trajectories but wrong operator product expansion (OPE) for the two-point vectorial QCD Green function. We modify the dilaton potential to comply OPE. We study also the axial two-point function using the same modified dilaton field and an additional scalar field to address chiral symmetry breaking. OPE is recovered adding a boundary term and low energy chiral parameters, $F_\pi$ and $L_{10}$, are well described analytically by the model in terms of Regge spacing and QCD condensates. The model nicely supports and extends previous theoretical analyses advocating Digamma function to study QCD two-point functions in different momentum regions.Comment: Major changes to improve the presentation of the paper but main results unchanged. Added appendix on Regge progressio

Standard Model prediction and new physics tests for D0 -> h+h-l+l- (h=\pi,K; l=e,\mu)

Motivated by the recent evidence for direct CP-violation in D0 -> h+h- decays, we provide an exhaustive study of both Cabibbo-favored and Cabibbo-suppressed (singly and doubly) D0 -> h1+h2-l+l- decays. In particular, we study the Dalitz plot for the long-distance contributions in the (m_{ll}^2,m_{hh}^2) parameter space. We find that near-resonant effects, i.e., D0 -> V(h1+h2-)l+l- with V=\rho,K*,\phi, are sizeable and even dominant (over Bremsstrahlung) for the \mu+\mu- decay modes, bringing the branching ratios close to the LHCb reach. We also provide a detailed study of the angular asymmetries for such decays and identify signatures for new physics detection. In particular, new physics signals can be neatly isolated in asymmetries involving the semileptonic operator Q_{10}, where for typical new physics scenarios the effects can be as sizeable as O(1%) for the doubly Cabibbo-suppressed modes.Comment: 19 pages, 5 figures. v2 (journal version) contains the new subsection III.B, where the potential size of new physics effects is discussed. Tables and figures slightly changed to match the journal version. Typos corrected and references added. Conclusions unchange

Antisymmetric tensors in holographic approaches to QCD

We study real (massive) antisymmetric tensors of rank two in holographic models of QCD based on the gauge/string duality. Our aim is to understand in detail how the AdS/CFT correspondence describes correlators with tensor currents in QCD. To this end we study a set of bootstrapped correlators with spin-1 vector and tensor currents, imposing matching to QCD at the partonic level. We show that a consistent description of this set of correlators yields a very predictive picture. For instance, it imposes strong constraints on infrared boundary conditions and precludes the introduction of dilatonic backgrounds as a mechanism to achieve linear confinement. Additionally, correlators with tensor currents turn out to be especially sensitive to chiral symmetry breaking, thus offering an ideal testing ground for genuine QCD effects. Several phenomenological consequences are explored, such as the nontrivial interplay between $1^{+-}$ states and conventional $1^{--}$ vector mesons.Comment: 15 pages, 2 figures. Minor changes to match the journal versio

Form factor in K+ --> pi+ pi0 gamma: interference versus direct emission

We analyze the effect of a form factor in the magnetic contribution to K+ --> pi+ pi0 gamma. We emphasize how this can show up experimentally: in particular we try to explore the difference between a possible interference contribution and a form factor in the magnetic part. The form factor used for K+ --> pi+ pi0 gamma is analogous to the one for KL --> pi+ pi- gamma, experimentally well established.Comment: 9 pages revtex, 10 eps figures; improved presentation of theoretical and experimental status; refs. adde

On off-shell bosonic string amplitudes

We give a simple prescription for computing, in the framework of the bosonic string theory, off-shell one-loop amplitudes with any number of external massless particles, both for the open and for the closed string. We discuss their properties and, in particular, for the two-string one-loop amplitudes we show their being transverse.Comment: 12 pages, Latex. One reference added. Introduction and conclusions expanded. Some other minor changes in the tex

Tunnelling Effects in a Brane System and Quantum Hall Physics

We argue that a system of interacting D-branes, generalizing a recent proposal, can be modelled as a Quantum Hall fluid. We show that tachyon condensation in such a system is equivalent to one particle tunnelling. In a conformal field theory effective description, that induces a transition from a theory with central charge c=2 to a theory with c=3/2, with a corresponding symmetry enhancement.Comment: 12 pages, no figures, Latex, some aspects clarified, sect.3 expanded, references adde