123 research outputs found
Klebanov-Witten theory with massive dynamical flavors
We consider the addition of a large number of massive dynamical flavors to
the Klebanov-Witten theory, the quiver gauge theory describing the low energy
dynamics of Nc D3-branes at the conifold singularity. Massive flavors are
introduced by means of Nf D7-branes which are holomorphically embedded and
smeared along the transverse directions. After some general comments on the
validity of the smearing procedure, we find the full backreacted supergravity
solution corresponding to a particular class of massive embeddings. The
solution depends on a running effective number of flavors, whose functional
form follows from the smeared embedding. The running reflects the integrating
in/out of massive degrees of freedom in the dual field theory as the energy
scale is changed. We study how the dynamics of the theory depends on the flavor
parameters, mainly focusing on the static quark-antiquark potential. As
expected, we find that the dynamical flavors tend to screen the static color
charges.Comment: 31 pages, 8 figures; v2: few comments and a reference added, minor
change
Dirac Fields on Spacelike Hypersurfaces, Their Rest-Frame Description and Dirac Observables
Grassmann-valued Dirac fields together with the electromagnetic field (the
pseudoclassical basis of QED) are reformulated on spacelike hypersurfaces in
Minkowski spacetime and then restricted to Wigner hyperplanes to get their
description in the rest-frame Wigner-covariant instant form of dynamics. The
canonical reduction to the Wigner-covariant Coulomb gauge is done in the rest
frame. It is shown, on the basis of a geometric incosistency, that the
description of fermions is incomplete, because there is no bosonic carrier of
the spin structure describing the trajectory of the electric current in
Minkowski spacetime, as it was already emphasized in connection with the first
quantization of spinning particles in a previous paper.Comment: 44 pages, revte
Non chiral dynamical flavors and screening on the conifold
We present a new class of string theory solutions which are conjectured to be
dual to the N=1 conifold theory by Klebanov and Witten coupled to non chiral
massive dynamical flavors. These are introduced, in the Veneziano limit, by
means of suitably embedded and smeared D7-branes whose full backreaction on the
background is taken into account. The string solutions are used to study
chromoelectric charge screening effects due to the dynamical flavors in the non
perturbative regime of the dual gauge theories.Comment: 6 pages, 1 figure, previously unpublished results presented.
Contribution to the proceedings of the RTN workshop "Constituents,
Fundamental Forces and Symmetries of the Universe", Varna, Bulgaria,
September 11-17, 200
Resonance contributions to nucleon spin structure in Holographic QCD
We study polarized inelastic electron-nucleon scattering at low momentum
transfer, in the Witten-Sakai-Sugimoto model of holographic QCD. We focus in
particular on resonance production contributions to the nucleon spin structure
functions. Our analysis includes both spin and spin low-lying
nucleon resonances with positive and negative parity. We determine, in turn,
the helicity amplitudes for nucleon-resonance transitions and the resonance
contributions to the neutron and proton generalized spin polarizabilities.
Extrapolating the model parameters to realistic QCD data, our analysis,
triggered by recent experimental results from Jefferson Lab, agrees with the
observation that the resonance gives the dominant contribution
to the forward spin polarizabilities at low momentum transfer. The contribution
is negative and increases towards zero as the momentum transfer increases. As
expected, the contribution of the to the longitudinal-transverse
polarizabilities is instead negligible. Our analysis shows that different spin
resonances give different contributions, in sign and magnitude, to the
generalized longitudinal-transverse spin polarizabilities. In the proton case
they globally give rise to a positive function which decreases towards zero as
the momentum transfer increases. In the neutron case, the net effect produces a
negative increasing function. These features are in qualitative agreement with
experimental data.Comment: 43 pages, 17 figures, 2 table
Neutron-proton mass difference from gauge/gravity duality
Using gauge/gravity duality as a tool, we compute the strong sector, isospin
breaking induced contribution to the neutron-proton mass difference in the
Witten-Sakai-Sugimoto model of large QCD with two non-degenerate light
flavors. The mass difference, for which we provide an analytic expression,
turns out to be positive and proportional to the down-up quark mass splitting,
consistently with expectations and previous estimates based on effective QCD
models. Extrapolating the model parameters to fit QCD hadronic observables, we
find that the strong sector contribution to the nucleon mass splitting
overcomes the electromagnetic contribution and is about of the average
nucleon mass in the model, a result which approaches recent lattice QCD
estimates. Our formula is extended to resonances and baryons. We thus
use it to compute the strong sector contribution to baryons mass
differences. Finally, we also provide details of how isospin breaking affects
the holographic instanton solution describing the baryons.Comment: Latex, 23 pages; v2 clarifications, comments and references added,
version published on PR
Holographic QCD with Dynamical Flavors
Gravity solutions describing the Witten-Sakai-Sugimoto model of holographic
QCD with dynamical flavors are presented. The field theory is studied in the
Veneziano limit, at first order in the ratio of the number of flavors and
colors. The gravity solutions are analytic and dual to the field theory either
in the confined, low temperature phase or in the deconfined, high temperature
phase with small baryonic charge density. The phase diagram and the flavor
contributions to vacuum (e.g. string tension and hadron masses) and
thermodynamical properties of the dual field theory are then deduced. The phase
diagram of the model at finite temperature and imaginary chemical potential, as
well as that of the unflavored theory at finite theta angle are also discussed
in turn, showing qualitative similarities with recent lattice studies.
Interesting degrees of freedom in each phase are discussed. Covariant
counterterms for the Witten-Sakai-Sugimoto model are provided both in the probe
approximation and in the backreacted case, allowing for a standard holographic
renormalization of the theory.Comment: 48 pages, 2 figures; v2: various comments added (e.g. on integration
constants and holographic renormalization), clarified discussion about
degrees of freedom, typos corrected, references added. Version published by
JHE
Neutron electric dipole moment from gauge/string duality
We compute the electric dipole moment of nucleons in the large QCD
model by Witten, Sakai and Sugimoto with degenerate massive flavors.
Baryons in the model are instantonic solitons of an effective five-dimensional
action describing the whole tower of mesonic fields. We find that the dipole
electromagnetic form factor of the nucleons, induced by a finite topological
angle, exhibits complete vector meson dominance. We are able to
evaluate the contribution of each vector meson to the final result - a small
number of modes are relevant to obtain an accurate estimate. Extrapolating the
model parameters to real QCD data, the neutron electric dipole moment is
evaluated to be . The
electric dipole moment of the proton is exactly the opposite.Comment: Latex, 4 pages; v2: minor corrections, few comments adde
Holographic Flavored Quark-Gluon Plasmas
Holography provides a novel method to study the physics of Quark Gluon
Plasmas, complementary to the ordinary field theory and lattice approaches. In
this context, we analyze the informations that can be obtained for strongly
coupled Plasmas containing dynamical flavors, also in the presence of a finite
baryon chemical potential. In particular, we discuss the jet quenching and the
hydrodynamic transport coefficients.Comment: 10 pages. Contribution to the proceedings of the workshop "The many
faces of QCD", Ghent (Belgium), November 1-5 201
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