808 research outputs found
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
D3-D7 Quark-Gluon Plasmas at Finite Baryon Density
We present the string dual to SU(Nc) N=4 SYM, coupled to Nf massless
fundamental flavors, at finite temperature and baryon density. The solution is
determined by two dimensionless parameters, both depending on the 't Hooft
coupling at the scale set by the temperature T:
, weighting the backreaction of the flavor
fields and , where is the
baryon density. For small values of these two parameters the solution is given
analytically up to second order. We study the thermodynamics of the system in
the canonical and grand-canonical ensembles. We then analyze the energy loss of
partons moving through the plasma, computing the jet quenching parameter and
studying its dependence on the baryon density. Finally, we analyze certain
"optical" properties of the plasma. The whole setup is generalized to non
abelian strongly coupled plasmas engineered on D3-D7 systems with D3-branes
placed at the tip of a generic singular Calabi-Yau cone. In all the cases,
fundamental matter fields are introduced by means of homogeneously smeared
D7-branes and the flavor symmetry group is thus a product of abelian factors.Comment: 27 pages; v2: 29 pages, 1 (new) figure, new section 4.4 on optical
properties, references, comments added; v3: eq. (3.19), comments and a
reference adde
Fission-track dating of Armenian and georgian obsidians: characterisation of potential sources of raw material and provenance studies of numerous artefacts from prehistoric sites
Holographic Duals of Quark Gluon Plasmas with Unquenched Flavors
We review the construction of gravitational solutions holographically dual to
N=1 quiver gauge theories with dynamical flavor multiplets. We focus on the
D3-D7 construction and consider the finite temperature, finite quark chemical
potential case where there is a charged black hole in the dual solution.
Discussed physical outputs of the model include its thermodynamics (with
susceptibilities) and general hydrodynamic properties.Comment: Lecture presented at the Workshop "AdS/CFT and Novel Approaches to
Hadron and Heavy Ion Physics", Kavli Institute of Theoretical Physics
(KITPC), Beijing, China, 13 October 2010. Review article to be published in
Communications in Theoretical Physics. 27 pages, 2 figure
Wilson Loop, Regge Trajectory and Hadron Masses in a Yang-Mills Theory from Semiclassical Strings
We compute the one-loop string corrections to the Wilson loop, glueball Regge
trajectory and stringy hadron masses in the Witten model of non supersymmetric,
large-N Yang-Mills theory. The classical string configurations corresponding to
the above field theory objects are respectively: open straight strings, folded
closed spinning strings, and strings orbiting in the internal part of the
supergravity background. For the rectangular Wilson loop we show that besides
the standard Luescher term, string corrections provide a rescaling of the field
theory string tension. The one-loop corrections to the linear glueball Regge
trajectories render them nonlinear with a positive intercept, as in the
experimental soft Pomeron trajectory. Strings orbiting in the internal space
predict a spectrum of hadronic-like states charged under global flavor
symmetries which falls in the same universality class of other confining
models.Comment: 52 pages, latex 3 times, v3: references adde
Hydrodynamics of fundamental matter
First and second order transport coefficients are calculated for the strongly
coupled N=4 SYM plasma coupled to massless fundamental matter in the Veneziano
limit. The results, including among others the value of the bulk viscosity and
some relaxation times, are presented at next-to-leading order in the flavor
contribution. The bulk viscosity is found to saturate Buchel's bound. This
result is also captured by an effective single-scalar five-dimensional
holographic dual in the Chamblin-Reall class and it is suggested to hold, in
the limit of small deformations, for generic plasmas with gravity duals,
whenever the leading conformality breaking effects are driven by marginally
(ir)relevant operators. This proposal is then extended to other relations for
hydrodynamic coefficients, which are conjectured to be universal for every
non-conformal plasma with a dual Chamblin-Reall-like description. Our analysis
extends to any strongly coupled gauge theory describing the low energy dynamics
of Nc>>1 D3-branes at the tip of a generic Calabi-Yau cone. The fundamental
fields are added by means of 1<<Nf<<Nc homogeneously smeared D7-branes.Comment: 24 pages. V2: Important improvements in the discussion of the results
in section 1. References adde
Back-reaction of Non-supersymmetric Probes: Phase Transition and Stability
We consider back-reaction by non-supersymmetric D7/anti-D7 probe branes in
the Kuperstein-Sonnenschein model at finite temperature. Using the smearing
technique, we obtain an analytical solution for the back-reacted background to
leading order in N_f/N_c. This back-reaction explicitly breaks the conformal
invariance and introduces a dimension 6 operator in the dual field theory which
is an irrelevant deformation of the original conformal field theory. We further
probe this back-reacted background by introducing an additional set of probe
brane/anti-brane. This additional probe sector undergoes a chiral phase
transition at finite temperature, which is absent when the back-reaction
vanishes. We investigate the corresponding phase diagram and the thermodynamics
associated with this phase transition. We also argue that additional probes do
not suffer from any instability caused by the back-reaction, which suggests
that this system is stable beyond the probe limit.Comment: 56 pages, 8 figures. References updated, improved discussion on
dimension eight operato
Holography and Unquenched Quark-Gluon Plasmas
We employ the string/gauge theory correspondence to study properties of
strongly coupled quark-gluon plasmas in thermal gauge theories with a large
number of colors and flavors. In particular, we analyze non-critical string
duals of conformal (S)QCD, as well as ten dimensional wrapped fivebrane duals
of SQCD-like theories. We study general properties of the dual plasmas,
including the drag force exerted on a probe quark and the jet quenching
parameter. We find that these plasma observables depend on the number of colors
and flavors in the ``QCD dual''; in particular, we find that the jet quenching
parameter increases linearly with N_f/N_c at leading order in the probe limit.
In the ten dimensional case we find a non trivial drag coefficient but a
vanishing jet quenching parameter. We comment on the relation of this result
with total screening and argue that the same features are shared by all known
plasmas dual to fivebranes in ten dimensions. We also construct new D5 black
hole solutions with spherical horizon and show that they exhibit the same
features.Comment: 30 pages. v2: Comments in section 2 and references updated, a typo
fixe
Probes on D3-D7 Quark-Gluon Plasmas
We study the holographic dual model of quenched flavors immersed in a
quark-gluon plasma with massless dynamical quarks in the Veneziano limit. This
is modeled by embedding a probe D7 brane in a background where the backreaction
of massless D7 branes has been taken into account. The background, and hence
the effects, are perturbative in the Veneziano parameter N_f/N_c, therefore
giving small shifts of all magnitudes like the constituent mass, the quark
condensate, and several transport coefficients. We provide qualitative results
for the effect of flavor degrees of freedom on the probes. For example, the
meson melting temperature is enhanced, while the screening length is
diminished. The drag force is also enhanced.Comment: 31 pages, 17 figure
D3/D7 Quark-Gluon Plasma with Magnetically Induced Anisotropy
We study the effects of the temperature and of a magnetic field in the setup
of an intersection of D3/D7 branes, where a large number of D7 branes is
smeared in the transverse directions to allow for a perturbative solution in a
backreaction parameter. The magnetic field sources an anisotropy in the plasma,
and we investigate its physical consequences for the thermodynamics and energy
loss of particles probing the system. In particular we comment on the
stress-energy tensor of the plasma, the propagation of sound in the directions
parallel and orthogonal to the magnetic field, the drag force of a quark moving
through the medium and jet quenching.Comment: 29 pages + appendices, 5 figures. v2 Version to appear in JHEP, with
minor revisions, references added and typos correcte
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