20 research outputs found
Finite momentum at string endpoints
We argue that classical strings, both bosonic and supersymmetric, can have
finite energy and momentum at their endpoints. We show that in a general curved
background, string endpoints must propagate along null geodesics as long as
their energy remains finite. Finite endpoint momentum allows strings with a
fixed energy to travel a greater distance in an AdS5-Schwarzschild background
than has been possible for classical solutions considered previously. We review
the relevance to heavy ion phenomenology of the dependence of this distance on
energy, and we propose a scheme for determining the instantaneous rate of
energy loss.Comment: 28 pages, 4 figure
Holographic light quark jet quenching at RHIC and LHC via the shooting strings
A new shooting string holographic model of jet quenching of light quarks in
strongly coupled plasmas is presented to overcome the phenomenological
incompatibilities of previous falling string holographic scenarios that emerged
when confronted with the recent LHC data. This model is based on strings with
finite momentum endpoints that start close to the horizon and lose energy as
they approach the boundary. This framework is applied to compute the nuclear
modification factor RAA of light hadrons at RHIC and LHC, showing that this
model improves greatly the comparison with the recent light hadron suppression
data. The effects of the Gauss-Bonnet quadratic curvature corrections to the
AdS5 geometry further improve the agreement with the data.Comment: 4 pages, 1 figure. Proceedings of the Hard Probes 2013 Conferenc
Shooting String Holography of Jet Quenching at RHIC and LHC
We derive a new formula for jet energy loss using finite endpoint momentum
shooting strings initial conditions in SYM plasmas to overcome the difficulties
of previous falling string holographic scenarios. We apply the new formula to
compute the nuclear modification factor RAA and the elliptic flow parameter v2
of light hadrons at RHIC and LHC. We show furthermore that Gauss-Bonnet
quadratic curvature corrections to the AdS5 geometry improve the agreement with
the recent data.Comment: 10 pages, 5 figures. Added references and two appendices. Several
claims clarified and other minor corrections. Published versio
Falling Strings and Light Quark Jet Quenching at LHC
We explore phenomenological signatures of light quark jet quenching within
the AdS/CFT correspondence. Numerical studies of the instantaneous energy loss
of light quarks, modeled as falling strings, suggest a linear path dependence.
We propose a phenomenological model for the energy loss and use it to compute
the nuclear modification factor for light quarks in an expanding
plasma with Glauber initial conditions. The results are compared to the light
hadron data at the LHC and, although they show qualitative agreement,
the quantitative disagreement we found motivated the exploration of effects
from higher order derivative corrections to on this observable.Comment: 4 pages, 1 figure. Proceedings for Hard Probes 201
Energy loss, equilibration, and thermodynamics of a baryon rich strongly coupled quark-gluon plasma
Lattice data for the QCD equation of state and the baryon susceptibility near
the crossover phase transition (at zero baryon density) are used to determine
the input parameters of a 5-dimensional Einstein-Maxwell-Dilaton holographic
model that provides a consistent holographic framework to study both
equilibrium and out-of-equilibrium properties of a hot and {\it baryon rich}
strongly coupled quark-gluon plasma (QGP). We compare our holographic equation
of state computed at nonzero baryon chemical potential, , with recent
lattice calculations and find quantitative agreement for the pressure and the
speed of sound for MeV. This holographic model is used to
obtain holographic predictions for the temperature and dependence of
the drag force and the Langevin diffusion coefficients associated with heavy
quark jet propagation as well as the jet quenching parameter and the
shooting string energy loss of light quarks in the baryon dense plasma. We find
that the energy loss of heavy and light quarks generally displays a nontrivial,
fast-varying behavior as a function of the temperature near the crossover.
Moreover, energy loss is also found to generally increase due to nonzero baryon
density effects even though this strongly coupled liquid cannot be described in
terms of well defined quasiparticle excitations. Furthermore, to get a glimpse
of how thermalization occurs in a hot and baryon dense QGP, we study how the
lowest quasinormal mode of an external massless scalar disturbance in the bulk
is affected by a nonzero baryon charge. We find that the equilibration time
associated with the lowest quasinormal mode decreases in a dense medium.Comment: 51 pages, 14 figures, corrected results for the Langevin
coefficients, appendix and references added. Version accepted for publication
in JHE
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Holographic Jet Quenching
In this dissertation we study the phenomenon of jet quenching in quark-gluon plasma using the AdS/CFT correspondence.
We start with a weakly coupled, perturbative QCD approach to energy loss, and present a Monte Carlo code for computation of the DGLV radiative energy loss of quarks and gluons at an arbitrary order in opacity. We use the code to compute the radiated gluon distribution up to n=9 order in opacity, and compare it to the thin plasma (n=1) and the multiple soft scattering (n=\infty) approximations. We furthermore show that the gluon distribution at finite opacity depends in detail on the screening mass and the mean free path.
In the next part, we turn to the studies of how heavy quarks, represented as "trailing strings" in AdS/CFT, lose energy in a strongly coupled plasma. We study how the heavy quark energy loss gets modified in a "bottom-up" non-conformal holographic model, constructed to reproduce some properties of QCD at finite temperature and constrained by fitting the lattice gauge theory results. The energy loss of heavy quarks is found to be strongly sensitive to the medium properties. We use this model to compute the nuclear modification factor R_AA of charm and bottom quarks in an expanding plasma with Glauber initial conditions, and comment on the range of validity of the model.
The central part of this thesis is the energy loss of light quarks in a strongly coupled plasma. Using the standard model of "falling strings", we present an analytic derivation of the stopping distance of light quarks, previously available only through numerical simulations, and also apply it to the case of Gauss-Bonnet higher derivative gravity. We then present a general formula for computing the instantaneous energy loss in non-stationary string configurations. Application of this formula to the case of falling strings reveals interesting phenomenology, including a modified Bragg-like peak at late times and an approximately linear path dependence. Based on these results, we develop a phenomenological model of light quark energy loss and use it compute the nuclear modification factor R_AA of light quarks in an expanding plasma. Comparison with the LHC pion suppression data shows that, although R_AA has the right qualitative structure, the overall magnitude is too low, indicating that the predicted jet quenching is too strong.
In the last part of the thesis we consider a novel idea of introducing finite momentum at endpoints of classical (bosonic and supersymmetric) strings, and the phenomenological consequences of this proposal on the energy loss of light quarks. We show that in a general curved background, finite momentum endpoints must propagate along null geodesics and that the distance they travel in an AdS5-Schwarzschild background is greater than in the previous treatments of falling strings. We also argue that this leads to a more realistic description of energetic quarks, allowing for an unambiguous way of distinguishing between the energy in the dual hard probe and the energy in the color fields surrounding it. This proposal also naturally allows for a clear and simple definition of the instantaneous energy loss. Using this definition and the "shooting string" initial conditions, we develope a new formula for light quark energy loss. Finally, we apply this formula to compute the nuclear modification factor R_AA of light hadrons at RHIC and LHC, which, after the inclusion of the Gauss-Bonnet quadratic curvature corrections to the AdS5 geometry, shows a reasonably good agreement with the recent data
Jet Quenching in Non-Conformal Holography
We use our non-conformal holographic bottom-up model for QCD described in
1012.0116 to further study the effect of the QCD trace anomaly on the energy
loss of both light and heavy quarks in a strongly coupled plasma. We compute
the nuclear modification factor for bottom and charm quarks in an
expanding plasma with Glauber initial conditions. We find that the maximum
stopping distance of light quarks in a non-conformal plasma scales with the
energy with a temperature (and energy) dependent effective power.Comment: 4 pages, 1 figure. Proceedings for Quark Matter 201