42 research outputs found
Off-center collisions in AdS_5 with applications to multiplicity estimates in heavy-ion collisions
We study the trapped surface produced by an off-center collision of
light-like, point-sourced shock waves in anti-de Sitter space. We find an
analytic expression for the shape of the trapped surface in the limit where the
energy of the shock waves is large and the impact parameter is not too large.
We use the area of the trapped surface to estimate a lower bound on the entropy
produced in the collision. We compare our results to particle multiplicity
measurements in heavy-ion collisions as interpreted through the Glauber model.
In an attempt to roughly simulate the effects of asymptotic freedom and
confinement in quantum chromodynamics, we also consider the effects of slicing
off parts of anti-de Sitter space.Comment: 43 pages, 12 figures, 2 appendice
Saturation physics and angular correlations at RHIC and LHC
We investigate the angular correlation between pions and photons produced in
deuteron-gold collisions at RHIC and proton-lead collisions at LHC using the
Color Glass Condensate formalism and make predictions for the dependence of the
production cross section on the angle between the pion and photon at different
rapidities and transverse momenta. Measuring this dependence would shed further
light on the role of high gluon density and saturation dynamics at RHIC and
LHC.Comment: 2-column EPJ C format, requires svjour.cls and svepj.clo; 4 pages, 4
figure
Pomeron loop and running coupling effects in high energy QCD evolution
Within the framework of a (1+1)-dimensional model which mimics evolution and
scattering in QCD at high energy, we study the influence of the running of the
coupling on the high-energy dynamics with Pomeron loops. We find that the
particle number fluctuations are strongly suppressed by the running of the
coupling, by at least one order of magnitude as compared to the case of a fixed
coupling, for all the rapidities that we have investigated, up to Y=200. This
reflects the slowing down of the evolution by running coupling effects, in
particular, the large rapidity evolution which is required for the formation of
the saturation front via diffusion. We conclude that, for all energies of
interest, processes like deep inelastic scattering or forward particle
production can be reliably studied within the framework of a mean-field
approximation (like the Balitsky-Kovchegov equation) which includes running
coupling effects.Comment: 23 pages, 8 figure
Asymmetric Collision of Two Shock Waves in AdS_5
We consider high energy collisions of two shock waves in AdS_5 as a model of
ultrarelativistic nucleus-nucleus collisions in the boundary theory. We first
calculate the graviton field produced in the collisions in the NLO and NNLO
approximations, corresponding to three- and four-graviton exchanges with the
shock waves. We then consider the asymmetric limit where the energy density in
one shock wave is much higher than in the other one. In the boundary theory
this setup corresponds to proton-nucleus collisions, with the nucleus being the
denser of the two shock waves and the proton being the less dense one.
Employing the eikonal approximation we find the exact high energy analytic
solution for the metric in AdS_5 for the asymmetric collision of two
delta-function shock waves. The solution resums all-order graviton exchanges
with the "nucleus" shock wave and a single-graviton exchange with the "proton"
shock wave. Using the holographic renormalization prescription we read off the
energy-momentum tensor of the matter produced in proton-nucleus collisions. We
show in explicit detail that in the boundary theory the proton is completely
stopped by strong-coupling interactions with the nucleus, in agreement with our
earlier results. We also apply the eikonal technique to the asymmetric
collision of two unphysical delta-prime shock waves, which we introduced in an
earlier work as a means of modeling nuclear collisions with weak coupling
initial dynamics. We obtain a surprising result that, for delta-prime shock
waves, the multiple bulk graviton exchange series giving the leading
energy-dependent contribution to the energy-momentum tensor terminates at the
order of two graviton exchanges with the nucleus.Comment: 44 pages, 8 figures; v2: typos corrected, references added; v3: a
subsection added, more typos correcte
Shockwaves and deep inelastic scattering within the gauge/gravity duality
Within the gauge/gravity correspondence, we discuss the general formulation
of the shockwave metric which is dual to a 'nucleus' described by the
strongly-coupled N=4 SYM theory in the limit where the number of colors Nc is
arbitrarily large. We emphasize that the 'nucleus' must possess Nc^2 degrees of
freedom per unit volume, so like a finite-temperature plasma, in order for a
supergravity description to exist. We critically reassess previous proposals
for introducing transverse inhomogeneity in the shockwave and formulate a new
proposal in that sense, which involves no external source but requires the
introduction of an 'infrared' cutoff which mimics confinement. This cutoff
however plays no role when the shockwave is probed by a highly virtual
projectile, so like in deep inelastic scattering. We consider two such
projectiles, the dilaton and the R-current, and compute the respective
structure functions including unitarity corrections. We find that there are no
leading-twist contributions to the structure functions at high virtuality,
meaning that there are no point-like constituents in the strongly coupled
'nucleus'. In the black-disk regime at low virtuality, the structure functions
are suggestive of parton saturation with occupation numbers of order one. The
saturation momentum Qs grows with the energy like Qs^2 ~ 1/x (with x the
Bjorken variable), which is the hallmark of graviton exchanges and is also
necessary for the fulfillment of the energy-momentum sum rules.Comment: 43 page
The dynamics of quark-gluon plasma and AdS/CFT
In these pedagogical lectures, we present the techniques of the AdS/CFT
correspondence which can be applied to the study of real time dynamics of a
strongly coupled plasma system. These methods are based on solving
gravitational Einstein's equations on the string/gravity side of the AdS/CFT
correspondence. We illustrate these techniques with applications to the
boost-invariant expansion of a plasma system. We emphasize the common
underlying AdS/CFT description both in the large proper time regime where
hydrodynamic dynamics dominates, and in the small proper time regime where the
dynamics is far from equilibrium. These AdS/CFT methods provide a fascinating
arena interrelating General Relativity phenomenae with strongly coupled gauge
theory physics.Comment: 35 pages, 3 figures. Lectures at the 5th Aegean summer school, `From
gravity to thermal gauge theories: the AdS/CFT correspondence'. To appear in
the proceedings in `Lecture Notes in Physics
Non-linear QCD dynamics and exclusive production in collisions
The exclusive processes in electron-proton () interactions are an
important tool to investigate the QCD dynamics at high energies as they are in
general driven by the gluon content of proton which is strongly subject to
parton saturation effects. In this paper we compute the cross sections for the
exclusive vector meson production as well as the deeply virtual Compton
scattering (DVCS) relying on the color dipole approach and considering the
numerical solution of the Balitsky-Kovchegov equation including running
coupling corrections. We show that the small- evolution given by this
evolution equation is able to describe the DESY-HERA data and is relevant for
the physics of the exclusive observables in future electron-proton colliders
and in photoproduction processes to be measured in coherent interactions at the
LHC.Comment: 6 pages, 4 figure
Non-linear QCD dynamics in two-photon interactions at high energies
Perturbative QCD predicts that the growth of the gluon density at high
energies should saturate, forming a Color Glass Condensate (CGC), which is
described in mean field approximation by the Balitsky-Kovchegov (BK) equation.
In this paper we study the interactions at high energies and
estimate the main observables which will be probed at future linear colliders
using the color dipole picture. We discuss in detail the dipole - dipole cross
section and propose a new relation between this quantity and the dipole
scattering amplitude. The total ,
cross-sections and the real photon structure function are
calculated using the recent solution of the BK equation with running coupling
constant and the predictions are compared with those obtained using
phenomenological models for the dipole-dipole cross section and scattering
amplitude. We demonstrate that these models are able to describe the LEP data
at high energies, but predict a very different behavior for the observables at
higher energies. Therefore we conclude that the study of
interactions can be useful to constrain the QCD dynamics.Comment: 11 pages, 5 figures. Version to be published in European Physical
Journal
Heavy-quark energy loss in pQCD and SYM plasmas
We consider heavy-quark energy loss and pT-broadening in a strongly-coupled
N=4 Super Yang Mills (SYM) plasma, and the problem of finite-extend matter is
addressed. When expressed in terms of the appropriate saturation momentum, one
finds identical parametric forms for the energy loss in pQCD and SYM theory,
while pT-broadening is radiation dominated in SYM theory and multiple
scattering dominated in pQCD.Comment: 5 pages, 2 figures, Proceedings of the IIIrd Workshop for Young
Scientists on the Physics of Ultrarelativistic Nucleus-Nucleus Collisions
(HotQuarks08), Estes Park, USA, August 18-23 200
Glauber - Gribov approach for DIS on nuclei in N=4 SYM
In this paper the Glauber-Gribov approach for deep-inelastic scattering (DIS)
with nuclei is developed in N=4 SYM. It is shown that the amplitude displays
the same general properties, such as geometrical scaling, as is the case in the
high density QCD approach. We found that the quantum effects leading to the
graviton reggeization, give rise to an imaginary part of the nucleon amplitude,
which makes the DIS in N=4 SYM almost identical to the one expected in high
density QCD. We concluded that the impact parameter dependence of the nucleon
amplitude is very essential for N=4 SYM, and the entire kinematic region can be
divided into three regions which are discussed in the paper. We revisited the
dipole description for DIS and proposed a new renormalized Lagrangian for the
shock wave formalism which reproduces the Glauber-Gribov approach in a certain
kinematic region. However the saturation momentum turns out to be independent
of energy, as it has been discussed by Albacete, Kovchegov and Taliotis. We
discuss the physical meaning of such a saturation momentum and argue
that one can consider only within the shock wave approximation.Comment: 40pp.,9 figures in eps file