355 research outputs found
Universality of the saturation scale and the initial eccentricity in heavy ion collisions
Recent estimates that Color Glass Condensate initial conditions may generate
a larger initial eccentricity for noncentral relativistic heavy ion collisions
(relative to the initial eccentricity assumed in earlier hydrodynamic
calculations) have raised the possibility of a higher bound on the viscosity of
the Quark Gluon Plasma. We show that this large initial eccentricity results in
part from a definition of the saturation scale as proportional to the number of
nucleons participating in the collision. A saturation scale proportional to the
nuclear thickness function (and therefore independent of the probe) leads to a
smaller eccentricity, albeit still larger than the value used in hydrodynamic
models. Our results suggest that the early elliptic flow in heavy ion
collisions (unlike multiplicity distributions) is sensitive to the universality
of the saturation scale in high energy QCD.Comment: 5 pages, 3 figures, RevTE
Wilson line correlator in the MV model: relating the glasma to deep inelastic scattering
In the color glass condensate framework the saturation scale measured in deep
inelastic scattering of high energy hadrons and nuclei can be determined from
the correlator of Wilson lines in the hadron wavefunction. These same Wilson
lines give the initial condition of the classical field computation of the
initial gluon multiplicity and energy density in a heavy ion collision. In this
paper the Wilson line correlator in both adjoint and fundamental
representations is computed using exactly the same numerical procedure that has
been used to calculate gluon production in a heavy ion collision. In particular
the discretization of the longitudinal coordinate has a large numerical effect
on the relation between the color charge density parameter g^2 mu and the
saturation scale Qs. Our result for this relation is Qs = 0.6 g^2 mu, which
results in the classical Yang-Mills value for the "gluon liberation
coefficient" c = 1.1.Comment: 8 pages, 10 figures, RevTEX4, V2: typo corrections, V3: small
clarifications, to be published in EPJ
Energy dependence of the saturation scale and the charged multiplicity in pp and AA collisions
A natural framework to understand the energy dependence of bulk observables
from lower energy experiments to the LHC is provided by the Color Glass
Condensate, which leads to a "geometrical scaling" in terms of an energy
dependent saturation scale Q_s. The measured charged multiplicity, however,
seems to grow faster (~\sqrt{s}^0.3) in nucleus-nucleus collisions than it does
for protons (~\sqrt{s}^0.2), violating the expectation from geometric scaling.
We argue that this difference between pp and AA collisions can be understood
from the effect of DGLAP evolution on the value of the saturation scale, and is
consistent with gluon saturation observations at HERA.Comment: RevTeX, 8 pages, 4 figures. V2: modified discussion of fragmentation,
published in EPJ
Non-perturbative computation of double inclusive gluon production in the Glasma
The near-side ridge observed in A+A collisions at RHIC has been described as
arising from the radial flow of Glasma flux tubes formed at very early times in
the collisions. We investigate the viability of this scenario by performing a
non-perturbative numerical computation of double inclusive gluon production in
the Glasma. Our results support the conjecture that the range of transverse
color screening of correlations determining the size of the flux tubes is a
semi-hard scale, albeit with non-trivial structure. We discuss our results in
the context of ridge correlations in the RHIC heavy ion experiments.Comment: 25 pages, 11 figures, uses JHEP3.cls V2: small clarifications,
published in JHE
Elliptic flow of thermal photons and dileptons
In this talk we describe the recently discovered rich phenomenology of
elliptic flow of electromagnetic probes of the hot matter created in
relativistic heavy-ion collisions. Using a hydrodynamic model for the
space-time dynamics of the collision fireball created in Au+Au collisions at
RHIC, we compute the transverse momentum spectra and elliptic flow of thermal
photons and dileptons. These observables are shown to provide differential
windows into various stages of the fireball expansion.Comment: 8 pages, including 9 figures. Invited talk at the Hard Probes 2006
Conference (Asilomar, June 9-16, 2006), to appear in the Proceedings
(Elsevier
Chemical composition of the decaying glasma
The the initial stage of a relativistic heavy ion collision can be described
by a classical color field configuration known as the Glasma. The production of
quark pairs from this background field is then computed nonperturbatively by
numerically solving the Dirac equation in the classical background. The result
seems to point towards an early chemical equilibration of the plasma.Comment: 8 pages, 5 figures, invited talk at Stangeness in Quark Matter 2006
(SQM06), UCLA, March 200
Exploring the properties of the phases of QCD matter - research opportunities and priorities for the next decade
This document provides a summary of the discussions during the recent joint
QCD Town Meeting at Temple University of the status of and future plans for the
research program of the relativistic heavy-ion community. A list of compelling
questions is formulated, and a number of recommendations outlining the greatest
research opportunities and detailing the research priorities of the heavy-ion
community, voted on and unanimously approved at the Town Meeting, are
presented. They are supported by a broad discussion of the underlying physics
and its relation to other subfields. Areas of overlapping interests with the
"QCD and Hadron Structure" ("cold QCD") subcommunity, in particular the
recommendation for the future construction of an Electron-Ion Collider, are
emphasized. The agenda of activities of the "hot QCD" subcommunity at the Town
Meeting is attached.Comment: 34 pages of text, 254 references,16 figure
On the use of a running coupling in the calculation of forward hadron production at next-to-leading order
We study a puzzle raised recently regarding the running coupling prescription used in the calculation of forward particle production in proton-nucleus collisions at next-to-leading order: using a coordinate space prescription which is consistent with the one used in the high energy evolution of the target leads to results which can be two orders of magnitude larger than the ones obtained with a momentum space prescription. We show that this is an artefact of the Fourier transform involved when passing between coordinate and momentum space and propose a new coordinate space prescription which avoids this problem.Peer reviewe
What is the Evidence for the Color Glass Condensate?
I introduce the concept of the Color Glass Condensate. I review data from
HERA and RHIC which suggest that such a universal form of matter has been
found
Resumming double logarithms in the QCD evolution of color dipoles
The higher-order perturbative corrections, beyond leading logarithmic
accuracy, to the BFKL evolution in QCD at high energy are well known to suffer
from a severe lack-of-convergence problem, due to radiative corrections
enhanced by double collinear logarithms. Via an explicit calculation of Feynman
graphs in light cone (time-ordered) perturbation theory, we show that the
corrections enhanced by double logarithms (either energy-collinear, or double
collinear) are associated with soft gluon emissions which are strictly ordered
in lifetime. These corrections can be resummed to all orders by solving an
evolution equation which is non-local in rapidity. This equation can be
equivalently rewritten in local form, but with modified kernel and initial
conditions, which resum double collinear logs to all orders. We extend this
resummation to the next-to-leading order BFKL and BK equations. The first
numerical studies of the collinearly-improved BK equation demonstrate the
essential role of the resummation in both stabilizing and slowing down the
evolution.Comment: 16 pages, 5 figure
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