1,125 research outputs found
Centrality dependence of multiplicity, transverse energy, and elliptic flow from hydrodynamics
The centrality dependence of the charged multiplicity, transverse energy, and
elliptic flow coefficient is studied in a hydrodynamic model, using a variety
of different initializations which model the initial energy or entropy
production process as a hard or soft process, respectively. While the charged
multiplicity depends strongly on the chosen initialization, the p_t-integrated
elliptic flow for charged particles as a function of charged particle
multiplicity and the p_t-differential elliptic flow for charged particles in
minimum bias events turn out to be almost independent of the initial energy
density profile.Comment: 11 pages RevTex, including 10 postscript figures. Slightly modified
discussion of Figs. 5 and 6, updated references. This version to appear in
Nuclear Physics
Hydrodynamics of nuclear collisions with initial conditions from perturbative QCD
We compute the longitudinal hydrodynamic flow in ultrarelativistic heavy ion
collisions at = 5500 GeV by using boost non-invariant initial
conditions following from perturbative QCD. The transfer of entropy and energy
from the central region to larger rapidities caused by boost non-invariance is
determined and the associated decrease in the lifetime of the system is
estimated.Comment: 12 pages, including 6 figure
Testing collinear factorization and nuclear parton distributions with pA collisions at the LHC
Global perturbative QCD analyses, based on large data sets from
electron-proton and hadron collider experiments, provide tight constraints on
the parton distribution function (PDF) in the proton. The extension of these
analyses to nuclear parton distributions (nPDF) has attracted much interest in
recent years. nPDFs are needed as benchmarks for the characterization of hot
QCD matter in nucleus-nucleus collisions, and attract further interest since
they may show novel signatures of non- linear density-dependent QCD evolution.
However, it is not known from first principles whether the factorization of
long-range phenomena into process-independent parton distribution, which
underlies global PDF extractions for the proton, extends to nuclear effects. As
a consequence, assessing the reliability of nPDFs for benchmark calculations
goes beyond testing the numerical accuracy of their extraction and requires
phenomenological tests of the factorization assumption. Here we argue that a
proton-nucleus collision program at the LHC would provide a set of measurements
allowing for unprecedented tests of the factorization assumption underlying
global nPDF fits.Comment: 4 pages, 5 figure
Thermal photons from fluctuating initial conditions
Event-by-event fluctuations of initial QCD-matter density produced in
heavy-ion collisions at RHIC enhance the production of thermal photons
significantly in the region GeV/ compared to a smooth
initial-state averaged profile in the ideal hydrodynamic calculation. This
enhancement is a an early time effect due to the presence of hotspots or
over-dense regions in the fluctuating initial state. The effect of fluctuations
is found to be stronger in peripheral than in central collisions.Comment: 4 pages, 3 figures. Talk given at Quark Matter 2011, 22-28 May 2011,
Annecy, Franc
Effects of shadowing on Drell-Yan dilepton production in high energy nuclear collisions
We compute cross sections for the Drell-Yan process in nuclear collisions at
next-to-leading order (NLO) in \alpha_s. The effects of shadowing on the
normalization and on the mass and rapidity dependence of these cross sections
are presented. An estimate of higher order corrections is obtained from
next-to-next-to-leading order (NNLO) calculation of the rapidity-integrated
mass distribution. Variations in these predictions resulting from choices of
parton distribution sets are discussed. Numerical results for mass
distributions at NLO are presented for RHIC and LHC energies, using appropriate
rapidity intervals. The shadowing factors in the dilepton mass range 2 < M < 10
GeV are predicted to be substantial, typically 0.5 - 0.7 at LHC, 0.7 - 0.9 at
RHIC, and approximately independent of the choice of parton distribution sets
and the order of calculation.Comment: 13 pages, 9 eps figure
Collision centrality and dependence of the emission of thermal photons from fluctuating initial state in ideal hydrodynamic calculation
Fluctuations in the initial QCD matter density distribution are found to
enhance the production of thermal photons significantly in the range 2 \leq pT
\leq 4 GeV/c compared to a smooth initial state averaged profile in ideal
hydrodynamic calculation for 200 AGeV Au+Au collisions at the Relativistic
Heavy Ion Collider (RHIC) and 2.76 ATeV Pb+Pb collisions at the Large Hadron
Collider (LHC). The thermal emission of photons is strongly dependent on the
initial temperature of the system where the presence of 'hotspots' in the
initial state translates into enhanced production of photons compared to a
smooth profile. The effect of fluctuations in the initial state is found to be
stronger for peripheral collisions and for lower beam energies. The pT spectra
are found to be quite sensitive to the value of the initial formation time of
the plasma which is not known unambiguously and which may vary with collision
centralities at a particular beam energy. Increase in the value of the
formation time lowers the production of thermal photons compared to the results
from a shorter formation time. However, the relative enhancement from
fluctuating initial tates (compared to a smooth initial state) is found to be
stronger for the larger values of formation time. The pT spectra alone are
found to be insufficient to quantify the fluctuations in the initial density
distribution due to the uncertainties in the initial conditions. A suitably
normalized ratio of central-to-peripheral yield as a function of collision
centrality and pT can be a useful measure of the fluctuation size scale.Comment: 10 pages, 10 figure
Hadron multiplicities, pT-spectra and net-baryon number in central Pb+Pb collisions at the LHC
We compute the initial energy density and net baryon number density in 5%
most central Pb+Pb collisions at TeV from pQCD + (final state)
saturation, and describe the evolution of the produced system with
boost-invariant transversely expanding hydrodynamics. In addition to the total
multiplicity at midrapidity, we give predictions for the multiplicity of
charged hadrons, pions, kaons and (anti)protons, for the total transverse
energy and net-baryon number, as well as for the -spectrum of charged
hadrons, pions and kaons. We also predict the region of applicability of
hydrodynamics by comparing these results with high- hadron spectra
computed from pQCD and energy losses.Comment: 2 pages, 2 figures, to be presented at the workshop "Heavy Ion
Collisions at the LHC: Last Call for Predictions" at CERN 29 May - 2 Jun
Transverse Spectra of Hadrons in Central Collisions at RHIC and LHC from pQCD+Saturation+Hydrodynamics and from pQCD+Energy Losses
We study the transverse spectra of hadrons in nearly central collisions
at RHIC and LHC in a broad transverse momentum range Low- spectra are
calculated by using boost-invariant hydrodynamics with initial energy and
net-baryon densities from the EKRT pQCD+saturation model. High- spectra
are obtained from pQCD jet calculation including the energy loss of the parton
in the matter prior to its fragmentation to final hadrons.Comment: 4 pages, 2 figures, proceedings for Quark Matter 200
Nuclear parton distributions in the DGLAP approach
Determination of the nuclear parton distributions within the framework of
perturbative QCD, the DGLAP equations in particular, is discussed. Scale and
flavour dependent nuclear effects in the parton distributions are compared with
the scale and flavour independent parametrizations of HIJING and of the Hard
Probe Collaboration. A comparison with the data from deep inelastic
lepton-nucleus scattering and the Drell-Yan process in proton-nucleus
collisions is shown.Comment: 19 pages, 6 eps-figures, to appear in the Proceedings of the Hard
Probe Collaboratio
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