2,805 research outputs found
Observable implications of geometrical and dynamical aspects of freeze-out in heavy ion collisions
Using an analytical parameterization of hadronic freeze-out in relativistic
heavy ion collisions, we present a detailed study of the connections between
features of the freeze-out configuration and physical observables. We focus
especially on anisotropic freeze-out configurations (expected in general for
collisions at finite impact parameter), azimuthally-sensitive HBT
interferometry, and final-state interactions between non-identical particles.
Model calculations are compared with data taken in the first year of running at
RHIC; while not perfect, good agreement is found, raising the hope that a
consistent understanding of the full freeze-out scenario at RHIC is possible,
an important first step towards understanding the physics of the system prior
to freeze-out.Comment: 36 pages, 56 figures, 2 tables; version accepted for publication in
PRC: some figures, references and discussion added; now also discusses
classical versus quantum statistic
Induced photon emission from quark jets in ultrarelativistic heavy-ion collisions
We study the induced photon bremsstrahlung from a fast quark produced in
AA-collisions due to multiple scattering in quark-gluon plasma. For RHIC and
LHC conditions the induced photon spectrum is sharply peaked at photon energy
close to the initial quark energy. In this region the contribution of the
induced radiation to the photon fragmentation function exceeds the ordinary
vacuum radiation. Contrary to previous analyses our results show that at RHIC
and LHC energies the final-state interaction effects in quark-gluon plasma do
not suppress the direct photon production, and even may enhance it at p_{T}
about 5-15 GeV.Comment: 11 pages, 4 figure
Prototype tests for the ALICE TRD
A Transition Radiation Detector (TRD) has been designed to improve the
electron identification and trigger capability of the ALICE experiment at the
Large Hadron Collider (LHC) at CERN. We present results from tests of a
prototype of the TRD concerning pion rejection for different methods of
analysis over a momentum range from 0.7 to 2 GeV/c. We investigate the
performance of different radiator types, composed of foils, fibres and foams.Comment: Presented at the IEEE Nuclear Science Symposium and Medical Imaging
Conference, Lyon, October 15-20, 2000 (accepted for publication in IEEE TNS),
Latex (IEEEtran.cls), 7 pages, 11 eps figure
Emission of thermal photons and the equilibration time in Heavy-Ion collisions
The emission of hard real photons from thermalized expanding hadronic matter
is dominated by the initial high-temperature expansion phase. Therefore, a
measurement of photon emission in ultrarelativistic heavy-ion collisions
provides valuable insights into the early conditions realized in such a
collision. In particular, the initial temperature of the expanding fireball or
equivalently the equilibration time of the strongly interacting matter are of
great interest. An accurate determination of these quantities could help to
answer the question whether or not partonic matter (the quark gluon plasma) is
created in such collisions. In this work, we investigate the emission of real
photons using a model which is based on the thermodynamics of QCD matter and
which has been shown to reproduce a large variety of other observables. With
the fireball evolution fixed beforehand, we are able to extract limits for the
equilibration time by a comparison with photon emission data measured by WA98.Comment: 12 pages, 5 figures, accepted for publication at Phys. Rev.
Transverse Momentum Spectra of Pions in Particle and Nuclear Collisions and Some Ratio-Behaviours: Towards A Combinational Approach
The nature of transverse momentum dependence of the inclusive cross-sections
for secondary pions produced in high energy hadronic(), hadronuclear()
and nuclear() collisions has here been exhaustively investigated for a
varied range of interactions in a unified way with the help of a master
formula. This formula evolved from a new combination of the basic Hagedorn's
model for particle(pion) production in PP scattering at ISR range of energies,
a phenomenological approach proposed by Peitzmann for converting the results of
reactions to those for either or collisions, and a specific
form of parametrization for mass number-dependence of the nuclear cross
sections. This grand combination of models(GCM) is then applied to analyse the
assorted extensive data on various high energy collisions. The nature of
qualitative agreement between measurements and calculations on both the
inclusive cross-sections for production of pions, and some ratios of them as
well, is quite satisfactory. The modest successes that we achieve here in
dealing with the massive data-sets are somewhat encouraging in view of the
diversity of the reactions and the very wide range of interaction energies.Comment: 19 pages, 19 figure
Is the analysis of flow at the CERN SPS reliable?
Several heavy ion experiments at SPS have measured azimuthal distributions of
particles with respect to the reaction plane. These distributions are deduced
from two-particle azimuthal correlations under the assumption that they result
solely from correlations with the reaction plane. In this paper, we investigate
other sources of azimuthal correlations: transverse momentum conservation,
which produces back-to-back correlations, resonance decays, HBT correlations
and final state interactions. These correlations increase with impact
parameter: most of them vary with the multiplicity N like 1/N. When they are
taken into account, the experimental results of the NA49 collaboration at SPS
are significantly modified. These correlations might also explain an important
fraction of the pion directed flow observed by WA98. Data should be reanalyzed
taking into account carefully these non--flow correlations.Comment: Revised version (minor corrections), 13 pages, LaTeX, 6 Postscript
figures included. Submitted to Physical Review
Measurement of the production of charm jets tagged with D mesons in pp collisions at = 7 TeV
The production of charm jets in proton-proton collisions at a center-of-mass
energy of TeV was measured with the ALICE detector at the CERN
Large Hadron Collider. The measurement is based on a data sample corresponding
to a total integrated luminosity of , collected using a
minimum-bias trigger. Charm jets are identified by the presence of a D
meson among their constituents. The D mesons are reconstructed from their
hadronic decay DK. The D-meson tagged jets are
reconstructed using tracks of charged particles (track-based jets) with the
anti- algorithm in the jet transverse momentum range
and pseudorapidity
. The fraction of charged jets containing a D-meson
increases with from to . The distribution of D-meson tagged jets as a
function of the jet momentum fraction carried by the D meson in the
direction of the jet axis () is reported for two ranges
of jet transverse momenta, and
in the intervals
and , respectively. The
data are compared with results from Monte Carlo event generators (PYTHIA 6,
PYTHIA 8 and Herwig 7) and with a Next-to-Leading-Order perturbative Quantum
Chromodynamics calculation, obtained with the POWHEG method and interfaced with
PYTHIA 6 for the generation of the parton shower, fragmentation, hadronisation
and underlying event.Comment: 29 pages, 8 captioned figures, 3 tables, authors from page 24,
published version, figures at http://alice-publications.web.cern.ch/node/525
First measurement of production in pp collisions at = 7 TeV
The production of the charm-strange baryon is measured for
the first time at the LHC via its semileptonic decay into e
in pp collisions at TeV with the ALICE detector. The transverse
momentum () differential cross section multiplied by the branching
ratio is presented in the interval 1 8 GeV/ at
mid-rapidity, 0.5. The transverse momentum dependence of the
baryon production relative to the D meson production is
compared to predictions of event generators with various tunes of the
hadronisation mechanism, which are found to underestimate the measured
cross-section ratio.Comment: 22 pages, 6 captioned figures, 1 table, authors from page 17,
published version, figures at
http://aliceinfo.cern.ch/ArtSubmission/node/412
Constraining the magnitude of the Chiral Magnetic Effect with Event Shape Engineering in Pb-Pb collisions at = 2.76$ TeV
In ultrarelativistic heavy-ion collisions, the event-by-event variation of
the elliptic flow reflects fluctuations in the shape of the initial state
of the system. This allows to select events with the same centrality but
different initial geometry. This selection technique, Event Shape Engineering,
has been used in the analysis of charge-dependent two- and three-particle
correlations in Pb-Pb collisions at TeV. The
two-particle correlator ,
calculated for different combinations of charges and , is
almost independent of (for a given centrality), while the three-particle
correlator
scales almost linearly both with the event and charged-particle
pseudorapidity density. The charge dependence of the three-particle correlator
is often interpreted as evidence for the Chiral Magnetic Effect (CME), a parity
violating effect of the strong interaction. However, its measured dependence on
points to a large non-CME contribution to the correlator. Comparing the
results with Monte Carlo calculations including a magnetic field due to the
spectators, the upper limit of the CME signal contribution to the
three-particle correlator in the 10-50% centrality interval is found to be
26-33% at 95% confidence level.Comment: 20 pages, 6 captioned figures, 1 tables, authors from page 15,
published version, figures at
http://aliceinfo.cern.ch/ArtSubmission/node/382
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