74 research outputs found
Performance of Calorimetry in ALICE
The ALICE experiment at LHC studies the strong interaction sector of the
Standard Model with pp, pA and AA collisions. Within the scope of the physics
program, measurements of photons, neutral mesons and jets in ALICE are
performed by two electromagnetic calorimeters. Precise and high-granularity
photon spectrometer (PHOS) composed of lead-tungstate crystals, along with a
wide-aperture lead-scintillator sampling calorimeter (EMCal) provide
complementary measurements of photon observables in a wide kinematic range. The
calorimeter trigger system allows the experiment to utilize efficiently the
full delivered luminosity, recording a data sample enhanced with high-energy
photons and jets. Performance of the ALICE calorimeters from proton-proton to
heavy-ion collision systems is discussed and illustrated by physics results
derived from data collected by ALICE with its electromagnetic calorimeter
system.Comment: 7 pages, 5 figures. Sixth Annual Conference on Large Hadron Collider
Physics (LHCP2018), 4-9 June 2018, Bologna, Ital
Elliptic flow of charged particles at midrapidity relative to the spectator plane in Pb-Pb and Xe-Xe collisions
Measurements of the elliptic flow coefficient relative to the collision plane
defined by the spectator neutrons {} in collisions of Pb
ions at center-of-mass energy per nucleon-nucleon pair =2.76
TeV and Xe ions at =5.44 TeV are reported. The results are
presented for charged particles produced at midrapidity as a function of
centrality and transverse momentum. The ratio between {}
and the elliptic flow coefficient relative to the participant plane {4},
estimated using four-particle correlations, deviates by up to 20% from unity
depending on centrality. This observation differs strongly from the magnitude
of the corresponding eccentricity ratios predicted by the TRENTo and the
elliptic power models of initial state fluctuations that are tuned to describe
the participant plane anisotropies. The differences can be interpreted as a
decorrelation of the neutron spectator plane and the reaction plane because of
fragmentation of the remnants from the colliding nuclei, which points to an
incompleteness of current models of initial state fluctuations. A significant
transverse momentum dependence of the ratio {}/{4} is
observed in all but the most central collisions, which may help to understand
whether momentum anisotropies at low and intermediate transverse momentum have
a common origin in initial state fluctuations. The ratios of {} and {4} to the corresponding initial state eccentricities for Xe-Xe
and Pb-Pb collisions at similar initial entropy density show a difference of
% with an additional variation of +1.8% when including RHIC data
in the TRENTo parameter extraction. These observations provide new experimental
constraints for viscous effects in the hydrodynamic modeling of the expanding
quark-gluon plasma.Comment: 15 pages, 4 captioned figures, submitted to PLB, figures at
http://alice-publications.web.cern.ch/node/784
PID performance of the ALICE-TOF detector in Run 2
In these proceedings we report on the status of the ALICE Time-Of-Flight
(TOF) detector. The running performance of the Run 1 (2009-2013) and Run 2
(2015-present) data taking campaigns are compared. The Particle IDentification
(PID) capabilities of the detector are presented and discussed in the light of
the improved detector calibration that allowed to reach a timing resolution of
56 ps
Measurement of Υ(1S) Elliptic Flow at Forward Rapidity in Pb-Pb Collisions at √sNN=5.02 TeV
The first measurement of the Υ(1S) elliptic flow coefficient (v2) is performed at forward rapidity (2.5 < y < 4) in Pb–Pb collisions at √sNN=5.02 TeV with the ALICE detector at the LHC. The results are obtained with the scalar product method and are reported as a function of transverse momentum (pT) up to 15 GeV/c in the 5%–60% centrality interval. The measured Υ(1S)v2 is consistent with 0 and with the small positive values predicted by transport models within uncertainties. The v2 coefficient in 2 < pT < 15 GeV/c is lower than that of inclusive J/ψ mesons in the same pT interval by 2.6 standard deviations. These results, combined with earlier suppression measurements, are in agreement with a scenario in which the Υ(1S) production in Pb–Pb collisions at LHC energies is dominated by dissociation limited to the early stage of the collision, whereas in the J/ψ case there is substantial experimental evidence of an additional regeneration component
Symmetry plane correlations in Pb-Pb collisions at TeV
A newly developed observable for correlations between symmetry planes, which
characterize the direction of the anisotropic emission of produced particles,
is measured in Pb-Pb collisions at TeV with ALICE.
This so-called Gaussian Estimator allows for the first time the study of these
quantities without the influence of correlations between different flow
amplitudes. The centrality dependence of various correlations between two,
three and four symmetry planes is presented. The ordering of magnitude between
these symmetry plane correlations is discussed and the results of the Gaussian
Estimator are compared with measurements of previously used estimators. The
results utilizing the new estimator lead to significantly smaller correlations
than reported by studies using the Scalar Product method. Furthermore, the
obtained symmetry plane correlations are compared to state-of-the-art
hydrodynamic model calculations for the evolution of heavy-ion collisions.
While the model predictions provide a qualitative description of the data,
quantitative agreement is not always observed, particularly for correlators
with significant non-linear response of the medium to initial state
anisotropies of the collision system. As these results provide unique and
independent information, their usage in future Bayesian analysis can further
constrain our knowledge on the properties of the QCD matter produced in
ultrarelativistic heavy-ion collisions.Comment: 21 pages, 5 captioned figures, submitted to EPJC, figures at
http://alice-publications.web.cern.ch/node/896
Real-time data processing in the ALICE High Level Trigger at the LHC
At the Large Hadron Collider at CERN in Geneva, Switzerland, atomic nuclei are collided at ultra-relativistic energies. Many final-state particles are produced in each collision and their properties are measured by the ALICE detector. The detector signals induced by the produced particles are digitized leading to data rates that are in excess of 48 GB/s. The ALICE High Level Trigger (HLT) system pioneered the use of FPGA- and GPU-based algorithms to reconstruct charged-particle trajectories and reduce the data size in real time. The results of the reconstruction of the collision events, available online, are used for high level data quality and detector-performance monitoring and real-time time-dependent detector calibration. The online data compression techniques developed and used in the ALICE HLT have more than quadrupled the amount of data that can be stored for offline event processing
Symmetry plane correlations in Pb–Pb collisions at √sNN = 2.76 TeV
A newly developed observable for correlations between symmetry planes, which characterize the direction of the anisotropic emission of produced particles, is measured in Pb–Pb collisions at √sNN = 2.76 TeV with ALICE. This so-called Gaussian Estimator allows for the first time the study of these quantities without the influence of correlations between different flow amplitudes. The centrality dependence of various correlations between two, three and four symmetry planes is presented. The ordering of magnitude between these symmetry plane correlations is discussed and the results of the Gaussian Estimator are compared with measurements of previously used estimators. The results utilizing the new estimator lead to significantly smaller correlations than reported by studies using the Scalar Product method. Furthermore, the obtained symmetry plane correlations are compared to state-of-the-art hydrodynamic model calculations for the evolution of heavy-ion collisions. While the model predictions provide a qualitative description of the data, quantitative agreement is not always observed, particularly for correlators with significant non-linear response of the medium to initial state anisotropies of the collision system. As these results provide unique and independent information, their usage in future Bayesian analysis can further constrain our knowledge on the properties of the QCD matter produced in ultrarelativistic heavy-ion collisions
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