2,506 research outputs found
GPU-based Online Track Reconstruction for the ALICE TPC in Run 3 with Continuous Read-Out
In LHC Run 3, ALICE will increase the data taking rate significantly to 50
kHz continuous read-out of minimum bias Pb-Pb collisions. The reconstruction
strategy of the online-offline computing upgrade foresees a first synchronous
online reconstruction stage during data taking enabling detector calibration
and data compression, and a posterior calibrated asynchronous reconstruction
stage. Many new challenges arise, among them continuous TPC read-out, more
overlapping collisions, no a priori knowledge of the primary vertex and of
location-dependent calibration in the synchronous phase, identification of
low-momentum looping tracks, and sophisticated raw data compression. The
tracking algorithm for the Time Projection Chamber (TPC) will be based on a
Cellular Automaton and the Kalman filter. The reconstruction shall run online,
processing 50 times more collisions per second than today, while yielding
results comparable to current offline reconstruction. Our TPC track finding
leverages the potential of hardware accelerators via the OpenCL and CUDA APIs
in a shared source code for CPUs and GPUs for both reconstruction stages. We
give an overview of the status of Run 3 tracking including performance on
processors and GPUs and achieved compression ratios.Comment: 8 pages, 7 figures, contribution to CHEP 2018 conferenc
Track Reconstruction in the ALICE TPC using GPUs for LHC Run 3
In LHC Run 3, ALICE will increase the data taking rate significantly to
continuous readout of 50 kHz minimum bias Pb-Pb collisions. The reconstruction
strategy of the online offline computing upgrade foresees a first synchronous
online reconstruction stage during data taking enabling detector calibration,
and a posterior calibrated asynchronous reconstruction stage. We present a
tracking algorithm for the Time Projection Chamber (TPC), the main tracking
detector of ALICE. The reconstruction must yield results comparable to current
offline reconstruction and meet the time constraints like in the current High
Level Trigger (HLT), processing 50 times as many collisions per second as
today. It is derived from the current online tracking in the HLT, which is
based on a Cellular automaton and the Kalman filter, and we integrate missing
features from offline tracking for improved resolution. The continuous TPC
readout and overlapping collisions pose new challenges: conversion to spatial
coordinates and the application of time- and location dependent calibration
must happen in between of track seeding and track fitting while the TPC
occupancy increases five-fold. The huge data volume requires a data reduction
factor of 20, which imposes additional requirements: the momentum range must be
extended to identify low-pt looping tracks and a special refit in uncalibrated
coordinates improves the track model entropy encoding. Our TPC track finding
leverages the potential of hardware accelerators via the OpenCL and CUDA APIs
in a shared source code for CPUs, GPUs, and both reconstruction stages. Porting
more reconstruction steps like the remainder of the TPC reconstruction and
tracking for other detectors will shift the computing balance from traditional
processors to GPUs.Comment: 13 pages, 10 figures, proceedings to Connecting The Dots Workshop,
Seattle, 201
Lepton-pair production in nuclear collisions - past, present, future
The key results on lepton-pair production in ultra-relativistic nuclear
collisions are shortly reviewed, starting at the roots of pp collisions in the
seventies, and ending at the perspectives of the colliders RHIC and LHC. The
presence is dominated by the recent precision results from NA60 at the CERN
SPS, culminating in the first measurement of the in-medium rho spectral
function and the transverse flow of the associated thermal radiation. The
seeming cut-off of the flow above the rho may well be the first direct hint for
thermal radiation of partonic origin in nuclear collisions. The major
milestones in the theoretical developments are also covered.Comment: Invited talk at INPC07, Tokyo, June 3-8, 200
Dileptons and Direct Photons at SPS
The study of dilepton and direct photon emission was one of the main topics
of the experimental program at the SPS devoted to the search of signals for QGP
formation. Three generations of experiments, Helios-3, NA38/NA50, CERES and
NA60 measured e+e- or mu+mu- production in various colliding systems and at
different energies. While lepton pair production in p-A collisions was found to
be reasonably well described by the expected sources, all experiments observed
in nuclear collisions an excess of the yield above the extrapolation from p-A.
As a result of this joint experimental effort we have currently a large amount
of information characterizing this excess: its mass spectrum over the full
range from 0.2 GeV/c^2 up to the J/psi, its transverse momentum spectra
including their mass dependence, its angular distributions, its dependence on
collision centrality over the complete range etc. Putting together all this
information leads to the conclusion that what we observe is the long-sought
thermal radiation from the fireball.Comment: 8 pages, 6 figures - To appear in the conference proceedings for
Quark Matter 2009, March 30 - April 4, Knoxville, Tennesse
NA60 results on spectra and the spectral function in In-In collisions
The NA60 experiment at the CERN SPS has studied low-mass muon pairs in 158
AGeV In-In collisions. A strong excess of pairs is observed above the yield
expected from neutral meson decays. The unprecedented sample size of close to
400K events and the good mass resolution of about 2% have made it possible to
isolate the excess by subtraction of the decay sources (keeping the ).
The shape of the resulting mass spectrum exhibits considerable broadening, but
essentially no shift in mass. The acceptance-corrected transverse-momentum
spectra have a shape atypical for radial flow and show a significant mass
dependence, pointing to different sources in different mass regions.Comment: 4 pages, 4 figures, Quark Matter 2006 conference proceeding
Study of the electromagnetic transition form-factors in \eta -> \mu^+\mu^-\gamma and \omega -> \mu^+\mu^-\pi^0 decays with NA60
The NA60 experiment at the CERN SPS has studied low-mass muon pairs in 158A
GeV In-In collisions. The mass and pT spectra associated with peripheral
collisions can quantitatively be described by the known neutral meson decays.
The high data quality has allowed to remeasure the electromagnetic transition
form factors of the Dalitz decays \eta -> \mu^+\mu^-\gamma and \omega ->
\mu^+\mu^-\pi^0. Using the usual pole approximation F =
(1-M_{\mu\mu}^{2}/\Lambda^{2})^{-1} for the form factors, we find \Lambda^{-2}
(in GeV^{-2}) to be 1.95+-0.17(stat.)+-0.05(syst.) for the \eta and
2.24+-0.06(stat.)+-0.02(syst.) for the \omega. While the values agree with
previous results from the Lepton-G experiment, the errors are greatly improved,
confirming now on the level of 10\sigma the strong enhancement of the \omega
form factor beyond the expectation from vector meson dominance. An improved
value of the branching ratio BR(\omega -> \mu^+\mu^-\pi^0) =
[1.73+-0.25(stat.)+-0.14(syst.)]*10^{-4} has been obtained as a byproduct.Comment: Submitted to Phys. Lett.
Study of dimuon production in Indium-Indium collisions with the NA60 experiment
The NA60 experiment at the CERN-SPS is devoted to the study of dimuon
production in heavy-ion and proton-nucleus collisions. We present preliminary
results from the analysis of Indium-Indium collisions at 158 GeV per nucleon.
The topics covered are low mass vector meson production, J/psi production and
suppression, and the feasibility of the open charm measurement from the dimuon
continuum in the mass range below the J/psi peak.Comment: Contribution at XXXXth Rencontres de Moriond, "QCD and High Energy
Hadronic Interactions
First results from the NA60 experiment at CERN
Since 1986, several heavy ion experiments have studied some signatures of the
formation of the quark-gluon plasma and a few exciting results have been found.
However, some important questions are still unanswered and require new
measurements. The NA60 experiment, with a new detector concept that vastly
improves dimuon detection in proton-nucleus and heavy-ion collisions, studies
several of those open questions, including the production of open charm. This
paper presents the experiment and some first results from data collected in
2002.Comment: Paper presented at the XXXVIII Rencontres de Moriond, QCD and High
Energy Hadronic Interactions, Les Arcs, March 22-29, 2003. 4 pages, 6 figure
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