A Large Ion Collider Experiment Detector Research Articles

Commissioning of the ALICE readout software for LHC Run 3

ALICE (A Large Ion Collider Experiment) is a heavy-ion detector studying the physics of strongly interacting matter and the quarkgluon plasma at the CERN LHC (Large Hadron Collider). During the second long shut-down of the LHC, the ALICE detector was upgraded to cope with an interaction rate of 50 kHz in Pb-Pb collisions, producing in the online computing system (O2) a sustained input throughput of 3.5 TB/s. In the past years, the O2/FLP project built the new data-acquisition system capable of handling this load. It consists of 200 readout nodes, collecting the data transferred from over 8000 detector links to PCs memory by dedicated PCI boards. The readout software manages the hardware and software memory buffers used for DMA and inter-process communication. It initiates the data flow, performs on-the-fly consistency checks, formats the data, reports performance, and finally pushes the data to the local processing pipeline. The output is then sent by the data distribution software over 100Gb/s links to a dedicated event processing farm. The readout software modular design allowed to address the manifold needs faced during the prototyping, installation and commissioning phases, which proved essential from the lab tests to physics production, like file replay and recording, or online multi-threaded LZ4 compression. We will describe the hardware and software implementation of the O2 readout system, and review the challenges met during the commissioning and first months of operation with LHC collisions in 2022.

Performance of the ALICE electromagnetic calorimeters in LHC Runs 1 and 2 and upgrade projects

ALICE (A Large Ion Collider Experiment) at the LHC is designed for studies of nuclear matter at extreme temperatures and energy densities, the so-called Quark-Gluon Plasma (QGP). Two detectors for measurements of electromagnetic signals, high-granularity photon spectrometer PHOS and large acceptance electromagnetic calorimeter EMCal/DCal, are incorporated into the ALICE detector. In order to enhance the data sample of the high-energy part of the spectra, dedicated triggers on high energetic photons or jets are used. High precision measurements of neutral mesons require fine energy and timing calibrations. Photon signal purity is improved using dedicated photon PID criteria based on cluster shape and anti-track matching with the ALICE central tracking system, as well as a special detector located in front of PHOS called CPV. PHOS and EMCal participated in LHC Run 1 (2009-2013) and Run 2 (2015-2018) during data taking periods of pp, p-Pb and Pb-Pb collisions. We give an overview of their performance in Runs 1 and 2 in low and high multiplicity environments as well as the upgrade plans for future LHC runs.

MAPS application in the STAR and ALICE Experiments

Abstract Monolithic Active Pixel Sensors (MAPS) offer a set of highly desirable characteristics for high precision tracking of charged particles. These characteristics include relatively high readout speed, low mass and radiation length, low power dissipation and cost and complexity savings over hybrid detectors. These features along with reasonable radiation tolerance make them ideal candidates for the current heavy ion experiments at colliders. We will describe the principles of operation and the design and performance of the MAPS used in the Soleniodal Tracker at RHIC (STAR) PiXeL (PXL) vertex detector and the A Large Ion Collider Experiment (ALICE) Inner Tracking System (ITS) upgrade. In addition, we will show that the performance at STAR demonstrates the value of the technology. We will also give the performance that is expected to be achieved with the ITS upgrade at the ALICE detector at the CERN.

Readout Software for the Alice Integrated Online-Offline (O2) System

ALICE (A Large Ion Collider Experiment) is a heavy -ion detector studying the physics of strongly interacting matter and the quarkgluon plasma at CERN’s LHC (Large Hadron Collider). During the second long shutdown of the LHC, the ALICE detector will be upgraded to cope with an interaction rate of 50 kHz in Pb–Pb collisions, producing in the online computing system (O2) a sustained input throughput of 3 TB/s. The readout software is in charge of the first step of data-acquisition, handling the data transferred from over 8000 detector links to PC memory by dedicated PCI boards, formatting and buffering incoming traffic until sent to the next components in the processing pipeline. On the 250 readout nodes where it runs, the software has to sustain a throughput which can locally exceed 100 Gb/s. We present the modular design used to cope with various data sources (hardware devices and software emulators), integrated with the central O2 components (logging, configuration, monitoring, data sampling, transport) and initiating the online data flow using the standard O2 messaging system. Performance considerations and measurements are also discussed.

A programming framework for data streaming on the Xeon Phi

ALICE (A Large Ion Collider Experiment) is the dedicated heavy-ion detector studying the physics of strongly interacting matter and the quark-gluon plasma at the CERN LHC (Large Hadron Collider). After the second long shut-down of the LHC, the ALICE detector will be upgraded to cope with an interaction rate of 50 kHz in Pb-Pb collisions, producing in the online computing system (O2) a sustained throughput of 3.4 TB/s. This data will be processed on the fly so that the stream to permanent storage does not exceed 90 GB/s peak, the raw data being discarded. In the context of assessing different computing platforms for the O2 system, we have developed a framework for the Intel Xeon Phi processors (MIC). It provides the components to build a processing pipeline streaming the data from the PC memory to a pool of permanent threads running on the MIC, and back to the host after processing. It is based on explicit offloading mechanisms (data transfer, asynchronous tasks) and basic building blocks (FIFOs, memory pools, C++11 threads). The user only needs to implement the processing method to be run on the MIC. We present in this paper the architecture, implementation, and performance of this system.

Online Calibration of the TPC Drift Time in the ALICE High Level Trigger

ALICE (A Large Ion Collider Experiment) is one of four major experiments at the Large Hadron Collider (LHC) at CERN. The High Level Trigger (HLT) is a compute cluster, which reconstructs collisions as recorded by the ALICE detector in real-time. It employs a custom online data-transport framework to distribute data and workload among the compute nodes. ALICE employs subdetectors sensitive to environmental conditions such as pressure and temperature, e.g. the Time Projection Chamber (TPC). A precise reconstruction of particle trajectories requires the calibration of these detectors. Performing the calibration in real time in the HLT improves the online reconstructions and renders certain offline calibration steps obsolete speeding up offline physics analysis. For LHC Run 3, starting in 2020 when data reduction will rely on reconstructed data, online calibration becomes a necessity. Reconstructed particle trajectories build the basis for the calibration making a fast online-tracking mandatory. The main detectors used for this purpose are the TPC and ITS (Inner Tracking System). Reconstructing the trajectories in the TPC is the most compute-intense step. We present several improvements to the ALICE High Level Trigger developed to facilitate online calibration. The main new development for online calibration is a wrapper that can run ALICE offline analysis and calibration tasks inside the HLT. On top of that, we have added asynchronous processing capabilities to support long-running calibration tasks in the HLT framework, which runs event-synchronously otherwise. In order to improve the resiliency, an isolated process performs the asynchronous operations such that even a fatal error does not disturb data taking. We have complemented the original loop-free HLT chain with ZeroMQ data-transfer components. [...]

Measurement of heavy-flavor production in Pb—Pb collisions at the LHC with ALICE

A Large Ion Collider Experiment (ALICE) at the Large Hadron Collider (LHC) has been built in order to study the Quark-Gluon Plasma (QGP) created in high-energy nuclear collisions. As heavy-flavor quarks are produced at the early stage of the collision, they serve as sensitive probes for the QGP. The ALICE detector with its capabilities such as particle identification, secondary vertexing and tracking in a high multiplicity environment can address, among other measurements, the heavy-flavor sector in heavy-ion collisions.We present latest results on the measurement of the nuclear modification factor of open heavy-flavors as well as on the measurement of open heavy-flavor azimuthal anisotropy v2 in Pb-Pb collisions at TeV.Open charmed hadrons are reconstructed in the hadronic decay channels D° K−−π+, D+ K−−π+π+, and D*+ D0π+ applying a secondary decay-vertex topology. Complementary measurements are performed by detecting electrons (muons) from semi-leptonic decays of open heavy-flavor hadrons in the central (forward) rapidity region.

The ALICE online data storage system

The ALICE (A Large Ion Collider Experiment) Data Acquisition (DAQ) system has the unprecedented requirement to ensure a very high volume, sustained data stream between the ALICE Detector and the Permanent Data Storage (PDS) system which is used as main data repository for Event processing and Offline Computing. The key component to accomplish this task is the Transient Data Storage System (TDS), a set of data storage elements with its associated hardware and software components, which supports raw data collection, its conversion into a format suitable for subsequent high-level analysis, the storage of the result using highly parallelized architectures, its access via a cluster file system capable of creating high-speed partitions via its affinity feature, and its transfer to the final destination via dedicated data links.We describe the methods and the components used to validate, test, implement, operate, and monitor the ALICE Online Data Storage system and the way it has been used in the early days of commissioning and operation for the ALICE Detector. We will also introduce the future developments needed from next year, when the ALICE Data Acquisition System will shift its requirements from those associated to the test and commissioning phase to those imposed by long-duration data taking periods alternated by shorter validation and maintenance tasks which will be needed to adequately operate the ALICE Experiment.

The ALICE experiment at the CERN LHC

ALICE (A Large Ion Collider Experiment) is a general-purpose, heavy-iondetector at the CERN LHC which focuses on QCD, the strong-interactionsector of the Standard Model. It is designed to address the physics ofstrongly interacting matter and the quark-gluon plasma at extreme valuesof energy density and temperature in nucleus-nucleus collisions. Besidesrunning with Pb ions, the physics programme includes collisions withlighter ions, lower energy running and dedicated proton-nucleus runs.ALICE will also take data with proton beams at the top LHC energy tocollect reference data for the heavy-ion programme and to address severalQCD topics for which ALICE is complementary to the other LHC detectors.The ALICE detector has been built by a collaboration including currentlyover 1000 physicists and engineers from 105 Institutes in 30 countries. Its overall dimensions are 16 × 16 × 26 m3 with a total weight of approximately10 000 t. The experiment consists of 18 different detector systems eachwith its own specific technology choice and design constraints, drivenboth by the physics requirements and the experimental conditions expectedat LHC. The most stringent design constraint is to cope with the extremeparticle multiplicity anticipated in central Pb-Pb collisions. Thedifferent subsystems were optimized to provide high-momentumresolution as well as excellent Particle Identification (PID) over a broadrange in momentum, up to the highest multiplicities predicted for LHC.This will allow for comprehensive studies of hadrons, electrons, muons,and photons produced in the collision of heavy nuclei.Most detector systems are scheduled to be installed and ready for data taking by mid-2008 when the LHC is scheduled to start operation, with the exception ofparts of the Photon Spectrometer (PHOS), Transition Radiation Detector(TRD) and Electro Magnetic Calorimeter (EMCal). These detectors will becompleted for the high-luminosity ion run expected in 2010.This paper describes in detail the detector components asinstalled for the first data taking in the summer of 2008.