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.
The ALICE Time-Of-Flight (TOF) detector at LHC is based on the Multigap Resistive Plate Chambers (MRPCs). The TOF performance during LHC Run 2 is here reported. Particular attention is given to the improved time resolution reached by TOF detector of $56$ ps, with the consequently improved particle identification capabilities.
The ALICE High-Level Trigger processes data online, to either select interesting (sub-) events, or to compress data efficiently by modeling techniques. Focusing on the main data source, the Time Projection Chamber, the architecure of the system and the current state of the tracking and compression methods are outlined.
The central detectors of the ALICE experiment at LHC will produce a data size of up to 75 MByte/event at an event rate <200 Hz resulting in a data rate of \~15 GByte/sec. This exceeds the foreseen mass storage bandwidth of 1.25 GByte/sec by one order of magnitude. Online processing of the data is necessary in order to select interesting (sub)events ("High Level Trigger"), or to compress data efficiently by modeling techniques. The largest computing challenge is imposed by the TPC, requiring realtime pattern recognition. The work carried out investigates the performance of various pattern recog…
The High Level Trigger (HLT) of the ALICE experiment requires massive parallel computing. One of the main tasks of the HLT system is two-dimensional cluster finding on raw data of the Time Projection Chamber (TPC), which is the main data source of ALICE. To reduce the number of computing nodes needed in the HLT farm, FPGAs, which are an intrinsic part of the system, will be utilized for this task. VHDL code implementing the Fast Cluster Finder algorithm, has been written, a testbed for functional verification of the code has been developed, and the code has been synthesized
As from the run 3 of CERN LHC scheduled in 2022, the upgraded ALICE experiment will use a Common Readout Unit (CRU) at the heart of the data acquisition system. The CRU, based on the PCIe40 hardware designed for LHCb, is a common interface between 3 main sub-systems: the front-end, the computing system, and the trigger and timing system. The 475 CRUs will interface 10 different sub-detectors and reduce the total data throughput from 3.5 TB/s to 635 GB/s. The ALICE common firmware framework supports data taking in continuous and triggered mode and forwards clock, trigger and slow control to the…
ALICE will significantly increase its Pb--Pb data taking rate from the 1\,kHz of triggered readout in Run 2 to 50 kHz of continuous readout for LHC Run 3. Updated tracking detectors are installed for Run 3 and a new two-phase computing strategy is employed. In the first synchronous phase during the data taking, the raw data is compressed for storage to an on-site disk buffer and the required data for the detector calibration is collected. In the second asynchronous phase the compressed raw data is reprocessed using the final calibration to produce the final reconstruction output. Traditional C…
The High Level Trigger (HLT) system of the ALICE experiment is an online event filter and trigger system designed for input bandwidths of up to 25 GB/s at event rates of up to 1 kHz. The system is designed as a scalable PC cluster, implementing several hundred nodes. The transport of data in the system is handled by an object-oriented data flow framework operating on the basis of the publisher-subscriber principle, being designed fully pipelined with lowest processing overhead and communication latency in the cluster. In this paper, we report the latest measurements where this framework has be…
The ALICE High Level Trigger has to process data online, in order to select interesting (sub)events, or to compress data efficiently by modeling techniques. Focusing on the main data source, the Time Projection Chamber (TPC), we present two pattern recognition methods under investigation: a sequential approach (cluster finder and track follower) and an iterative approach (track candidate finder and cluster deconvoluter). We show, that the former is suited for pp and low multiplicity PbPb collisions, whereas the latter might be applicable for high multiplicity PbPb collisions of dN/dy>3000. Bas…
The ALICE High Level Trigger has to process data online, in order to select interesting (sub)events, or to compress data efficiently by modeling techniques.Focusing on the main data source, the Time Projection Chamber (TPC), we present two pattern recognition methods under investigation: a sequential approach "cluster finder" and "track follower") and an iterative approach ("track candidate finder" and "cluster deconvoluter"). We show, that the former is suited for pp and low multiplicity PbPb collisions, whereas the latter might be applicable for high multiplicity PbPb collisions, if it turns…
The ALICE experiment at the LHC is equipped with an electromagnetic calorimeter (EMCal) designed to enhance its capabilities for jet, photon and electron measurement. In addition, the EMCal enables triggering on jets and photons with a centrality dependent energy threshold. After its commissioning in 2010, the EMCal Level 1 (L1) trigger was officially approved for physics data taking in 2011. After describing the L1 hardware and trigger algorithms, the commissioning and the first year of running experience, both in proton and heavy ion beams, are reviewed. Additionally, the upgrades to the ori…
The Muon Spectrometer (MS) of the ALICE experiment at LHC is equipped with a HLT (High Level Trigger), whose aim is to improve the accuracy of the trigger cuts delivered at the L0 stage. A computational challenge of real-time event reconstruction is satisfied to achieve this software trigger cut of the HLT. After the description of the online algorithms, the performance of the online tracker is compared with that of the offline tracker using the measured pp collisions at $\sqrt{s}=7$ TeV.
Silicon Drift Detectors (SDD) equip the two central layers of the Inner Tracking System of the ALICE experiment at the LHC. Main results of systematic studies of detector performance including noise, gain, drift speed and charge collection measurements will be reported
The ALICE detector at the Large Hadron Collider (LHC) consists of a central barrel, a muon spectrometer, zero degree calorimeters and additional detectors which are used for trigger purposes and for event classification. The main detector systems of relevance for measuring diffractive and electromagnetic processes are described. The trigger strategy for such measurements is outlined. The physics potential of studying diffractive and electromagnetic processes at the LHC is presented by discussing possible signatures of the Odderon.
The ALICE experiment at the LHC is equipped with an electromagnetic calorimeter (EMCal) designed to enhance its capabilities for jet measurement. In addition, the EMCal enables triggering on high energy jets. Based on the previous development made for the Photon Spectrometer (PHOS) level-0 trigger, a specific electronic upgrade was designed in order to allow fast triggering on high energy jets (level-1). This development was made possible by using the latest generation of FPGAs which can deal with the instantaneous incoming data rate of 26 Gbit/s and process it in less than 4 μs.
A dedicated front-end electronics has been developed for the trigger chambers of the ALICE muon spectrometer which will work at the future LHC at CERN. These trigger chambers are based on RPCs (Resistive Plate Chambers) working in streamer mode. The number of electronics channels (about 21000) and the fact that RPC pulsed signals have specific characteristics have led to design a 8 channels front-end ASIC using a new discrimination technique. The principle of the ASIC is described and the radiation hardness is discussed. Special emphasis is put on production characteristics of about 4000 chips…
The ALICE Central Trigger Processor (CTP) is going to be upgraded for LHC Run 3 with completely new hardware and a new Trigger and Timing Control (TTC-PON) system based on a Passive Optical Network (PON) system. The new trigger system has been designed as dead time free and able to transmit trigger data at 9.6 Gbps. A new universal trigger board has been designed, where by changing the FMC card, it can function as a CTP or as a LTU. It is based on the Xilinx Kintex Ultrascale FPGA and upgraded TTC-PON. The new trigger system and the prototype of the trigger board will be presented.
Particle identification is an important feature of the ALICE detector at the LHC. In particular, for particle identification via the time-of-flight technique, the precise determination of the event collision time represents an important ingredient of the quality of the measurement. In this paper, the different methods used for such a measurement in ALICE by means of the T0 and the TOF detectors are reviewed. Efficiencies, resolution and the improvement of the particle identification separation power of the methods used are presented for the different LHC colliding systems (pp , p-Pb and Pb-Pb)…
The ALICE muon spectrometer studies the production of quarkonia and open heavy- flavour particles. It is equipped with a Trigger System composed of Resistive Plate Chambers which, by applying a transverse-momentum-based muon selection, minimises the background from light-hadron decays. The system has been continuously taking data throughout the LHC Run I; it has undergone maintenance and consolidation operations during the LHC shutdown period 1. In the first year of the LHC Run II, the system, fully recommissioned, has participated in data taking in pp and Pb/en-dashPb collisions. The performa…
ALICE (A Large Ion Collider Experiment) studies the transition of nuclear matter to a deconfined phase known as Quark Gluon Plasma, in ultra-relativistic heavy-ion collisions at the LHC. ALICE is equipped with a muon spectrometer for the detection of quarkonia and heavy flavour particles. The trigger system of the spectrometer consists of 72 RPCs arranged in four detection planes, with a total area of 140 m^{2}. In the first three years of LHC operation, the muon trigger system was fully operational in data-taking in pp, Pb-Pb and p-Pb collisions. The RPC performance and stability throughout t…
This document describes the plans of the ALICE Collaboration for a major upgrade of its detector, referred to as ALICE 3, which is proposed for physics data-taking in the LHC Run 5 and beyond. ALICE 3 will enable an extensive programme to fully exploit the LHC for the study of the properties of strongly interacting matter with high-energy nuclear collisions. The proposed detector layout, based on advanced silicon sensors, features superb pointing resolution, excellent tracking and particle identification over a large acceptance and high readout-rate capabilities. This document discusses the pr…
During the upcoming Run 3 and Run 4 at the LHC the upgraded ALICE (A Large Ion Collider Experiment) will operate at a significantly higher luminosity and will collect two orders of magnitude more events than in Run 1 and Run 2. A part of the ALICE upgrade is the new Fast Interaction Trigger (FIT). This thoroughly redesigned detector combines, in one system, the functionality of the four forward detectors used by ALICE during the LHC Run 2: T0, V0, FMD and AD. The FIT will monitor luminosity and background, provide feedback to the LHC, and generate minimum bias, vertex and centrality triggers, …
This article documents the main design choices and the close to 20 years of preparation, detector R&D, construction and installation of ALICE, the dedicated heavy ion experiment at the CERN LHC accelerator.
The Silicon Strip Detector (SSD) is a fundamental part of the Inner Tracking System (ITS) for the ALICE experiment. Since the early phase of proton-proton collisions at LHC, the SSD is fully operational and participating in the charged particle detection and identification carried out by ALICE. The performance of the SSD during the 900 GeV and 7 TeV collision data taking is presented here. The stability of the system is monitored through the time evolution of its calibration parameters and their correlation with the environmental conditions. The intrinsic noise of the 2.6 million channels comp…
The ALICE (A Large Ion Collider Experiment) detector yields a huge sample of data from different sub-detectors. On-line data processing is applied to select and reduce the volume of the stored data. ALICE applies a multi-level hardware trigger scheme where fast detectors are used to feed a three-level (L0, L1, and L2) deep chain. The High-Level Trigger (HLT) is a fourth filtering stage sitting logically between the L2 trigger and the data acquisition event building. The EMCal detector comprises a large area electromagnetic calorimeter that extends the momentum measurement of photons and neutra…
ALICE is the CERN LHC experiment optimised for the study of the strongly interacting matter produced in heavy-ion collisions and devoted to the characterisation of the quark-gluon plasma. To achieve the physics program for LHC Run 3, a major upgrade of the experimental apparatus is ongoing. A key element of the upgrade is the substitution of the Inner Tracking System (ITS) with a completely new silicon-based detector whose features will allow the reconstruction of rare physics channels, not accessible with the previous layout. The enabling technology for such a performance boost is the adoptio…
The current conceptual design of proposed Large EMCal of ALICE at LHC is based largely on the scintillating mega-tile/fiber technology implemented in CDF Endplug upgrade project and in both barrel and endcap electromagnetic calorimeters of the STAR. The cost of scintillating material leads us to the choice of extruded polystyrene based scintillator, which is available in new FNAL-NICADD facility. Result of optical measurements, such as light yield and light yield variation, show that it is possible to use this material as active media of Large EMCal of ALICE at LHC.
ALICE (A Large Ion Collider Experiment) experiment at CERN LHC is designed to study the properties of the Quark-Gluon Plasma (QGP) in heavy-ion collisions. In 2019-2020 the upgrade of CERN LHC will increase the luminosity and the collision rate beyond the design parameters of the current ALICE setup. To be able to benefit from the improved performance of the LHC, ALICE will upgrade several of its key detector systems including the Fast Interaction Trigger (FIT) [2]. FIT is designed to provide the functionality of the old forward detectors while retaining or even improving their performance. It…
A major upgrade of the ALICE experiment is in progress and will result in high-rate data taking during LHC Run 3 (2022-2024). The LHC interaction rate at Point 2 where the ALICE experiment is located will be increased to $50\ \mathrm{kHz}$ in Pb--Pb collisions and $1\ \mathrm{MHz}$ in pp collisions. The ALICE experiment will be able to read out data at these interaction rates leading to an increase of the collected luminosity by a factor of up to about 100 with respect to LHC Runs 1 and 2. To satisfy these requirements, a new readout system has been developed for most of the ALICE detectors, a…
ALICE is the CERN LHC experiment optimised for the study of the strongly interacting matter produced in heavy-ion collisions and devoted to the characterisation of the quark-gluon plasma. To achieve the physics program for LHC Run 3, a major upgrade of the experimental apparatus is ongoing. A key element of the upgrade is the substitution of the Inner Tracking System (ITS) with a completely new silicon-based detector (ITS 2) whose features will allow the reconstruction of rare physics channels not accessible with the previous layout. The enabling technology for such a performance boost is the …
The ALICE Muon IDentifier is composed of 72 single-gap bakelite Resistive Plate Chambers, which have been operational since 2009 in maxi-avalanche mode (discrimination threshold:7 mV without amplification) with a Tetrafluoroethane/Isobutane/Sulfur Hexafluoride gas mixture, undergoing counting rates of the order of tens of Hz/cm^2. In this talk, the long-term performance and stability of the RPC system will be discussed, in terms of efficiency, dark current and dark rate. An assessment of potential signs of ageing observed on the detectors will be presented, together with a summary of the most …
We discuss how a level-1 trigger, about 8 us after a hadron-hadron collision, can be derived from the Transition Radiation Detector (TRD) in A Large Ion Collider Experiment (ALICE) at the LHC. Chamber-wise track segments from fast on-detector reconstruction are read out with position, angle and electron likelihood. In the Global Tracking Unit up to 6 tracklets from a particle traversing the detector layers are matched and used for the reconstruction of transverse momentum and electron identification. Such tracks form the basis for versatile and flexible trigger conditions, e.g. single high-pt …
After close to 20 years of preparation, the dedicated heavy ion experiment ALICE took first data with proton collisions at the LHC at the end of 2009. This article recalls the main design choices made for the detector and summarizes initial operation and performance of ALICE at the LHC; first physics results are covered elsewhere in these proceedings.
The ALICE HMPID (High Momentum Particle IDentification) detector has been designed to identify charged pions and kaons in the range 1 < p < 3 GeV/c and protons in the range 1.5 < p < 5 GeV/c. It consists of seven identical proximity focusing RICH (Ring Imaging Cherenkov) counters, covering in total 11 m2, which exploit large area MWPC equipped with CsI photocathodes for Cherenkov light imaging emitted in a liquid C6F14 radiator. The ALICE detector has been widely commissioned using cosmics and LHC beam from December 2007 until October 2008. During the cosmics data taking the HMPID detector col…
Measurements of the cross sections of the reference processes seen by the ALICE trigger system were obtained based on beam properties measured from van der Meer scans. The measurements are essential for absolute cross section determinations of physics processes. The paper focuses on instrumental and technical aspects of detectors and accelerators, including a description of the extraction of beam properties from the van der Meer scan. As a result, cross sections of reference processes seen by the ALICE trigger system are given for proton-proton collisions at two energies; $\sqrt{s}$=2.76 TeV a…
Cross sections of reference trigger processes were obtained based on beam property measurements in dedicated luminosity calibration experiments (van der Meer scans). These cross-sections are essential for absolute cross section determinations of physics processes. The reference cross sections are presented for pp collisions at two center of mass energies; 2.76 TeV and 7 TeV, and for Pb-Pb collisions at 2.76 TeV together with a discussion of the systematic uncertainty originating from beam intensity and rate measurement uncertainties.
