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. [...]Comment: 8 pages, 10 figures, proceedings to 2016 IEEE-NPSS Real Time Conferenc

Implementation of a Custom Muon High Level Trigger Monitoring System

A Large Ion Collider Experiment (ALICE) is one of the 4 major experiments at the Large Hadron Collider (LHC) at CERN. Its main aim is to investigate the physics of strongly interacting matter in proton-proton, nucleus-nucleus and nucleus-proton or proton-nucleus collisions at ultra high energy densi- ties, where the Quark Gluon Plasma (QGP) is expected to form. The experiment is expected to produce data at very high rates of about 25 Gbytes/s however the bandwidth to permanent storage is limited to about 10% (1.25 Gbyte/s) of the total expected data rates. In order to reduce data to permanent storage a special level of selecting interesting/relevant physics events is required. In ALICE the trigger (selection) of signals is issued based on a series of levels varying from levels 0 (L0) up to the High Level Trigger (HLT). For the ALICE muon spectrometer, the role of the trigger is to select events containing muon tracks, with the transverse momentum (pt) above a given threshold. Due to the limited spatial resolution of the muon trigger chambers a pt cut above a few GeV with the L0 trigger is not possible. While the L0 signal for the muon spectrometer is issued at about 700 - 800 ns, the HLT is delivered at about 1 ms. The role of the HLT is to perform online and o ine reconstruction of the ALICE muon spectrometer data in order to improve the measured (L0) pT resolution. In this way a better separation between relevant physics events and unwanted events (background) can be attainable, which could eventu- ally lead to lower trigger rates. The HLT is designed to improve signal- to-background ratio in the raw data transferred to the storage. In order to facilitate online/o ine data analysis the HLT monitoring system which will enable the user to graphically view the events during the reconstruc- tion phase was developed in this study. The system will read and decode the reconstructed events from the HLT analysis chain using the HLT Online Monitoring Environment including ROOT (HOMER) and displaying them on the ALICE Event Visualization Environment (ALIEVE). In addition, the ii utility, dHLTdumpraw, that inspects, with ner detail, the contents of all muon HLT internal data blocks and the detector data link (DDL) raw data stream is also described

The ALICE detector and trigger strategy for diffractive and electromagnetic processe

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.Comment: 6 pages, 8 figures, Proceedings workshop on "High energy photon collisions at the LHC", CERN, apr 22-25, 200

Real-time data analysis at the LHC: present and future

The Large Hadron Collider (LHC), which collides protons at an energy of 14 TeV, produces hundreds of exabytes of data per year, making it one of the largest sources of data in the world today. At present it is not possible to even transfer most of this data from the four main particle detectors at the LHC to "offline" data facilities, much less to permanently store it for future processing. For this reason the LHC detectors are equipped with real-time analysis systems, called triggers, which process this volume of data and select the most interesting proton-proton collisions. The LHC experiment triggers reduce the data produced by the LHC by between 1/1000 and 1/100000, to tens of petabytes per year, allowing its economical storage and further analysis. The bulk of the data-reduction is performed by custom electronics which ignores most of the data in its decision making, and is therefore unable to exploit the most powerful known data analysis strategies. I cover the present status of real-time data analysis at the LHC, before explaining why the future upgrades of the LHC experiments will increase the volume of data which can be sent off the detector and into off-the-shelf data processing facilities (such as CPU or GPU farms) to tens of exabytes per year. This development will simultaneously enable a vast expansion of the physics programme of the LHC's detectors, and make it mandatory to develop and implement a new generation of real-time multivariate analysis tools in order to fully exploit this new potential of the LHC. I explain what work is ongoing in this direction and motivate why more effort is needed in the coming years.Comment: Contribution to the proceedings of the HEPML workshop NIPS 2014. 20 pages, 5 figure

Commissioning and Prospects for Early Physics with ALICE

The ALICE detector has been commissioned and is ready for taking data at the Large Hadron Collider. The first proton-proton collisions are expected in 2009. This contribution describes the current status of the detector, the results of the commissioning phase and its capabilities to contribute to the understanding of both pp and PbPb collisionsComment: 8 pages, 7 figures, To appear in the proceedings for Quark Matter 2009, March 30 - April 4, Knoxville, Tennesse

Data processing and online reconstruction

In the upcoming upgrades for Run 3 and 4, the LHC will significantly increase Pb--Pb and pp interaction rates. This goes along with upgrades of all experiments, ALICE, ATLAS, CMS, and LHCb, related to both the detectors and the computing. The online processing farms must employ faster, more efficient reconstruction algorithms to cope with the increased data rates, and data compression factors must increase to fit the data in the affordable capacity for permanent storage. Due to different operating conditions and aims, the experiments follow different approaches, but there are several common trends like more extensive online computing and the adoption of hardware accelerators. This paper gives an overview and compares the data processing approaches and the online computing farms of the LHC experiments today in Run 2 and for the upcoming LHC Run 3 and 4.Comment: 6 pages, 0 figures, contribution to LHCP2018 conferenc

ALICE diffractive physics in p-p and Pb-Pb collisions at the LHC

The ALICE detector is introduced and a double gap trigger is presented. The interest in studying double pomeron induced events both in proton-proton and in lead-lead reactions is discussed.Comment: 5 pages, 5 figures, to appear in proceedings conference Diffraction 2008, La Londe-les-Maures, France, sep 9-14, 200

The Transition Radiation Detector for ALICE at LHC

The Transition Radiation Detector (TRD) for the ALICE experiment at the Large Hadron Collider (LHC) identifies electrons in p+p and in the challenging high multiplicity environment of heavy-ion collisions and provides fast online tracking for the ALICE Level1 trigger. The TRD is designed to have excellent position resolution and pion rejection capability. Presently, six of the 18 TRD supermodules are installed in the ALICE central barrel. In 2008, four supermodules were installed and commissioning of the detector using cosmic ray tracks was successfully performed. We briefly describe the design of the detector and report on the performance and current understanding of the detector based on these data.Comment: 4 pages, 6 figures - To appear in the conference proceedings for Quark Matter 2009, March 30 - April 4, Knoxville, Tennesse