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Figure 1: One of the first heavy ion events seen in ALICE and processed and visualized in the HLT
Figure 2: The ALICE HLT Cluster at CERN
Figure 3: The first on-line reconstructed vertex position by the ALICE HLT, showing counter clockwise from bottom left the distributions in x, y, z and the x-y plane.
Figure 4: The first on-line proton-proton collision in the full ALICE detector, on-line reconstructed by the HLT
Figure 5: The first on-line calculated invariant mass distribution of K0s mesons by the ALICE HLT.

The ALICE High-Level Trigger

Introduction

The High-Level Trigger (HLT) [1] of the ALICE experiment (A Large Ion Collider Experiment) [2] at the LHC, CERN is designed to reduce the readout data from 25 GB/s to 1.25 GB/s. A large high performance PC cluster (see Fig.2) has been built to reconstruct and analyse the collision data on-line and allow advanced trigger algorithms to decide on-line if a given event should be rejected or permanently stored. Its main physics goals are

  • Trigger Accept or reject events on the basis of on-line reconstruction and analysis.
  • Select Select physics region of interest within the event
  • Compress Minimize the event size without loss of physics information.

The reconstruction is organized in several hierarchical levels, according to the detector readout structure. First-Level reconstruction is directly performed on Front-End-Processor nodes, hosting the data receiving PCI-X cards (HRORCs), or in the FPGAs of the HRORCs itself. The next reconstruction levels, the event-by-event analysis and the triggering itself is processed on the hundreds of Computing Nodes following a layered data flow. Finally the HLT trigger decision is sent back to the Data Acquisition (DAQ) accompanied by the reconstructed event in the form of a off-line Event-Summary-Data structure (ESD).

First Collisions at LHC

On November 23rd, 2009 450 GeV protons where collided in inside the ALICE detector for the first time. The High-Level Trigger was running on-line recon-struction algorithms for Inner Tracking System (ITS). In order to get a fast estimate of the beam position, the vertex position was reconstructed on-line from data of the Silicon Pixel Detector (SPD) as shown in Fig 3 [3].

When LHC achieved stable beam conditions on December 6th, 2009 the HLT was running full on-line reconstruction of the central barrel detectors and the muon arm for the first time. Fig. 4 shows an the first on-line reconstructed proton-proton collision in the HLT.

In the following ALICE pp data-taking, the HLT was not only performing full on-line reconstruction, but also finding of primary and secondary vertices. Invariant mass distributions of L0 and K0s (see Fig. 5) have been on-line calculated to improve the on-line quality assurance.

In November and December 2010 ALICE had its first running period of heavy-ion collisions. During this period the High Level Trigger processed the data from the collisions and produced quasi-live online event displays in the main ALICE control room, showing the particles passing through the experiment. Fig. 1 shows the online display of one of the first heavy-ion collisions in ALICE.

References

[1]  Technical Report ALICE TDR 10, CERN/LHCC 2003-062 (2004)
[2]  K. Aamodt et al. The ALICE Collaboration, The ALICE Experiment at the CERN LHC. JINST 3 S08002, 259pp. (2008)
[3]  K. Aamodt et al. The ALICE Collaboration, First proton--proton collisions at the LHC as observed with the ALICE detector: measurement of the charged particle pseudorapidity density at sqrt(s) = 900 GeV. Eur. Phys. J. C 65, 111-125 (2010)

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