ALICE First paper Size 16 x 26 meters

ALICE – First paper

Size: 16 x 26 meters Weight: 10, 000 tons TOF TRD HMPID ITS PMD Muon Arm PHOS ALICE Set-up TPC 2

ALICE TPC • • • Large volume gas detector Drift volume and MPWC at the end caps 3 -dim. “continuous” tracking device for charged particles • • • x, y of pad z derived from drift time Designed to record up to 20000 tracks Event rate: about 1 k. Hz Typical event size for a central Pb+Pb collision: about 75 MByte 3 3

ALICE TPC: 5 years of construction 4

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Trigger system • Minimal requirements tr ig g er de te ct or • Detect collisions • Initialise readout of detectors • Initialise data transfer to data acquisition (DAQ) • Protection against pile-up trigger system • Select interesting events • Needs real-time processing of raw data and extraction of physics observables Detectors Readout electronics raw data DAQ • High level requirements high-level trigger processed data Why? interaction rate (e. g. 8 k. Hz for Pb+Pb) > detector readout rate (e. g. 1 k. Hz for TPC) > DAQ archiving rate (50 - 100 Hz) 6

What to trigger on? • Every central Pb+Pb collisions produces a QGP - no need for a QGP-trigger • But hard probes are (still) rare at high momentum • In addition, the reconstruction efficiency of heavy quark probes is very low • E. g. Detection of hadronic charm decays: D 0 K– + + • about 1 D 0 per event (central Pb-Pb) in ALICE acceptance • after cuts • signal/event = 0. 001 • background/event = 0. 01 trigger 7

PHOS L 0 trigger Pb. O 4 W- crystal calorimeter for photons, neutral mesons, 1 to > 100 Ge. V Array of crystals + APD + preamp + trigger logic + readout DAQ L 0 trigger • tasks • shower finder L 0/L 1 trigger • energy sum • implementation • FPGA • VHDL firmware 8 8

PHOS – muon tracks 9

D 0 trigger • Detection of hadronic charm decays: D 0 K– + + (6. 75%), c = 124 m • HLT code • TPC tracker TPC+ITS track fitter displaced decay vertex finder ITS TPC • D 0 finder: cut on d 0(K)*d 0( ) Preliminary result: invariant mass resolution is within a factor of two compared to offline 10

Introducing the High Level Trigger ALICE data rates (example TPC) ● Central Pb+Pb collisions ● event rates: ~200 Hz (past/future protected) event sizes: (after zero-suppression) ● data rates: ● ~75 Mbyte TPC is the largest data source with 570132 channels, 512 timebins and 10 bit ADC value. ~ 15 Gbyte/sec TPC data rate alone exceeds by far the total DAQ bandwidth of 1. 25 Gbyte/sec HLT tasks • Event selection based on software trigger • Efficient data compression 11

HLT requirements • Full event reconstruction in real-time • Main task: reconstruction of up to 10000 charged particle trajectories • Method: Pattern recognition in the TPC • Cluster finder • Track fit Global track fit ITS-TPC-TRD Vertex finder • Event analysis • Trigger decision 12

HLT architecture • HLT is a generic high performance cluster Detectors HLT DAQ Mass storage

HLT building blocks (1) • Hardware • Nodes • Sufficient computing power for p+p • 121 Front-End PCs: 968 CPU cores, 1. 935 TB RAM, equipped with custom PCI card for receiving detector data • 51 Computing PCs: 408 CPU cores, 1. 104 TB RAM • Network • Infiniband backbone, Giga. Bit ethernet • Infrastructure • 20 redundant servers for all critical systems

HLT building blocks (2) • Software • Cluster management and monitoring • Data transport and process synchronisation framework • Interfaces to online systems: Experiement control system, Detector control system, Offline DB, . . . • Event reconstruction and trigger applications

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Planning pp run • November 200 collision @ 900 Ge. V • December 106 collisions @ 900 Ge. V • Some collisions @ 2. 4 Te. V • February-> collisions @ 7 Te. V 21