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The ATLAS First Level Calorimeter Trigger
L1Calo Collaboration The ATLAS First Level Calorimeter Trigger, Steve Hillier
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The ATLAS First Level Calorimeter Trigger, Steve Hillier
Outline LHC and ATLAS Trigger and L1Calo architecture L1Calo subsystems: design and algorithms Preprocessor system Processor systems L1Calo performance Pile-up in 2012 Future prospects The ATLAS First Level Calorimeter Trigger, Steve Hillier
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The ATLAS First Level Calorimeter Trigger, Steve Hillier
LHC and ATLAS The ATLAS First Level Calorimeter Trigger, Steve Hillier
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The Large Hadron Collider
pp collisions, √s up to 14 TeV * Bunch spacing: 25ns * Nominal luminosity: cm-2s-1 Collisions per crossing: ~30 ** The trigger challenge for the ‘General Purpose Detectors’: Roughly 1 GHz known physics Large event sizes: O(Mbytes) Typically small rate of ‘new’ physics channels * Eventually ** Now! The ATLAS First Level Calorimeter Trigger, Steve Hillier
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The ATLAS First Level Calorimeter Trigger, Steve Hillier
The ATLAS detector Muon spectrometers Monitored Drift Tubes (MDT) Cathode Strip Chambers (CSC) Resistive Plate Chambers (RPC) Thin Gap Chambers (TGC) 8 toroïdal magnets Calorimeters Electromagnetic Hadronic Solénoïd 2T Inner detectors Pixel Semi-Conductor Tracker (SCT) Transition Radiation Tracker (TRT) Specifications Length: 44 m, Diameter: 22 m weight: 7000 t Interaction point The ATLAS First Level Calorimeter Trigger, Steve Hillier
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Trigger and L1Calo Architecture
The ATLAS First Level Calorimeter Trigger, Steve Hillier
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The ATLAS First Level Calorimeter Trigger, Steve Hillier
ATLAS Calorimeters Liquid Argon Calorimeter (LArg) Accordion shaped with lead/copper absorbers Forms all EM layers plus Hadronic end-cap Hadronic Tile Calorimeter Scintillating Tiles with steel absorbers Forms barrel part of Hadronic layer Physically all of outer layer The ATLAS First Level Calorimeter Trigger, Steve Hillier
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Formation of Trigger Towers
LAr Tiles (semi-projective segmentation ) 0.1x0.2 3.1 <||< 3.2 0.4x0.4125 3.2 <||< 4.9 0.2x0.2 2.5 <||< 3.1 0.1x0.1 ||< 2.5 x Position Analogue summation of calorimeter cells
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The ATLAS First Level Calorimeter Trigger, Steve Hillier
Triggering in ATLAS Calorimeters Muon Detectors Three-stage triggering system Level-1: custom-built hardware, fixed latency – target rate 75 kHz Level-2: mostly software, RoI-based selection – target rate 5000 Hz Event Filter: software, full detector – target rate 400 Hz All data buffered at bunch-crossing rate of 40 MHz for 2.5 ms Level-1 has three sub-systems: Calorimeter Trigger Muon Trigger Central Trigger (CTP) Calorimeter Trigger Muon Trigger e/γ tau jet ET ΣET μ Central Trigger Processor Level-1 Trigger Trigger to Front-end Buffers Regions of Interest (RoI) to Level-2 The ATLAS First Level Calorimeter Trigger, Steve Hillier
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Calorimeter Trigger Architecture
Digitized Energies Cluster Processor 56 modules Merging 8 modules Features: Real-time Path: Fixed Latency (~1μs) Many processing stages Massive parallelism Multi-purpose modules Heavily FPGA based Analogue Calorimeter signals (>7000) Merged Results To CTP Readout Driver (ROD) 14 modules Data Region of Interest ROD 6 modules Interest Data Preprocessor 124 modules Jet/Energy Processor 32 modules Merging 4 modules Real-time Data Path Five Main Types of Custom 9U Modules PPM CPM JEM CMM ROD The ATLAS First Level Calorimeter Trigger, Steve Hillier
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The ATLAS First Level Calorimeter Trigger, Steve Hillier
USA15 Installation The ATLAS First Level Calorimeter Trigger, Steve Hillier
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The ATLAS First Level Calorimeter Trigger, Steve Hillier
L1Calo Subsystems Preprocessor The ATLAS First Level Calorimeter Trigger, Steve Hillier
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The ATLAS First Level Calorimeter Trigger, Steve Hillier
Pre-Processor System Digitization 40 MHz, 10-bit flash-ADC 0.25 GeV/count Timing adjustable at nanosecond level PPr a0 a1 a2 a3 a4 + FIR filter Bunch-crossing Identification Applies Finite Impulse Response (FIR) filter Then peak-finding criteria Assigns energy to correct bunch-crossing Independent logic for saturated pulses 10 bit Pedestal Ethres 8 bit Look-up table Pedestal subtraction Noise suppression Final energy calibration (to 1 GeV/count) 8 bit output transmitted to processors The ATLAS First Level Calorimeter Trigger, Steve Hillier
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Preprocessor hardware
1 ASIC 4 FADCs 3 LVDS Tx 124 PPM modules in all 64 towers per PPM Each contain 16 MCMs The ASIC is the ‘heart’ of the MCM MCMs Input Conditioning Readout and Control The ATLAS First Level Calorimeter Trigger, Steve Hillier
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Installation: Analogue Cables
496 cables into 8 crates Four cables just fit front of one 9U module The ATLAS First Level Calorimeter Trigger, Steve Hillier
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Timing of incoming signals
Timing variation of input channels wrt beam collision is up to +/- 200ns Due mostly to cable lengths For the trigger to work, must line these up precisely For BCID to work at low energies requires nano-second precision Timing originally estimated using pulse shapes and calibration data Comparison of ‘pulse-fitted’ timing with known position in timing scan Final timing established with signals from colliding beam Gauss Landau The ATLAS First Level Calorimeter Trigger, Steve Hillier
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The ATLAS First Level Calorimeter Trigger, Steve Hillier
Timing status in 2011 Offset from ideal timing derived by fit to pulse shape seen in beam data At the beginning of 2011, most channels already timed at +/- 2ns level Remaining large differences due to hardware repaired during 2010/2011 shutdown Corrections applied in April 2011, and small adjustments ever since Timing in 2012 close to perfect from the start The ATLAS First Level Calorimeter Trigger, Steve Hillier
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FIR Filter Calibration
Initial FIR Filters derived from calibration data But pulse shapes differ in real pulse in collision data Therefore determine normalized pulse shape per tower during collisions Separate into regions of similar pulse shape via a simple ‘pulse width’ parameter Calculate Optimal Filter Coefficients for each region (making maximal use of LUT range) To get final Energy calibration, measure Look-Up Table slope from collision data S1 + S3 S1 S2 S3 Identified regions in EM layer: , , , , , The ATLAS First Level Calorimeter Trigger, Steve Hillier
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Bunch-Crossing Identification (BCID) Efficiency
The toughest test of filter is the reliability of identifying small signals The turn-on observed are in line with our best expectations Note better performance at higher eta Electronics noise ~constant in E but suppressed by sin(θ) factor in Et conversion The ATLAS First Level Calorimeter Trigger, Steve Hillier
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The ATLAS First Level Calorimeter Trigger, Steve Hillier
Energy correlation L1Calo PPM tower ET vs calorimeter precision readout Excellent correlation measured with collision data Requires constant attention/re-calibration to react to detector changes (HV, masked cells) The ATLAS First Level Calorimeter Trigger, Steve Hillier
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Processor Architecture
L1Calo Subsystems Processor Architecture The ATLAS First Level Calorimeter Trigger, Steve Hillier
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‘Sliding’ Trigger Algorithms
Processor input is a matrix of tower energies (up to 50x64) Trigger algorithms use 4x4 grid Sliding by 1 tower in each direction To process each location, an outer environment is required The ATLAS First Level Calorimeter Trigger, Steve Hillier
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Jet/Energy Module (4x8) 32:45
Core and Environment Jet/Energy Module Core Each processor has a core of towers to be processed ‘Processor’ could be crate, module or even individual chip Requires extra ‘environment’ input Achieved by fan-out Ratio of core:environment dependant on size Smaller (more parallel) system requires more fan-out Sub-dividing makes connectivity more difficult Algorithm Environment 32 Core cells 45 Environment cells Jet/Energy Module (4x8) 32:45 Cluster Module (4x16) 64:69 The ATLAS First Level Calorimeter Trigger, Steve Hillier
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Solution and implications
Entirely Parallel Preprocessor Size governed by input cabling Eight 9U VME crates High bandwidth digital cabling ‘spaghetti’ to: Parallel Processor Four 9U VME crates for e/gamma trigger Two 9U VME crates for jet/energy trigger Necessary fan-out performed via: Digital cables to processors (~30%) Custom backplane in processor (~75%) Preprocessor crate High speed digital links 480 Mbit/s Processor crate The ATLAS First Level Calorimeter Trigger, Steve Hillier
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Processor custom Backplane
Dense, high bandwidth backplane Up to 1,150 pins per slot About 20,000 pins in all Common to CP and JEP systems Fastest signal speeds: 480 MHz differential (LVDS input) 160 MHz single ended CP system 80 MHz single ended JEP system The ATLAS First Level Calorimeter Trigger, Steve Hillier
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Installation: Digital Cabling
Up to 1400 individual LVDS signals into one crate More than 500 Gbit/s data input The ATLAS First Level Calorimeter Trigger, Steve Hillier
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Trigger Algorithms Jet algorithm e/γ or τ/hadron algorithm
Jet/Energy-sum Processor Cluster Processor ECAL+HCAL Jet algorithm Programmable size Energy in (em+had) > threshold 8 size/threshold sets 8 Missing-ET, 8 Sum ET plus 8 Missing-ET significance thresholds e/γ or τ/hadron algorithm Central cluster > threshold Isolation requirements in surrounding rings Local ET maximum 16 thresholds possible The ATLAS First Level Calorimeter Trigger, Steve Hillier
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Hardware Implementation
Multiple ‘layers’ of FPGA processing Data reception and fanout Algorithmic processing Result merging Final stages in common CMM The ATLAS First Level Calorimeter Trigger, Steve Hillier
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The ATLAS First Level Calorimeter Trigger, Steve Hillier
Our favourite plot Any digital error is seen here Very rarely any entry This is from a run with >200 pb-1 No errors in 20 million events in major physics streams The ATLAS First Level Calorimeter Trigger, Steve Hillier
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L1Calo Performance and Pile-Up
Selected highlights L1Calo contributes to all but muon triggers! The ATLAS First Level Calorimeter Trigger, Steve Hillier
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L1Calo Rates: some facts of life
A trigger is always a balance between rates and efficiencies Physics groups want more data, lower thresholds Detectors can only handle 75 kHz Level-1 Accept Rate From 2009 to early 2011 we could afford a rather loose trigger This all changed during 2011 as LHC really started to deliver In several cases Level-1 is the bottleneck Problems come from both linear scaling with luminosity and non-linear scaling with pile-up But we do have some tricks up our sleeves Hadronic veto for electrons (introduced in 2011) Isolation for electrons (not yet required at Level-1) Noise cuts at various levels to reduce effects of pile-up The ATLAS First Level Calorimeter Trigger, Steve Hillier
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L1Calo Rates: the Good, the Bad and Missing Energy
Well behaved triggers scale with luminosity Also useful to study trigger ‘cross-section’ as function of pile-up factor <μ> Triggers affected by pile-up very dependent on LHC bunch structure Typically missing energy, forward jets, multi-jets The ATLAS First Level Calorimeter Trigger, Steve Hillier
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Efficiencies: electron triggers
Early 2011 main single electron trigger: e20 seeded by L1_EM14 During 2012 we use EF_em25isolated, seeded by L1_EM18VH The ATLAS First Level Calorimeter Trigger, Steve Hillier
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Efficiencies: tau triggers
2011 HLT cuts Tau triggers have ‘softer’ turn-ons HLT heavily tuned to offline selection Pile-up in 2011 caused inefficiency in HLT, but not Level-1 Cuts tuned for higher pile-up in 2012 2012 HLT cuts The ATLAS First Level Calorimeter Trigger, Steve Hillier
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Efficiencies: jet triggers
Jet triggers (at all levels) on EM scale in 2011 Absolute value of threshold is not important, only the turn-on behaviour is significant Lowest un-prescaled threshold L1_J75 Lower thresholds down to L1_J10 used for multi-jet triggers: 4J10, 5J10 In 2012, multijets moved to L1_J15 The ATLAS First Level Calorimeter Trigger, Steve Hillier
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Missing Energy: the Pile-Up effect
Missing Energy (and Sum Energy) affected by pile-up and signal shaping Typical Calorimeter signal has about 125ns positive, long negative tail LHC bunch separation is 25 ns (50 ns in 2011 and 2012) L1Calo therefore experiences pedestal shifts due to unbalanced overlaying of signals at the start of the train Also increased noise RMS in bulk of train FCAL and high-eta regions strongly affected More small minimum bias energy deposits The ATLAS First Level Calorimeter Trigger, Steve Hillier
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FCAL pile-up noise and cuts
Noise RMS increases with pile-up <μ> Measured here in ‘ZeroBias’ collisions Typical electronics noise of MeV For most of FCAL pile-up noise now dominates (> 1 GeV) Consequently increase noise cuts Numbers shown here from 2011 analysis Cuts now optimised for <μ> = 25 Compared to original noise cuts: Trigger rates reduced enormously Efficiencies essentially unaffected The ATLAS First Level Calorimeter Trigger, Steve Hillier
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Missing Energy Performance
Noise cuts control Missing Energy rates Almost linear with luminosity except at very beginning of fill Level-1 thresholds now lower than in 2011 Also new Level-2 Missing Energy algorithm Overall ATLAS Missing Energy trigger in 2012 is better than in 2011 In spite of the tougher pile-up conditions The ATLAS First Level Calorimeter Trigger, Steve Hillier
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Upgrade could fill another talk…
Future Prospects Upgrade could fill another talk… The ATLAS First Level Calorimeter Trigger, Steve Hillier
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L1Calo, the next ten years
Long Shutdown 1 (2013/14) LHC goes to 13+ TeV, luminosity to 2x1034 MCM becomes nMCM Add Topology Trigger Long Shutdown 2 (2018) LHC luminosity goes to 3-4x1034 New digital high granularity trigger towers (super-cells) Digital eFEX and jFEX run initially in parallel with legacy-L1Calo Long Shutdown 3 (2022) Level-1 split into two stage trigger (L0 and L1) eFEX and jFEX fully populated as L0Calo New L1Calo fed from full detector readout information The ATLAS First Level Calorimeter Trigger, Steve Hillier
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L1Calo Upgrade in a Nutshell
New parallel digital trigger towers: approximately factor of 10 more data Some legacy signals fed to new trigger processor system via new daughterboards nMCM with FPGA: better noise, more flexible filters and energy calibration CMM->CMX allows extra thresholds and addition of Topology Triggers Updates in Muon and Central Trigger Processors also planned The ATLAS First Level Calorimeter Trigger, Steve Hillier
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Phase 1: High Granularity Trigger Towers
In fact 1/4/4/1 is now favoured High-granularity trigger towers digitized on detector Processed through new real-time path: DPS to eFEX and jFEX Extra eta granularity improves electron/jet disambiguation Finer isolation parameters should give extra factor 3 jet rejection The ATLAS First Level Calorimeter Trigger, Steve Hillier
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The ATLAS First Level Calorimeter Trigger, Steve Hillier
Conclusions L1Calo is doing a vital job for ATLAS And it’s working very well! Operation requires constant vigilance Reacting to changing detector conditions Optimising algorithms in the face of challenging demands from LHC and physics analyses Upgrade is becoming increasingly important The ATLAS First Level Calorimeter Trigger, Steve Hillier
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