the atlas first level calorimeter...
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The ATLAS First Level Calorimeter Trigger, Steve Hillier
The ATLAS First Level Calorimeter Trigger
L1Calo Collaboration
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
LHC and ATLAS
The ATLAS First Level Calorimeter Trigger, Steve Hillier
The Large Hadron Collider
• pp collisions, √s up to 14 TeV * • Bunch spacing: 25ns * • Nominal luminosity: 1034 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
The ATLAS First Level Calorimeter Trigger, Steve Hillier
* Eventually ** Now!
Specifications Length: 44 m, Diameter: 22 m weight: 7000 t
Interaction point
The ATLAS detector
The ATLAS First Level Calorimeter Trigger, Steve Hillier
Trigger and L1Calo Architecture
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
The ATLAS First Level Calorimeter Trigger, Steve Hillier
• Hadronic Tile Calorimeter – Scintillating Tiles with steel
absorbers • Forms barrel part of Hadronic
layer – Physically all of outer layer
Analogue summation of calorimeter cells
LAr Tiles (semi-projective segmentation )
0.1x0.2 3.1 <|η|< 3.2
0.4x0.4125 3.2 <|η|< 4.9
0.2x0.2 2.5 <|η|< 3.1
0.1x0.1 |η|< 2.5
∆η x ∆ϕ Position
Formation of Trigger Towers
Triggering in ATLAS
• 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 µs
• Level-1 has three sub-systems: – Calorimeter Trigger – Muon Trigger – Central Trigger (CTP)
Calorimeter Trigger
e/γ tau jet ET
ΣET
Muon Trigger
μ
Level-1 Trigger
Central Trigger
Processor
Trigger to Front-end
Buffers
Calorimeters Muon Detectors
Regions of Interest (RoI)
to Level-2
The ATLAS First Level Calorimeter Trigger, Steve Hillier
Features: Real-time Path: Fixed
Latency (~1μs) Many processing stages Massive parallelism Multi-purpose modules Heavily FPGA based
Calorimeter Trigger Architecture
Preprocessor 124 modules
Cluster Processor
56 modules
Jet/Energy Processor
32 modules
Merging 8 modules
Merging 4 modules
Digitized Energies
Analogue Calorimeter
signals (>7000)
Merged Results To CTP
Real-time Data Path
Readout Driver (ROD) 14 modules
Readout Data
Region of Interest ROD
6 modules
Region of Interest Data
PPM CPM JEM CMM ROD Five Main Types of Custom 9U Modules
The ATLAS First Level Calorimeter Trigger, Steve Hillier
USA15 Installation
The ATLAS First Level Calorimeter Trigger, Steve Hillier
L1Calo Subsystems
Preprocessor
The ATLAS First Level Calorimeter Trigger, Steve Hillier
Pre-Processor System
The ATLAS First Level Calorimeter Trigger, Steve Hillier
• Digitization – 40 MHz, 10-bit flash-ADC – 0.25 GeV/count – Timing adjustable at nanosecond level
• Bunch-crossing Identification – Applies Finite Impulse Response (FIR) filter – Then peak-finding criteria – Assigns energy to correct bunch-crossing – Independent logic for saturated pulses
• Look-up table – Pedestal subtraction – Noise suppression – Final energy calibration (to 1 GeV/count) – 8 bit output transmitted to processors
10 bit
Pedestal
Ethres 8 bit
Preprocessor hardware
• 124 PPM modules in all • 64 towers per PPM • Each contain 16 MCMs • The ASIC is the ‘heart’ of
the MCM
4 FADCs
1 ASIC
3 LVDS Tx
Input Conditioning
MCMs Readout and Control
The ATLAS First Level Calorimeter Trigger, Steve Hillier
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
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
The ATLAS First Level Calorimeter Trigger, Steve Hillier
Gauss Landau
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
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
The ATLAS First Level Calorimeter Trigger, Steve Hillier
Identified regions in EM layer: 0.0-0.8, 0.8-1.4, 1.4-1.5, 1.5-2.5, 2.5-3.2, 3.2-4.9
S1 S2 S3
S1 + S3
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
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
L1Calo Subsystems
Processor Architecture
The ATLAS First Level Calorimeter Trigger, Steve Hillier
‘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
Core and Environment
• 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
Jet/Energy Module Core 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
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
Processor crate
High speed digital links
LVDS @ 480 Mbit/s
The ATLAS First Level Calorimeter Trigger, Steve Hillier
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
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
Trigger Algorithms
Cluster Processor Jet/Energy-sum Processor
ECAL
+HCA
L
• e/γ or τ/hadron algorithm – Central cluster > threshold – Isolation requirements in
surrounding rings – Local ET maximum – 16 thresholds possible
• 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
The ATLAS First Level Calorimeter Trigger, Steve Hillier
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
Our favourite plot
The ATLAS First Level Calorimeter Trigger, Steve Hillier
• 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
L1Calo Performance and Pile-Up
Selected highlights L1Calo contributes to all but muon triggers!
The ATLAS First Level Calorimeter Trigger, Steve Hillier
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
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>
The ATLAS First Level Calorimeter Trigger, Steve Hillier
Triggers affected by pile-up very dependent on LHC bunch structure – Typically missing energy,
forward jets, multi-jets
Efficiencies: electron triggers
• Early 2011 main single electron trigger: – e20 seeded by L1_EM14
The ATLAS First Level Calorimeter Trigger, Steve Hillier
• During 2012 we use EF_em25isolated, seeded by L1_EM18VH
Efficiencies: tau triggers
The ATLAS First Level Calorimeter Trigger, Steve Hillier
• 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
2011 HLT cuts
2012 HLT cuts
Efficiencies: jet triggers
The ATLAS First Level Calorimeter Trigger, Steve Hillier
• 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
Missing Energy: the Pile-Up effect
The ATLAS First Level Calorimeter Trigger, Steve Hillier
• 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
FCAL pile-up noise and cuts
The ATLAS First Level Calorimeter Trigger, Steve Hillier
• Noise RMS increases with pile-up <μ> – Measured here in ‘ZeroBias’ collisions – Typical electronics noise of 300-350 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
Missing Energy Performance
The ATLAS First Level Calorimeter Trigger, Steve Hillier
• 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
Future Prospects
Upgrade could fill another talk…
The ATLAS First Level Calorimeter Trigger, Steve Hillier
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
• New parallel digital trigger towers: approximately factor of 10 more data
• Some legacy signals fed to new trigger processor system via new daughterboards
L1Calo Upgrade in a Nutshell
• nMCM with FPGA: better noise, more flexible filters and energy calibration
• CMM->CMX allows extra thresholds and addition of Topology Triggers
The ATLAS First Level Calorimeter Trigger, Steve Hillier
• Updates in Muon and Central Trigger Processors also planned
Phase 1: High Granularity Trigger Towers
• 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
In fact 1/4/4/1 is now favoured
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