1. The ATLAS First Level Calorimeter Trigger
L1Calo Collaboration
The ATLAS First Level Calorimeter Trigger, Steve Hillier

2. 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

3. The ATLAS First Level Calorimeter Trigger, Steve Hillier
LHC and ATLAS
The ATLAS First Level Calorimeter Trigger, Steve Hillier

4. 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

5. 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

6. Trigger and L1Calo Architecture
The ATLAS First Level Calorimeter Trigger, Steve Hillier

7. 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

8. 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

9. 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

10. 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

11. The ATLAS First Level Calorimeter Trigger, Steve Hillier
USA15 Installation
The ATLAS First Level Calorimeter Trigger, Steve Hillier

12. The ATLAS First Level Calorimeter Trigger, Steve Hillier
L1Calo Subsystems
Preprocessor
The ATLAS First Level Calorimeter Trigger, Steve Hillier

13. 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

14. 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

15. 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

16. 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

17. 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

18. 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

19. 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

20. 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

21. Processor Architecture
L1Calo Subsystems
Processor Architecture
The ATLAS First Level Calorimeter Trigger, Steve Hillier

22. ‘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

23. 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

24. 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

25. 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

26. 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

27. 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

28. 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

29. 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

30. L1Calo Performance and Pile-Up
Selected highlights
L1Calo contributes to all but muon triggers!
The ATLAS First Level Calorimeter Trigger, Steve Hillier

31. 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

32. 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

33. 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

34. 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

35. 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

36. 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

37. 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

38. 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

39. Upgrade could fill another talk…
Future Prospects
Upgrade could fill another talk…
The ATLAS First Level Calorimeter Trigger, Steve Hillier

40. 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

41. 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

42. 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

43. 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