The ATLAS First Level Calorimeter...

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


LHC and ATLAS


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


* Eventually ** Now!

Specifications Length: 44 m, Diameter: 22 m weight: 7000 t

Interaction point

The ATLAS detector


Trigger and L1Calo Architecture


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

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


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


USA15 Installation


L1Calo Subsystems

Preprocessor


Pre-Processor System


• 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


Installation: Analogue Cables

496 cables into 8 crates Four cables just fit front of one 9U module


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

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


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


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


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)


L1Calo Subsystems

Processor Architecture


‘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


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


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


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


Installation: Digital Cabling

Up to 1400 individual LVDS signals into one crate

More than 500 Gbit/s data input


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


Hardware Implementation

• Multiple ‘layers’ of FPGA processing • Data reception and fanout • Algorithmic processing • Result merging • Final stages in common CMM


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

L1Calo Performance and Pile-Up

Selected highlights L1Calo contributes to all but muon triggers!


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


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

Efficiencies: electron triggers

• Early 2011 main single electron trigger: – e20 seeded by L1_EM14


• During 2012 we use EF_em25isolated, seeded by L1_EM18VH

Efficiencies: tau triggers


• 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


• 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


• 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


• 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


• 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…


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


• 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


• 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


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...

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