1. The First-Level Trigger of ATLAS
International Europhysics Conference on High Energy Physics, July 21st-27th 2005, Lisbon, Portugal
The First-Level Trigger of ATLAS
Johannes Haller (CERN)
on behalf of the
ATLAS First-Level Trigger Groups

2. The First Level Trigger of ATLAS
Triggering at the LHC σ
rate
bunch crossing rate: 40 MHz
total interaction rate:~ 1 GHz
event size: ~ 1.5 MB
total interaction rate
affordable: ~ 300 MB/s
storage rate: ~ 200 Hz
→ online rejection: %
e.g.: Higgs → ZZ → 2e+2m
23 min. bias events:
~ 1725 particles/BC
storage rate
discoveries
powerful trigger needed
enormous rate reduction
retaining the rare events in the very tough LHC environment ET
Johannes Haller
The First Level Trigger of ATLAS

3. The First Level Trigger of ATLAS
ATLAS Trigger System
3-Level Trigger System:
hardware
LVL1 decision based on data from calorimeters and muon trigger chambers; synchronous at 40 MHz; bunch crossing identification
LVL2 uses Regions of Interest (identified by LVL1) data (ca. 2%) with full granularity from all detectors
Event Filter has access to full event and can perform more refined event reconstruction
2.5 ms
software
~ 10 ms
~ sec.
this talk:
LVL1: Calorimeter Trigger Muon Trigger Central Trigger
Johannes Haller
The First Level Trigger of ATLAS

4. LVL1 Calorimeter Trigger
to DAQ
Input/output data
DAQ
RODs
example: e/g algorithm:
goal: good discrimination e/g ↔ jets
identify 2x2 RoI with local ET maximum
cluster/ isolation cuts on various ET sums
2 ROD crates
Jet / ET
(JEP)
2 JEP crates
to LVL2
RoI
RODs
0.2 x 0.2
Analogue
tower sums 0.1 x 0.1
(~7200)
to CTP
Pre-
Processor
(PPr)
feature
types/
positions
e/, /had
Clusters
(CP)
to CTP
8 PPr crates
0.1 x 0.1
4 CP crates
electronic components (installed in counting room heavily FPGA based  flexibility):
PPr: digitisation of analogue signals from calorimeters and bunch crossing ID
JEP: jet finding and energy sums
CP: e/g and t/ had. cluster finding
output:
at 40 MHz: multiplicities for e/g, jets, t/had and flags for energy sums to Central Trigger (CTP)
accepted events: position of objects (RoIs) to LVL2 and additional information to DAQ
Johannes Haller
The First Level Trigger of ATLAS

5. The First Level Trigger of ATLAS
LVL1 Muon Trigger
algorithm:
dedicated muon chambers with good timing resolution for trigger:
Barrel |η|<1.0 : Resistive Plate Chambers (RPCs)
End-caps 1.0<|η|<2.4 : Thin Gap Chambers (TGCs)
local track finding for LVL1 done on- detector (ASICs)
looking for coincidences in chamber layers
programmable widths of 6 coincidence windows determines pT threshold
Johannes Haller
The First Level Trigger of ATLAS

6. LVL1 Central Trigger CTP: (one 9U VME64x crate, FPGA based)
Calorimeter trigger
Muon trigger
Cluster Processor (e/g, t/h)
Pre-Processor (analogue  ET)
Jet / Energy-sum Processor
Muon-CTP Interface (MuCTPI)
Muon Barrel Trigger (RPC)
Muon End-cap Trigger (TGC)
multiplicities of e/g, t/h, jet for 8 pT thresholds each; flags for SET, SET j, ETmiss over thresholds
multiplicities of m for 6 pT thresholds
Central Trigger Processor (CTP)
example trigger menu:
LVL1 Menu
2x1033cm-2s-1
MU20
0.8
2MU6
0.2
EM25i
12.0
2EM15i
4.0
J200
3J90
4J65
J60+xE60
0.4
TAU25+xE30
2.0
MU10+EM15i
0.1
Others
5.0
Total rate (kHz)
~ 25
CTP: (one 9U VME64x crate, FPGA based)
central part of LVL1 trigger system
combination of up to 160 input bits (plus internal bits) to 256 triggers (with prescale factors)
calculation of trigger decision based on inputs from L1Calo and L1Muon according to trigger menu
ATLAS LVL1 trigger strategy is as inclusive as possible to
reduce bias and be open for new physics
big uncertainties on predicted rates
Johannes Haller
The First Level Trigger of ATLAS

7. ATLAS Combined Test Beam
setup at CERN’s SPS H8 beam-line: (2004)
beam
(π, μ, e, p, g)
Ebeam =
(1 to 360) GeV
L1Muon setup
full scale ATLAS slice, all sub- detectors
test of prototypes and final modules
periods of 25ns structured beam
(like LHC)
aim to establish full trigger and data acquisition chain
end-cap chambers
barrel chambers
Johannes Haller
The First Level Trigger of ATLAS

8. LVL1 Trigger at the Test-Beam
all trigger, timing, control and readout paths successfully established:
full LVL1 trigger chain established for the first time
ATLAS
run control
LVL1 triggered the readout of all sub-detectors
LVL1 latency projected to ATLAS: 2.13 μs
signal distribution at test-beam:
Muon Trigger
Calo Trigger
all
sub-
detec-
tors
CTP latency: 95 ns
at test-beam ~125 ns (not optimized)
Johannes Haller
The First Level Trigger of ATLAS

9. Test-Beam Results: Muon Trigger
end-caps (TGCs):
barrel (RPCs):
threshold efficiency after
chamber shifting
position in precision muon
chambers vs. position in RPCs
efficiency and BCID
Triggered Bunch
Next Bunch
Previous Bunch
total efficiency
pT threshold 6
 pT threshold 5
 pT threshold 4
nice correlation between RPC and MDT position measurement
trigger efficiency at test-beam (3/4, phi): 99.4%
efficiency for correct identification of bunch crossing: 99.5%
chamber was shifted to emulate the effect of deflection in magnetic field
coincidence algorithm works
big timing margin where e(correct bunch) high and e(bunches before and after) tiny
Johannes Haller
The First Level Trigger of ATLAS

10. Test-Beam Results: Calorimeter Trigger
a full slice of the calorimeter trigger system was installed: ~1% of final capacity
checks of data consistency very successful
Correlation of energy
in LAr calo. and CPM
counting room reality:
ROD
PreProcessor
Receivers
CPMs/JEMs
L1Calo setup
good correlation of energy values measured in calorimeter and received in CP module
no event below e.m. trigger threshold of 20 GeV
 calorimeter trigger did work
Johannes Haller
The First Level Trigger of ATLAS

11. The First Level Trigger of ATLAS
Summary
The trigger and its performance are of paramount importance at the LHC
The First Level Trigger of ATLAS is based on calorimeters and dedicated muon chambers and reduces the event rate to ~75 kHz
Successful test of the First Level Trigger system at the ATLAS Combined Test Beam
Status: Prototypes of all types of modules and all ASICs validated; mass production started
Road to data-taking at the LHC:
muon trigger chamber integration already started
CTP installation: September 2005
calorimeter trigger installation starts in September 2005
first cosmic ray runs with a subset of detectors early 2006
ATLAS expects to be ready for first pp collisions in 2007
Johannes Haller
The First Level Trigger of ATLAS