1. High Level Triggering
Fred Wickens

2. High Level Triggering (HLT)
Introduction to triggering and HLT systems
Why do we Trigger
Why do we use Multi-Level Triggering
Brief description of “typical” 3 level trigger
Case study of ATLAS HLT (+ some comparisons with other experiments)
Summary

3. Why do we Trigger and why multi-level
Over the years experiments have focussed on rarer processes
Need large statistics of these rare events
DAQ system (and off-line analysis capability) under increasing strain
limiting useful event statistics
Aim of the trigger is to record just the events of interest
i.e. Trigger = system which selects the events we wish to study
Originally - only read-out the detector if Trigger satisfied
Larger detectors and slow serial read-out => large dead-time
Also increasingly difficult to select the interesting events
Introduced: Multi-level triggers and parallel read-out
At each level apply increasingly complex algorithms to obtain better event selection/background rejection
These have:
Led to major reduction in Dead-time – which was the major issue
Managed growth in data rates – this remains the major issue

4. Summary of ATLAS Data Flow Rates
From detectors > 1014 Bytes/sec
After Level-1 accept ~ 1011 Bytes/sec
Into event builder ~ 109 Bytes/sec
Onto permanent storage ~ 108 Bytes/sec  ~ 1015 Bytes/year

5. The evolution of DAQ systems

6. TDAQ Comparisons

7. Level 1 Time: few microseconds Hardware based
Using fast detectors + fast algorithms
Reduced granularity and precision
calorimeter energy sums
tracking by masks
During Level-1 decision time store event data in front-end electronics
at LHC use pipeline - as collision rate shorter than Level-1 decision time
For details of Level-1 see Dave Newbold lecture
2 weeks ago

8. Level 2 Previously - few milliseconds (1-100)
Dedicated microprocessors, adjustable algorithm
3-D, fine grain calorimetry
tracking, matching
Topology
Different sub-detectors handled in parallel
Primitives from each detector may be combined in a global trigger processor or passed to next level
few milliseconds (10-100)
Processor farm with Linux PC’s
Partial events received via high-speed network
Specialised algorithms
Each event allocated to a single processor, large farm of processors to handle rate

9. Level 3 millisecs to seconds processor farm
Previously microprocessors/emulators
Now standard server PC’s
full or partial event reconstruction
after event building (collection of all data from all detectors)
Each event allocated to a single processor, large farm of processors to handle rate

10. Summary of Introduction
For many physics analyses, aim is to obtain as high statistics as possible for a given process
We cannot afford to handle or store all of the data a detector can produce!
The Trigger
selects the most interesting events from the myriad of events seen
I.e. Obtain better use of limited output band-width
Throw away less interesting events
Keep all of the good events(or as many as possible)
must get it right
any good events thrown away are lost for ever!
High level Trigger allows:
More complex selection algorithms
Use of all detectors and full granularity full precision data

11. Case study of the ATLAS HLT system
Concentrate on issues relevant for ATLAS (CMS very similar issues), but try to address some more general points

12. Starting points for any HLT system
physics programme for the experiment
what are you trying to measure
accelerator parameters
what rates and structures
detector and trigger performance
what data is available
what trigger resources do we have to use it

13. Physics at the LHC
Interesting events are buried in a sea of soft interactions
B physics
High energy QCD jet production
top physics
Higgs production

14. The LHC and ATLAS/CMS LHC has This results in
design luminosity 1034 cm-2s-1 (In 2010 from ?)
bunch separation 25 ns (bunch length ~1 ns)
This results in
~ 23 interactions / bunch crossing
~ 80 charged particles (mainly soft pions) / interaction
~2000 charged particles / bunch crossing
Total interaction rate sec-1
b-physics fraction ~ sec-1
t-physics fraction ~ sec-1
Higgs fraction ~ sec-1

15. Physics programme
Higgs signal extraction important - but very difficult
There is lots of other interesting physics
B physics and CP violation
quarks, gluons and QCD
top quarks
SUSY
‘new’ physics
Programme will evolve with: luminosity, HLT capacity and understanding of the detector
low luminosity (first ~2 years)
high PT programme (Higgs etc.)
b-physics programme (CP measurements)
high luminosity
searches for new physics

16. Trigger strategy at LHC
To avoid being overwhelmed use signatures with small backgrounds
Leptons
High mass resonances
Heavy quarks
The trigger selection looks for events with:
Isolated leptons and photons,
-, central- and forward-jets
Events with high ET
Events with missing ET

17. Example Physics signatures
Objects
Physics signatures
Electron 1e>25, 2e>15 GeV
Higgs (SM, MSSM), new gauge bosons, extra dimensions, SUSY, W, top
Photon 1γ>60, 2γ>20 GeV
Higgs (SM, MSSM), extra dimensions, SUSY
Muon 1μ>20, 2μ>10 GeV
Jet 1j>360, 3j>150, 4j>100 GeV
SUSY, compositeness, resonances
Jet >60 + ETmiss >60 GeV
SUSY, exotics
Tau >30 + ETmiss >40 GeV
Extended Higgs models, SUSY

18. ARCHITECTURE Trigger DAQ 40 MHz ~1 PB/s (equivalent) ~ 200 Hz
Three logical levels
Hierarchical data-flow
~1 PB/s (equivalent)
LVL1 - Fastest: Only Calo and Mu Hardwired
On-detector electronics: Pipelines
~2.5 ms
LVL2 - Local: LVL1 refinement + track association
Event fragments buffered in parallel
~40 ms
LVL3 - Full event: “Offline” analysis
Full event in processor farm
~4 sec.
~ 200 Hz
~ 300 MB/s
Physics

19. Selected (inclusive) signatures

20. Trigger design – Level-1
sets the context for the HLT
reduces triggers to ~75 kHz
Uses limited detector data
Fast detectors (Calo + Muon)
Reduced granularity
Trigger on inclusive signatures
muons;
em/tau/jet calo clusters; missing and sum ET
Hardware trigger
Programmable thresholds
CTP selection based on multiplicities and thresholds

21. Level-1 Selection The Level-1 trigger
an “or” of a large number of inclusive signals
set to match the current physics priorities and beam conditions
Precision of cuts at Level-1 is generally limited
Adjust the overall Level-1 accept rate (and the relative frequency of different triggers) by
Adjusting thresholds
Pre-scaling (e.g. only accept every 10th trigger of a particular type) higher rate triggers
Can be used to include a low rate of calibration events
Menu can be changed at the start of run
Pre-scale factors may change during the course of a run

22. Example Level-1 Menu for 2x10^33
Level-1 signature
Output Rate (Hz)
EM25i
12000
2EM15i
4000
MU20
800
2MU6
200
J200
3J90
4J65
J60 + XE60
400
TAU25i + XE30
2000
MU10 + EM15i
100
Others (pre-scaled, exclusive, monitor, calibration)
5000
Total
~25000

23. Trigger design - Level-2
Level-2 reduce triggers to ~2 kHz
Note CMS does not have a physically separate Level-2 trigger, but the HLT processors include a first stage of Level-2 algorithms
Level-2 trigger has a short time budget
ATLAS ~40 milli-sec average
Note for Level-1 the time budget is a hard limit for every event, for the High Level Trigger it is the average that matters, so OK for a small fraction of events to take times much longer than this average
Full detector data available, but to minimise resources needed:
Limit the data accessed
Only unpack detector data when it is needed
Use information from Level-1 to guide the process
Analysis in steps with possibility to reject event after each step
Use custom algorithms

24. Regions of Interest
The Level-1 selection is dominated by local signatures (I.e. within Region of Interest - RoI)
Based on coarse granularity data from calo and mu only
Typically, there are 1-2 RoI/event
ATLAS uses RoI’s to reduce network b/w and processing power required

25. Trigger design - Level-2 - cont’d
Processing scheme
In each RoI extract features from sub-detector
combine features from one RoI into object
combine objects to test event topology
Precision of Level-2 cuts
Emphasis is on very fast algorithms with reasonable accuracy
Do not include many corrections which may be applied off-line
Calibrations and alignment available for trigger not as precise as ones available for off-line

26. ARCHITECTURE Trigger DAQ LVL1 H L T ~ 1 PB/s 40 MHz 75 kHz LVL2 ROS
Calo MuTrCh
Other detectors
~ 1 PB/s
40 MHz
40 MHz
75 kHz
~2 kHz
~ 200 Hz
LVL1
2.5 ms
Calorimeter
Trigger
Muon
FE Pipelines
2.5 ms
LVL1 accept
Read-Out Drivers
ROD
ROS
Read-Out Sub-systems
Read-Out Buffers
ROB
GB/s
Read-Out Links
RoI’s H L T
LVL2
~ 10 ms
L2P
L2SV
L2N
ROIB
RoI
requests
RoI data = 1-2%
~2 GB/s
Event Builder EB
~3 GB/s
LVL2 accept
Event Filter
EFP
~ 1 sec
EFN
~3 GB/s
~ 300 MB/s

27. CMS Event Building CMS perform Event Building after Level-1
This simplifies the architecture, but places much higher demand on technology:
Network traffic ~100 GB/s
Use Myrinet instead of GbE for the EB network
Plan a number of independent slices with barrel shifter to switch to a new slice at each event
Time will tell which philosophy is better

28. Example for Two electron trigger
EM20i +
Level1 seed 
t i m e
LVL1 triggers on two isolated
e/m clusters with pT>20GeV
(possible signature: Z–>ee)
Cluster
shape
STEP 1
ecand +
Signature 
HLT Strategy:
Validate step-by-step
Check intermediate signatures
Reject as early as possible
track
finding
STEP 2 e +
Signature 
pt>
30GeV
STEP 3
e30 +
Signature 
Sequential/modular approach facilitates early rejection
Iso–
lation
STEP 4
e30i +
Signature 

29. Trigger design - Event Filter / Level-3
Event Filter reduce triggers to ~200 Hz
Event Filter budget ~ 4 sec average
Full event detector data is available, but to minimise resources needed:
Only unpack detector data when it is needed
Use information from Level-2 to guide the process
Analysis proceeds in steps with possibility to reject event after each step
Use optimised off-line algorithms

30. Trigger design - HLT strategy
Level 2
confirm Level 1, some inclusive, some semi-inclusive, some simple topology triggers, vertex reconstruction (e.g. two particle mass cuts to select Zs)
Level 3
confirm Level 2, more refined topology selection, near off-line code

31. Example HLT Menu for 2x10^33 HLT signature Output Rate (Hz) e25i 40
<1
gamma60i 25
2gamma20i 2
mu20i
2mu10 10
j400
3j165
4j110
j70 + xE70 20
tau35i + xE45 5
2mu6 with vertex, decay-length and mass cuts (J/psi, psi’, B)
Others (pre-scaled, exclusive, monitor, calibration)
Total
~200

32. Example B-physics Menu for 10^33
LVL1 :
MU6 rate 24kHz (note there are large uncertainties in cross-section)
In case of larger rates use MU8 => 1/2xRate
2MU6
LVL2:
Run muFast in LVL1 RoI ~ 9kHz
Run ID recon. in muFast RoI mu6 (combined muon & ID) ~ 5kHz
Run TrigDiMuon seeded by mu6 RoI (or MU6)
Make exclusive and semi-inclusive selections using loose cuts
B(mumu), B(mumu)X, J/psi(mumu)
Run IDSCAN in Jet RoI, make selection for Ds(PhiPi)
EF:
Redo muon reconstruction in LVL2 (LVL1) RoI
Redo track reconstruction in Jet RoI
Selections for B(mumu) B(mumuK*) B(mumuPhi), BsDsPhiPi etc.

33. Matching problem
Background
Physics channel
Off-line
On-line

34. Matching problem (cont.)
ideally
off-line algorithms select phase space which shrink-wraps the physics channel
trigger algorithms shrink-wrap the off-line selection
in practice, this doesn’t happen
need to match the off-line algorithm selection
For this reason many trigger studies quote trigger efficiency wrt events which pass off-line selection
BUT off-line can change algorithm, re-process and recalibrate at a later stage
So, make sure on-line algorithm selection is well known, controlled and monitored

35. Selection and rejection
as selection criteria are tightened
background rejection improves
BUT event selection efficiency decreases

36. Selection and rejection
Example of a ATLAS Event Filter (I.e. Level-3) study of the effectiveness of various discriminants used to select 25 GeV electrons from a background of dijets

37. Other issues for the Trigger
Efficiency and Monitoring
In general need high trigger efficiency
For many analyses need a well known efficiency
Monitor efficiency by various means
Overlapping triggers
Pre-scaled samples of triggers in tagging mode (pass-through)
To assist with overall normalisation ATLAS divides each run into periods of a few minutes called a luminosity block.
During each block the beam luminosity should be constant
Can also exclude any blocks where there is a known problem
Final detector calibration and alignment constants not available immediately - keep as up-to-date as possible and allow for the lower precision in the trigger cuts when defining trigger menus and in subsequent analyses
Code used in trigger needs to be very robust - low memory leaks, low crash rate, fast

38. Other issues for the Trigger – cont’d
Beam conditions and HLT resources will evolve over several years (for both ATLAS and CMS)
In 2010 luminosity low, but also HLT capacity will be < 50% of full system
For details of the current ideas on ATLAS Menu evolution see
Gives details of menu for Startup (including single beam running), 10^31, 10^32, 10^33
Description of current menu is very long !!
Corresponding information for CMS is at
The expected performance of ATLAS for different physics channels (including the effects of the trigger) is documented in (beware ~2000 pages)

39. Summary
High-level triggers allow complex selection procedures to be applied as the data is taken
Thus allow large samples of rare events to be recorded
The trigger stages - in the ATLAS example
Level 1 uses inclusive signatures
muons; em/tau/jet; missing and sum ET
Level 2 refines Level 1 selection, adds simple topology triggers, vertex reconstruction, etc
Level 3 refines Level 2 adds more refined topology selection
Trigger menus need to be defined, taking into account:
Physics priorities, beam conditions, HLT resources
Include items for monitoring trigger efficiency and calibration
Try to match trigger cuts to off-line selection
Trigger efficiency should be as high as possible and well monitored
Must get it right - events thrown away are lost for ever!
Triggering closely linked to physics analyses – so enjoy!

40. ATLAS L1 and HLT trigger rates Dec 2009
for a typical run with stable beam flag.
Beam injection, record collision events. HLT algorithms off.
HLT active after LHC declares stable beam
Rejection factor of ~104 looking for space points in the Inner Detector at Level 2 trigger
~20
BPTX prescaled by x20 as input to L2

41. Description of ATLAS L1 + HLT rate plot
L1 and HLT trigger rates for a typical run with stable beam flag.
Also shown are a collision trigger at L1, requiring hits on both the A and the C side of the minimum bias scintillator counters and filled bunches for both beams.
The line labled L2 Inner Detector activity represents a filtering algorithm at the L2 trigger, which accepts events based on space point counts in the Inner Detector.
This L2 algorithm receives 5% of all filled bunches as input from L1. Assuming both the L1 collision trigger and the space point counting are highly efficient for collision events, the difference in the two lines should reflect this fraction, even though the acceptance of both triggers is different.
The moment the L2 algorithm is enabled is clearly visible as the jump of output L1 rate, and the start of event rate on the L2 line.
The dips in HLT and L1 output rates just before this moment are due to the short pause needed to change trigger setup.
The HLT output rate (which represents the rate of events recorded to disk) does not visibly change, as it is dominated by a constant rate of monitor triggers.

42. Additional Foils

43. ATLAS HLT Hardware Each rack of HLT (XPU) processors contains
~30 HLT PC’s (PC’s very similar to Tier-0/1 compute nodes)
2 Gigabit Ethernet Switches
a dedicated Local File Server
Final system will contain ~2300 PC’s

44. LFS nodes
UPS for CFS
XPUs
CFS nodes
SDX1|2nd floor|Rows 3 & 2

45. Naming Convention MU 20 I mu 20 i _ passEF
First Level Trigger (LVL1) Signatures in capitals e.g.
LVL1
HLT
type EM e
electron g
photon MU mu
muon HA
tau FJ fj
forward jet JE je
jet energy JT jt
jet TM xe
missing energy
name
threshold
isolated
MU 20 I
HLT in lower case:
name
threshold
isolated
mu 20 i _ passEF
EF in tagging mode
New in :
Threshold is cut value applied
previously was ~95% effic. point.
More details : see :

46. ATLAS Beam Pick-up detectors
The ATLAS BPTX detectors are simple electrostatic beam pick-up detectors 175 m on either side of ATLAS.
The BPTX signals are fed into a discriminator and input into the CTP to provide “filled bunch triggers” (one CTP input per beam).
These “filled bunch trigger” can be used to indicate when there are particle bunches in the interaction region from each beam.
By requiring a coincidence between the filled bunch triggers from both beams, a filled bunch crossing trigger signal can be formed.
Optionally, these trigger signals can be used in combination with other triggers (e.g. the minimum bias trigger scintillators).

47. What is a minimum bias event ?
- event accepted with the only requirement being activity in the detector
with minimal pT threshold [100 MeV] (zero bias events have no requirements)
- e.g. Scintillators at L1 + (> 40 SCT S.P. or > 900 Pixel clusters) at L2
- a miminum bias event is most likely to be either:
- a low pT (soft) non-diffractive event
- a soft single-diffractive event
- a soft double diffractive event
(some people do not include the diffractive events in the definition !)
- it is characterised by:
- having no high pT objects : jets; leptons; photons
- being isotropic
- see low pT tracks at all phi in a tracking detector
- see uniform energy deposits in calorimeter as function of rapidity
- these events occur in % of collisions. So if any given crossing
has two interactions and one of them has been triggered due to a high pT
component then the likelihood is that the accompanying event will
be a dull minimum bias event.

48. Each set of counters is segmented in phi (x8) and eta (x2).
ATLAS Mininum Bias Trigger Scintillators ?
The Minimum Bias Trigger Scintillators (MBTS) were designed to function only during initial data-taking at low luminosities.
After 3-4 months of higher luminosity operation the scintillators will yellow due to radiation damage.
Sixteen scintillator counters are installed on the inner face of the end-cap calorimeter cryostats on each side of ATLAS
Each set of counters is segmented in phi (x8) and eta (x2).
They are located at |z| = 3560 mm
the innermost set covers the region 2.82 < |eta| < 3.84
the outermost set covers the region 2.09 < |eta| < 2.82.
Signals from each scintillator are fed to NIM discriminators, the output of which goes into the CTP, which calculates a multiplicity for each side of ATLAS
In the longer term other detectors will be used for MinBias trigger: Beam Condition Monitor (BCM), LUCID, ZDC

49. L1 Rates 1031 14.4.0 Multi EM 6400 Multi Object 5500 Single EM
Trigger Group
Rate (Hz)
Multi EM
6400
Multi Object
5500
Single EM
Single Muon
1700
Multi Tau
470
Single Tau
150
Jets 80
Multi Muon 70 XE 50
TOTAL
20000
Removing overlaps between single+multi EM gives 18 kHz
Total estimated L1 rate with all overlaps removed is ~ 12 kHz 49

50. L2 Rates 1031 14.4.0 Electrons 310 Muons 210* Taus+X 180 XE+ 82
Trigger Group
Rate (Hz)
Electrons
310
Muons
210*
Taus+X
180
XE+ 82
Photons 46
B Phys 43
Jets 22
TOTAL
900
X=anything;
+ includesJE,
TE, anything
with MET
except taus;
Bphys includes
Bjet
* Manually prescaled off pass-through triggers mu4_tile, mu4_mu6
Total estimated L2rate with all overlaps removed is 840 Hz 50

51. EF Rates 1031 14.4.0 Muons 80 Electrons 67 Tau+X 56 B Phys 37 Jets 25
Trigger Group
Rate (Hz)
Muons 80
Electrons 67
Tau+X 56
B Phys 37
Jets 25
Photons 18
XE+ 13
Misc
TOTAL
310
91 Hz total is in prescaled triggers;
51 Hz of prescaled triggers is unique rate
Total estimated EF Rate with overlaps removed is 250 Hz 51

52. L1 Rates
Trigger Group
Rate (Hz)
Multi Object
30000
Single Muon
17000
Multi EM
11000
Single EM
8100
Multi Tau
4300
Single Tau
870
Multi Muon
690
Jets
300 XE
TOTAL
73000
Total estimated L1 rate with all overlaps removed is 46 kHz 52

53. L2 Rates
Trigger Group
Rate (Hz)
Tau+X
820
XE+
590
Electrons
390
Muons
280
3 Objects
270
Photons
120
B Phys
110
Jets 33
Misc 28
TOTAL
2600
Total estimated L2 with all overlaps removed is (too high!) 53

54. EF Rates
Trigger Group
Rate (Hz)
Tau+X
187
Electrons 77
Muons 46
Photons
BPhys 45
3 Objects
XE+ 42
Jets 11
Misc
TOTAL
510
Total estimated EF rates with all overlaps removed is 390 Hz
(Fixing L2 will likely come close to fixing EF as well) 54