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High Level Triggering

High Level Triggering. Fred Wickens. High Level Triggering (HLT). Introduction to triggering and HLT systems What is Triggering What is High Level Triggering Why do we need it Case study of ATLAS HLT (+ some comparisons with other experiments) Summary.

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High Level Triggering

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  1. High Level Triggering Fred Wickens

  2. High Level Triggering (HLT) • Introduction to triggering and HLT systems • What is Triggering • What is High Level Triggering • Why do we need it • 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 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 talk

  8. High Level Trigger - Levels 2 + 3 • Level-2 : Few milliseconds (10-100) • Partial events received via high-speed network • Specialised algorithms • 3-D, fine grain calorimetry • tracking, matching • Topology • Level-3 : Up to a few seconds • Full or partial event reconstruction • after event building (collection of all data from all detectors) • Level-2 + Level-3 • Processor farm with Linux server PC’s • Each event allocated to a single processor, large farm of processors to handle rate

  9. 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

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

  11. Starting points for any Trigger 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 • Particularly network b/w + cpu performance

  12. Physics at the LHC 7 TeV Interesting events are buried in a seaof soft interactions B physics High energy QCD jet production top physics Higgs production

  13. The LHC and ATLAS/CMS • LHC has • Design luminosity 1034 cm-2s-1 • In 2010 from 1027 – 2x1032 ; 2011 up to 2x1033 • Design 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 109 sec-1 • b-physics fraction ~ 10-3 106 sec-1 • t-physics fraction ~ 10-8 10 sec-1 • Higgs fraction ~ 10-11 10-2 sec-1

  14. 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 (2010 - 2011) • high PT programme (Higgs etc.) • b-physics programme (CP measurements) • high luminosity (2013 or 2014?) • high PT programme (Higgs etc.) • searches for new physics

  15. 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

  16. Example Physics signatures

  17. ~ 200 Hz Physics ~ 300 MB/s ARCHITECTURE Trigger DAQ 40 MHz ~1 PB/s(equivalent) Three logical levels Hierarchical data-flow LVL1 - Fastest:Only Calo and MuHardwired 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.

  18. Selected (inclusive) signatures

  19. Trigger design – Level-1 • 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

  20. 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

  21. 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

  22. 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 is 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 proceeds in steps with possibility to reject event after each step • Use custom algorithms

  23. 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

  24. Trigger design - Level-2 - cont’d • Processing scheme • extract features from sub-detectors in each RoI • combine features from one RoI into object • combine objects to test event topology • Precision of Level-2 cuts • Limited (although better than at Level-1) • 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

  25. Trigger DAQ Calo MuTrCh Other detectors ~ 1 PB/s 40 MHz 40 MHz LVL1 2.5 ms LVL1 accept Calorimeter Trigger Muon Trigger ROD ROD ROD Read-Out Drivers 75 kHz RoI’s 120 GB/s Read-Out Links RoI requests LVL2 ROB ROB ROB ~ 10 ms ROS Read-Out Buffers ROIB L2SV Read-Out Sub-systems RoI data = 1-2% ~2 GB/s L2P L2P L2P ~2 kHz ~3 GB/s L2N LVL2 accept Event Builder Event Filter ~ 1 sec EB ~3 GB/s EFN EFP EFP EFP ~ 300 MB/s ~ 200 Hz ~ 300 MB/s ARCHITECTURE FE Pipelines 2.5 ms H L T

  26. CMS Event Building • CMS perform Event Building after Level-1 • Simplifies the architecture, but places much higher demand on technology: • Network traffic ~100 GB/s • 1st stage use Myrinet • 2nd stage has 8 GbE slices • Time will tell which is better

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

  28. 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

  29. EM ROI Execution of a Trigger Chain L2 calorim. Level1: Region of Interest is found and position in EM calorimeter is passed to Level 2 Electromagnetic clusters cluster? L2 tracking track? • Level 2 seeded by Level 1 • Fast reconstruction algorithms • Reconstruction within RoI match? E.F.calorim. E.F.tracking track? • Ev.Filter seeded by Level 2 • Offline reconstruction algorithms • Refined alignment and calibration e/ reconst. e/ OK?

  30. Phys.Lett.B 688, Issue 1, 2010 Minimum Bias Trigger Minbias Trigger Scintillator: 32 sectors on LAr cryostat Main trigger for initial running h coverage 2.1 to 3.8 LHC collision rate (nb=4) LHC collision rate (nb=2) • Soft QCD studies • Provide control trigger on p-p collisions; discriminate against beam-related backgrounds (using signal time) • Minimum Bias Scintillators (MBTS) installed in each end-cap; • Example: MBTS_1 – at least 1 hit in MBTS • Also check nr. of hits in Inner Detector in Level-2

  31. e/γ Trigger L1 EM trigger pT > 5GeV pT≈3-20 GeV: b/c/tau decays, SUSY pT≈20-100 GeV: W/Z/top/Higgs pT>100 GeV: exotics Level 1: local ET maximum in ΔηxΔφ = 0.2x0.2 with possible isolation cut Level 2: fast tracking and calorimeter clustering – use shower shape variables plus track-cluster matching Event Filter: high precision offline algorithms wrapped for online running

  32. Discriminate against hadronic showers based on shower shape variables Use fine granularity of LAr calorimeter Resolution improved in Event Filter with respect to Level 2

  33. Muon Trigger 80% acceptance due to support structures etc. • Low PT: J/Y, U and B-physics • High PT: H/Z/W/τ➝μ, SUSY, exotics • Level 1: look for coincidence hits in muon trigger chambers • Resistive Plate Chambers (barrel) and Thin Gap Chambers (endcap) • pT resolved from coincidence hits in look-up table • Level 2: refine Level 1 candidate with precision hits from Muon Drift Tubes (MDT) and combine with inner detector track • Event Filter: use offline algorithms and precision; complementary algorithm does inside-out tracking and muon reconstruction

  34. Hadronic Tau Trigger • W/Z ➝t, SM &MSSM Higgs, SUSY, Exotics • Level 1: start from hadronic cluster – local maximum in ΔηxΔφ = 0.2x0.2 – possible to apply isolation • Level 2: track and calorimeter information are combined – narrow cluster with few matching tracks • Event Filter: 3D cluster reconstruction suppresses noise; offline ID algorithms and calibration used • Typical background rejection factor of ≈5-10 from Level 2+Event Filter • Right: fake rate for loose tau trigger with pT > 12 GeV – aka tau12_loose • MC is Pythia with no LHC-specific tuning

  35. Jet Trigger Note in preparation QCD multijet production, top, SUSY, generic BSM searches Level 1: look for local maximum in ET in calorimeter towers of ΔηxΔφ = 0.4x0.4 to 0.8x0.8 Level 2: simplified cone clustering algorithm (3 iterations max) on calorimeter cells Event Filter: anti-kT algorithm on calorimeter cells; currently running in transparent mode (no rejection)

  36. Missing ET Trigger Level 1 5 GeV threshold Level 1 20 GeV threshold SUSY, Higgs Level 1: ETmiss and ET calculated from all calorimeter towers Level 2: only muon corrections possible (at present) Event Filter: re-calculate from calorimeter cells and reconstructed muons

  37. The Trigger Menu • Collection of trigger signatures • In LHC GPD’s menus there can be 100’s of algorithm chains – defining which objects, thresholds and algorithms, etc should be used • Selections set to match the current physics priorities and beam conditions within the bandwidth and rates allowed by the TDAQ system • Includes calibration & monitoring chains • Principal mechanisms to adjust the accept rate (and the relative frequency of different triggers) • 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

  38. Example use of thresholds/prescales at Level-1 L1 trigger items and estimated rates at 10^31 cm−2 s−1 for jets Jet ET spectrum at 10^31 cm−2 s−1 before (dashed) and after (solid) pre-scaling at L1

  39. Trigger Menu cont’d • Basic Menu is defined at the start of a run • Pre-scale factors can be changed during the course of a run • Adjust triggers to match current luminosity • Turn triggers on/off

  40. Trigger Commissioning in ATLAS • First Collisions : L1 only • Since June : gradual activation of HLT

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

  42. 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

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

  44. Other issues for the Trigger • Efficiency and Monitoring • In general need high trigger efficiency • Also 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) • 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

  45. 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 had < 50% of full system • For details of the current ideas on ATLAS Menu evolution see • https://twiki.cern.ch/twiki/bin/view/Atlas/TriggerPhysicsMenu • Gives details of menu since Startup and for 2011 • Corresponding information for CMS is at • https://twiki.cern.ch/twiki/bin/view/CMS/TriggerMenuDevelopment • The expected performance of ATLAS for different physics channels (including the effect of the trigger) is documented in http://arxiv.org/abs/0901.0512 (beware - nearly 2000 pages)

  46. 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 (mu’s; 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!

  47. ATLAS works! Top-pair candidate - e-mu + 2b-tag

  48. CMS works!

  49. Additional Foils

  50. 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

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