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> Bytes/sec After Level-1 accept~ Bytes/sec Into event builder~ 10 9 Bytes/sec Onto permanent storage~ 10 8 Bytes/sec  ~ 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 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

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

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

13 The LHC and ATLAS/CMS LHC has –Design luminosity cm -2 s -1 In 2010 from – 2x10 32 ; 2011 up to 2x10 33 –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 rate10 9 sec -1 –b-physicsfraction ~ sec -1 –t-physicsfraction ~ sec -1 –Higgsfraction ~ 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 ( ) 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 E T –Events with missing E T

16 ObjectsPhysics signatures Electron 1e>25, 2e>15 GeVHiggs (SM, MSSM), new gauge bosons, extra dimensions, SUSY, W, top Photon 1γ>60, 2γ>20 GeVHiggs (SM, MSSM), extra dimensions, SUSY Muon 1μ>20, 2μ>10 GeVHiggs (SM, MSSM), new gauge bosons, extra dimensions, SUSY, W, top Jet 1j>360, 3j>150, 4j>100 GeVSUSY, compositeness, resonances Jet >60 + E T miss >60 GeVSUSY, exotics Tau >30 + E T miss >40 GeVExtended Higgs models, SUSY Example Physics signatures

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

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

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 –1 st stage use Myrinet –2 nd stage has 8 GbE slices Time will tell which is better

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

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

30 Phys.Lett.B 688, Issue 1, 2010 LHC collision rate (n b =4) LHC collision rate (n b =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 Minimum Bias Trigger Minbias Trigger Scintillator: 32 sectors on LAr cryostat Main trigger for initial running  coverage 2.1 to 3.8

31 e/γ Trigger p T ≈3-20 GeV: b/c/tau decays, SUSY p T ≈ GeV: W/Z/top/Higgs p T >100 GeV: exotics Level 1: local E T 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 L1 EM trigger p T > 5GeV

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 80% acceptance due to support structures etc. Muon Trigger Low P T : J/ ,  and B-physics High P T : 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) –p T 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 ➝ , 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 p T > 12 GeV – aka tau12_loose –MC is Pythia with no LHC-specific tuning

35 Jet Trigger QCD multijet production, top, SUSY, generic BSM searches Level 1: look for local maximum in E T 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-k T algorithm on calorimeter cells; currently running in transparent mode (no rejection) Note in preparation

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

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 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 Example use of thresholds/prescales at Level-1

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 Matching problem Background Physics channel Off-line On-line

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 – Gives details of menu since Startup and for 2011 Corresponding information for CMS is at – The expected performance of ATLAS for different physics channels (including the effect of the trigger) is documented in (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 E T ) –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

51 SDX1|2 nd floor|Rows 3 & 2 CFS nodes UPS for CFS LFS nodes

52 Naming Convention First Level Trigger (LVL1) Signatures in capitals e.g. LVL1 HLTtype EM eelectron gphoton MUmumuon HAtau FJ fj forward jet JEjejet energy JTjtjet TMxemissing energy HLT in lower case: name threshold isolated mu 20 i _ passEF EF in tagging mode name threshold isolated MU 20 I New in : Threshold is cut value applied previously was ~95% effic. point. More details : see :

53 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.

54 Example Level-1 Menu for 2x10^33 Level-1 signatureOutput Rate (Hz) EM25i EM15i4000 MU MU6200 J J J65200 J60 + XE60400 TAU25i + XE MU10 + EM15i100 Others (pre-scaled, exclusive, monitor, calibration)5000 Total~25000

55 L1 Rates Removing overlaps between single+multi EM gives 18 kHz Total estimated L1 rate with all overlaps removed is ~ 12 kHz Trigger GroupRate (Hz) Multi EM6400 Multi Object5500 Single EM5500 Single Muon1700 Multi Tau470 Single Tau150 Jets80 Multi Muon70 XE50 TOTAL20000

56 L2 Rates Total estimated L2rate with all overlaps removed is 840 Hz Trigger GroupRate (Hz) Electrons310 Muons210* Taus+X180 XE+82 Photons46 B Phys43 Jets22 TOTAL900 * Manually prescaled off pass-through triggers mu4_tile, mu4_mu6 X=anything; + includesJE, TE, anything with MET except taus; Bphys includes Bjet

57 EF Rates Total estimated EF Rate with overlaps removed is 250 Hz Trigger GroupRate (Hz) Muons 80 Electrons67 Tau+X56 B Phys37 Jets25 Photons18 XE+13 Misc13 TOTAL Hz total is in prescaled triggers; 51 Hz of prescaled triggers is unique rate

58 L1 Rates Total estimated L1 rate with all overlaps removed is 46 kHz Trigger GroupRate (Hz) Multi Object30000 Single Muon17000 Multi EM11000 Single EM8100 Multi Tau4300 Single Tau870 Multi Muon690 Jets300 XE300 TOTAL73000

59 L2 Rates Total estimated L2 with all overlaps removed is 1700 (too high!) Trigger GroupRate (Hz) Tau+X820 XE+590 Electrons390 Muons280 3 Objects270 Photons120 B Phys110 Jets33 Misc28 TOTAL2600

60 EF Rates Total estimated EF rates with all overlaps removed is 390 Hz (Fixing L2 will likely come close to fixing EF as well) Trigger GroupRate (Hz) Tau+X187 Electrons77 Muons46 Photons46 BPhys45 3 Objects45 XE+42 Jets11 Misc11 TOTAL510

61 End of pp trigger operations in 2010 Trigger group Trigger chainRate [Hz] Single- muon EF_mu13_tight24 Di-muonEF_2mu628 Single- electron EF_e15_mediu m 38 Di-electronEF_2e10_loos e 2.4 Single- photon EF_g40_loose9 Di-photonEF_2g15_loos e 2.1 Single jetEF_L1J95_No Alg 11 METEF_xe40_noM u 6 Single-tauEF_tau84_loos e 6.8 Di-tauEF_2tau29_loo se1 2.6 Trigger Report61 Run record peak luminosity 2.1x10 32 cm -2 s -1 For a given threshold tighten selection Loose->medium->tight Non-isolation->isolation For a given threshold tighten selection Loose->medium->tight Non-isolation->isolation Go higher in p T Trigger evolution in 2010 L1 output 35kHz, L2 output 5kHz, EF output 400Hz Due to lack of time no physics data collected with 50ns BS

62 Example rates for different objects