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 : – 2x10 32 ; 2011: up to 3.6x10 33 ; 2012: up to 6x10 33 –Design bunch separation 25 ns (bunch length ~1 ns) Currently running with 50 ns This results in – ~ 23 interactions / bunch crossing (Already exceeded!) ~ 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 evolving with: luminosity and HLT capacity –i.e. Balance between high PT programme (Higgs etc.) b-physics programme (CP measurements) 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 Limited detector data –Calo + Muon only –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 ~4 kHz (was ~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 - can 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 ~4 kHz ~ 400 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 ~6 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 ~6 GB/s ~ 600 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

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 ~400 Hz –(was ~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 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

31 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

32 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

33 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

34 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

35 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

36 Trigger Evolution in ATLAS

37 Matching problem Ideally –off-line algorithms select all the physics channel and no background –trigger algorithms select all the physics accepted by the off-line selection (and no background) In practice, neither of these happen –Need to optimise the combined selection For this reason many trigger studies quote trigger efficiency wrt events which pass off-line selection –BUT remember 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 Background Physics channel Off-line On-line

38 Other issues for the Trigger Optimisation of cuts –Balance background rejection vs efficiency 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 for the trigger –keep as up-to-date as possible –allow for the lower precision in the trigger cuts Code used in trigger needs to be fast + very robust –low memory leaks, low crash rate

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 (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!

40 ATLAS 2e2μ candidate with m 2e2μ = GeV p T (e +, e -, μ -, μ + )= 41.5, 26.5, 24.7, 18.3 GeV m (e + e - )= 76.8 GeV, m(μ + μ - ) = 45.7 GeV

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42 Excluded at 95% CL Putting all channels together  combined constraints H  γγ, H  ττ H  WW (*)  lνlν H  ZZ (*)  4l, H  ZZ  llνν H  ZZ  llqq, H  WW  lνqq W/ZH  lbb+X not included Excluded at 99% CL Expected if no signal < m H < GeV 131 <m H < 453 GeV, except GeV GeV 133 <m H < 230 GeV, 260 < m H < 437 GeV LEP ATLAS+CMS Combination ATLAS today

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45 Additional Foils

46 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

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

48 Price to pay for the high luminosity: larger-than-expected pile-up Z  μμ Period A: up to end August Period B: Sept-Oct Pile-up = number of interactions per crossing Tails up to ~20  comparable to design luminosity (50 ns operation; several machine parameters pushed beyond design) LHC figures used over the last 20 years: ~ 2 (20) events/crossing at L=10 33 (10 34 ) Challenging for trigger, computing resources, reconstruction of physics objects (in particular E T miss, soft jets,..) Precise modeling of both in-time and out-of-time pile-up in simulation is essential Event with 20 reconstructed vertices (ellipses have 20 σ size for visibility reasons)

49 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 :

50 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.

51 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

52 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

53 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

54 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

55 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

56 End of pp trigger operations in 2010 Trigger groupTrigger chainRate [Hz] Single-muonEF_mu13_tight24 Di-muonEF_2mu628 Single-electronEF_e15_medium38 Di-electronEF_2e10_loose2.4 Single-photonEF_g40_loose9 Di-photonEF_2g15_loose2.1 Single jetEF_L1J95_NoAlg11 METEF_xe40_noMu6 Single-tauEF_tau84_loose6.8 Di-tauEF_2tau29_loose12.6 Trigger Report 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

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58 Other issues for the Trigger – cont’d 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)

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

60 CMS works!