High Level Triggering Fred Wickens
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
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
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
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
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 • 2009 - 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
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
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 forATLAS (CMS very similar issues), but try to address some more general points
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
Physics at the LHC Interesting events are buried in a seaof soft interactions B physics High energy QCD jet production top physics Higgs production
The LHC and ATLAS/CMS • LHC has • design luminosity 1034 cm-2s-1 (In 2010 from 1031 - 1033 ?) • 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
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 • high PT programme (Higgs etc.) • searches for new physics
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
~ 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.
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
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
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
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
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
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
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 whichphilosophy is better
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
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
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
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.
Background Off-line Physics channel On-line Matching problem
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
Selection and rejection • as selection criteria are tightened • background rejection improves • BUT event selection efficiency decreases
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
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
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 • https://twiki.cern.ch/twiki/bin/view/Atlas/TriggerPhysicsMenu • 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 • https://twiki.cern.ch/twiki/bin/view/CMS/TriggerMenuDevelopment • The expected performance of ATLAS for different physics channels (including the effects of the trigger) is documented in http://arxiv.org/abs/0901.0512 (beware ~2000 pages)
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!
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 x20as input to L2
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.
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
LFS nodes UPS for CFS XPUs CFS nodes SDX1|2nd floor|Rows 3 & 2
threshold threshold MU 20 I name name isolated isolated EF in tagging mode mu 20 i _ passEF Naming Convention First Level Trigger (LVL1) Signatures in capitals e.g. HLT in lower case: • New in 13.0.30: • Threshold is cut value applied • previously was ~95% effic. point. • More details : see :https://twiki.cern.ch/twiki/bin/view/Atlas/TriggerPhysicsMenu
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).
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 99.999% 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.
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
L1 Rates 1031 14.4.0 Removing overlaps between single+multi EM gives 18 kHz Total estimated L1 rate with all overlaps removed is ~ 12 kHz
L2 Rates 1031 14.4.0 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