The CMS High-Level Trigger Leonard Apanasevich University of Illinois at Chicago On behalf of the CMS collaboration

Outline Introduction to the CMS Detector Trigger Challenges at the LHC Trigger Architecture Level-1 Trigger High Level Trigger HLT Processing Times Trigger Commissioning with Cosmic Data Trigger Rates and Menus for Startup 29 Sept, 2009 ICCMSE 2009

The Compact Muon Solenoid (CMS) General purpose pp collider experiment: Searches for Higgs bosons, other new particles (SUSY,...) and new phenomena; Precision measurement of SM parameters like top and W masses, ...; Heavy ion program. For details see e.g.: The CMS experiment at the CERN LHC, JINST 0803:S08004,2008; 29 Sept, 2009 ICCMSE 2009

The Large Hadron Collider 29 Sept, 2009 ICCMSE 2009

Trigger Challenges at the LHC At design L = 1034 cm-2s-1 Interaction rate: ~1 GHz ~20 pp events per 25 ns crossing 1 kHz W events 10 Hz top events < 104 detectable Higgs decays/year Event size: ~1 MB (≈75M channels) → 1000 TB/sec for 1 GHz input rate ~300 MB/sec affordable Enormous rate reduction necessary! but without losing interesting physics Select in stages: Level-1 Triggers 1 GHz to 100 kHz (1/10000 reduction) High Level Triggers 100 kHz to 100 Hz (1/1000 reduction) 29 Sept, 2009 ICCMSE 2009

Trigger Architecture CMS has a two-tiered system to handle the LHC challenge Level-1 trigger reduces rate from 40 MHz (xing freq) to 100 kHz (max) High-Level triggers reduce rate from 100 kHz to O(100 Hz) Front end pipelines Readout buffers Processor farms Switching network Detectors Lvl-1 HLT Lvl-2 Lvl-3 Front end pipelines “Traditional”: 3 physical levels CMS: 2 physical levels Progress in networking/switching has justified CMS choice 29 Sept, 2009 ICCMSE 2009

Level 1: Only Calorimeter & Muon Complex Algorithms Huge amounts of data Simple Algorithms Small amounts of data 29 Sept, 2009 ICCMSE 2009

The Level-1 Trigger Electrons, Photons, Jets, MET Muons Reduce data rate from 40 MHz to 100 kHz while keeping the interesting physics events Custom electronic boards and chips Selects muons, electrons, photons, jets ET and location in detector Also Missing ET, Total ET, HT, and jet counts Total decision latency: 3.2 s 128 Level-1 trigger bits. Bits can be set using logical combinations of Level-1 objects. 3<||<5 ||<3 ||<3 ||<2.1 0.9<||<2.4 ||<1.2 29 Sept, 2009 ICCMSE 2009

Calorimeter Trigger Geometry Trigger towers:  =  = 0.087 HCAL ECAL HF EB, EE, HB, HE map to 18 RCT crates Provide e/g and jet, t, ET triggers 29 Sept, 2009 ICCMSE 2009

Muon Trigger Geometry DT: drift-tube system CSC: cathode strip chamber system RPC: resistive plate chamber system Initial coverage of RPC is staged to <1.6 Initial coverage of CSC 1st station is staged to <2.1 29 Sept, 2009 ICCMSE 2009

Level-1 Trigger Menu Determined by the physics priorities of the experiment L1 Menu optimized to fit within the L1 bandwidth Designed with a safety factor of 3 in mind underestimation of input cross sections, poor beam conditions, detector performance, etc. Realistic menu including double and mixed triggers for specific physics channels Two L1 menus currently available L = 8e29 cm-2s-1 (day-1 menu) L = 1e31 cm-2s-1 (MC studies) 29 Sept, 2009 ICCMSE 2009

High Level Trigger Strategy 40 MHz 100 kHz ~150 Hz 29 Sept, 2009 ICCMSE 2009

High Level Trigger (HLT) 1 PB/s High-Level triggers reduce rate from 100 kHz to O(150 Hz) HLT does event reconstruction “on demand” seeded by the L1 objects found, using full detector resolution Algorithms are essentially offline quality but optimized for fast performance Farm of Processors ONE event, ONE processor High Latency Simpler I/O Sequential programming 300 MB/s 29 Sept, 2009 ICCMSE 2009

L2 unpacking (MUON/ECAL/HCAL) HLT Algorithm Design L1 seeds L2 unpacking (MUON/ECAL/HCAL) Local Reco (RecHit) L2 Algorithm Filter L2.5 unpacking (Pixels) L2.5 Algorithm Each HLT trigger path is a sequence of modules Processing of the trigger path stops once a module returns false Reconstruction time is significantly improved by doing regional data-unpacking and local reconstruction across HLT All algorithms regional Seeded by previous levels (L1, L2, L2.5) “Local”: using one sub-detector only “Regional”: using small (η, φ) region 29 Sept, 2009 ICCMSE 2009

CMS Trigger/DAQ Evolution CMS DAQ is a number of functionally identical, parallel, small DAQ systems Build up 512512 switch from 8 6464 switches (DAQ slices) HLT filter farm is currently comprised of 720 dual-quad core, 2.6 GHz, 16 GB memory Dell PE 1950 PC’s 7 out of 8 cores (~5000) dedicated to HLT processing Remaining processor does event building, acts a resource broker, and other services. 29 Sept, 2009 ICCMSE 2009

m triggers: performance Signal efficiency (eHLT/[eL1*A]) Background rates at L = 1032 cm-2 s-1 Threshold: single m isolated m non-isolated Threshold: double m non-isolated Di-muon Single muon 29 Sept, 2009 ICCMSE 2009

e/g triggers: performance Background rates at L = 1032 cm-2 s-1 Efficiency on benchmark channels (estimated from MC simulation) Electrons: Photons: Electrons Photons 29 Sept, 2009 ICCMSE 2009

Jets and Missing ET Jets: Rates for L = 1032 cm-2 s-1 ETmiss: At HLT level, using an iterative cone algorithm with cone radius: out of calorimeter “towers” (HCAL + projection on ECAL - deposits above certain thresholds) ETmiss: Algebraic sum of “transverse energies” (ET = E sinq) of calorimeter objects Objects can be individual calorimeter cells or jets Can be used in combination with 1 or more jets Rates for L = 1032 cm-2 s-1 29 Sept, 2009 ICCMSE 2009

t- and b-tagging at HLT Hadronic t decays: b-tagging triggers: Level 1: seed from calorimeter towers in a narrow region Level 2: HLT jet reconstructions and ECAL isolation Level 3: Track isolation using pixel tracks in a smaller rectangular region or full tracks in a larger region b-tagging triggers: b-lifetime: Jet requirements + global pixel track reconstruction to determine primary vertex at HLT level  requirement of at least 2 tracks with transverse impact parameter significance above a certain threshold b → μ : Jet requirements + L2/L3 muon reconstruction  requirements on DR(m-jet) and pT(m) relative to the jet direction 29 Sept, 2009 ICCMSE 2009

HLT Timing Considerations HLT Timing is influenced by: Trigger menu (L1T & HLT) Determined by physics priorities Input L1Trigger rate Limited by bandwidth Parameters of various L1T algorithms, e.g., H/E HLT algorithms and configuration Standard trigger paths at HLT seeded by L1 trigger bits Order of modules and filters in a path Parameters of the modules and filters Allowable CPU performance: L1 input rate / number of processors @ 50 kHz: 100 ms/evt @ 100 kHz: 50 ms/ evt 29 Sept, 2009 ICCMSE 2009

HLT Processing Times Average time needed to run full Trigger Menu on L1-accepted events: 43 ms/event Core 2 5160 Xeon processor running at 3.0 GHz CPU times strongly dependent on HLT input “Tails” have a significant impact on the average time Will eliminate with time-out mechanism 29 Sept, 2009 ICCMSE 2009

Trigger Commissioning CMS has collected over 300 million cosmic ray events Can use these data for: compare data and L1 emulation check synchronization of signals from the various sub-detectors Measure trigger rates from cosmic rays and from noise Evaluate trigger efficiencies Compare resolutions between trigger and reconstructed objects 29 Sept, 2009 ICCMSE 2009

Trigger Commissioning (II) Synchronization of signals from different detector sub-systems demonstrated L1 emulator: bit by bit software emulation of the L1 trigger Excellent agreement between emulation and data L1 trigger efficiency as a function of offline reconstructed pT 29 Sept, 2009 ICCMSE 2009

Trigger Menu for Startup Muons: L1/L2/L3 muons run unprescaled at pT=20/9/3 GeV Electrons: No isolation requirements at L1 Unprescaled at pT=10 GeV with large pixel-matching window (LW) Photons: Unprescaled at pT=15 GeV with no isolation requirement Jets: No L1/HLT jet corrections applied at startup Unprescaled at pT=30 GeV (~60 GeV corr.) MinBias: Several algorithms installed based upon HF-tower ET over threshold, HF ET ring sums, Ecal ET, and pixel triplets Muon: 33 Hz Electron: 30 Hz Photon: 9 Hz Jet: 36 Hz MET & HT: 5 Hz B-Tau: 4 Hz MinBias: 14 Hz Cosmics/Halo: 7 Hz Total: 138 Hz Trigger Rates by Object Rates for L=8e29 cm-2s-1 29 Sept, 2009 ICCMSE 2009

Summary LHC Trigger is Challenging A new crossing every 25 ns with ~ 20 events at design luminosity  1 GHz of events input All data stored 3 s then all but 50-100 kHz rejected Rejection of 99.99% of data without losing discovery physics! Remaining event filtering done on a farm of CPUs running offline quality reconstruction algorithms Rate of storage to archive is ~ 150 Hz Rejection of 99.99997% of data without losing discovery physics!! Fast selection and high efficiency is obtained for the physics objects and processes of interest Commissioning work already going on using Cosmic data HLT and L1 have been developed for early luminosities The overall CPU requirement is within the system capabilities CMS trigger system is ready for collisions!!! 29 Sept, 2009 ICCMSE 2009