Il Trigger di Alto Livello di CMS

1. Il Trigger di Alto Livello di CMS N. Amapane – CERN Workshop su Monte Carlo, la Fisica e le simulazioni a LHC Frascati, 25 Ottobre 2006

2. SUPERCONDUCTING COIL Total weight : 12,500 t Overall diameter : 15 m Overall length : 21.6 m Magnetic field : 4 Tesla Resistive Plate Cathode Strip Chambers (CSC) Drift Tube Resistive Plate Chambers (RPC) Chambers (RPC) Chambers (DT) The Compact Muon Solenoid CALORIMETERS ECAL Scintillating PbWO4 HCAL Plastic scintillator Crystals brass sandwich IRON YOKE TRACKER MUON ENDCAPS MUON BARREL Silicon Microstrips Pixels The CMS High Level Trigger

3. pp interactions Max DAQ 100 kHz LHC Event Rates srate @ nominal LHC luminosity Pile-up Machine Rate: 40 MHz On-line trigger selection Select 1:4x105 Decide every 25 ns! Acceptable storage rate: 100 Hz Off-line analysis Signals Particle mass (GeV/c2) The CMS High Level Trigger

4. Trigger Architecture • Start from 40 MHz → Decision every 25 ns • Too small even to read raw data • Selection in multiple levels, each taking a decision using only part of the available data • The first level (L1) is only feasible with dedicated, synchronous (clock driven) hardware 40 MHz 100 kHz • CMS choice: All further selection in a single phisical step (HLT) • Build full events and analyze them “as in offline” • Invest in networking (rather than in dedicated L2 hardware) 100 GB/s!! 100 Hz The CMS High Level Trigger

5. Level-1 Trigger • Custom programmable processors • To minimise latency • Synchronous decision every 25 ns • delayed by 3.2 ms = 128 BX (Max depth of pipeline memories) • Max output  max DAQ input • Design: 100 kHz; at startup: 50 kHz • Only m detectors and calorimeters • e/g, m, t jets, jets, ETmiss, SET • Selection by the “Global Trigger” • 128 simultaneous, programmable algorithms, each allowing: • Thresholds on single and multiple objects of different type • Correlations, topological conditions • Prescaling The CMS High Level Trigger

6. Trigger detectors • ECAL up to |h|<3 • HCAL: |h|< 3 (HB, HE); 3<|h|<5.191 (HF) • Muon (DT, CSC, RPC): |h|<2.4 • But trigger electronics only up |n|<2.1 The CMS High Level Trigger

7. L1 Trigger Table For L= 2x1033 cm-2s-1 (CMS Physics TDR v.2) Assume 50 KHz DAQ available at low luminosity + factor 3 safety The CMS High Level Trigger

8. DAQ L1 Event building Modular, 8 “slices” 4 to be installed at startup HLT farm (O(2000 CPU) The CMS High Level Trigger

9. CMS HLT • Run on farm of commercial CPUs: a single processor analyzes one event at a time and comes up with a decision • Has access to full granularity information • Freedom to implement sophisticated reconstruction algorithms, complex selection requirements, exclusive triggers… Constraints: • CPU time (Cost of filter farm) • Reject events ASAP: set up internal “logical” selection steps • L2: muon+ calorimeter only • L3: use full information including tracking • Must be able to measure efficiency from data • Use inclusive selction whenever possible • Single/double object above pT/ET, etc. • Define HLT selection paths from the L1 • Keep output rate limited (obvious…) The CMS High Level Trigger

10. Example: Muon HLT Integral rate (ℒ = 1034 cm-2s-1) • Key is to achieve the best pT resolution (and suppress non-prompt muons and b,c decays) c,b Rate(Hz) p/K W Z/g* 100 Hz KL t Threshold on generated pT (GeV/c) The CMS High Level Trigger

11. HLT Muon Reconstruction • Level-2: “confirm” L1 refitting hits in the muon chambers with full granularity • Regional reconstruction seeded by L1 muons • Kalman filtering iterative technique • pT resolution: 10% to 16% depending on h (muons from W decays) • Level-3: Inclusion of Tracker Hits • Regional tracker reconstruction seeded by L2 muons • pT resolution: achieve full CMS resolution of 1% to 1.7% depending on h(muons from W decays) • Isolation in calorimeters (at L2) and tracker (L3) to suppress b,c decays and non-prompt muons The CMS High Level Trigger

12. overlap endcaps barrel  = 0.12  = 0.14  = 0.17  = 0.013  = 0.015  = 0.018 1/pT Resolution Level-2: Improve L1 barr. ovr. end. 0.17 0.22 0.20 Level-3: Full resolution 10x scale The CMS High Level Trigger

13. Single Muon Rates ℒ = 1034 cm-2s-1 • L2,L3 reduce the rate by improving the pT resolution • L2 is justified as it reduces the rate to allow more time for processing data from the tracker 100 Hz The CMS High Level Trigger

14. HLT Reconstruction • g • L2: cluster ECAL deposits into “superclusters” and apply ET threshold • L3: isolation in HCAL and tracker • e • L2 common with g • L2.5: match the supercluster with a track in the pixel detector • L3: isolation in HCAL and tracker, cut on E/p • Jets • Iterative cone algorithm in calorimeters + energy corrections (non-linearity) • MET • Vector sum of transverse energy deposit in calorimeters, incl. muons • Tau • Look for isolated “narrow” jet, either: • Isolation in ECAL+pixel • Isolation in the tracker • B-tagging • L2.5: impact parameter with pixel track stubs • L3: with regional track reconstruction The CMS High Level Trigger

15. Setting trigger tables • HLT trigger paths start from corresponding L1 paths • Tresholds are set distributing bandwidth to the various paths in order to maximize efficiencies • There can be significant overlaps • Iterative process • Thresholds (and streams) will change with luminosity • And according to the physics of interest at the time of operation • Reference: 2x1033 cm-2 s-1 • Evolution of selection with luminosity is a delicate issue, up to now studied in detail only for jet (with prescales) The CMS High Level Trigger

16. HLT Trigger Table L= 2x1033 cm-2s-1 (CMS Physics TDR v.2) contd… The CMS High Level Trigger

17. HLT Trigger Table (cont). L= 2x1033 cm-2s-1 (CMS Physics TDR v.2) 120 Hz The CMS High Level Trigger

18. Some HLT Efficiencies At low luminosity, relative to events in detector acceptance: W en 68% W mn 69% Z mm 92% Z ee 90% tt m+X 72% H(115 GeV)gg 77% H(150) ZZ4m 98% H(120) ZZ4e 90% A/H(200 GeV)2t 45% H+(200-400)t+n 58% The CMS High Level Trigger

19. Triggers and offline analysis • The HLT selection can have an impact on analysis • May reduce signal efficiency and phase-space • Unless off-line selection is tighter than HLT • Simulation of the HLT selection is a part of analysis! • Specific exclusive triggers can be implemented for channels where the default trigger tables are not enough, but: • How much the selection costs in term of rate and CPU? • Is it possible to understand the selection efficiency from the data? The CMS High Level Trigger

20. Conclusions • Trigger at LHC is an integral part of the event selection • CMS uses a single physical step after L1, to achieve a rejection factor of ~1000 • HLT algorithms have the full event data available and no limitation on complexity, except for CPU time • Inclusive triggers based on the presence on one or more objects above pT/ET thresholds are normally sufficient to get good efficiency on most signal • More sophisticated selections are possible if necessary The CMS High Level Trigger

21. References • CMS DAQ/HLT TDR, 2002, CERN-LHCC-2002-026 • Full study of HLT rates, timing, benchmark signal efficiencies • CMS Physics TDR Volume 1 (2006), CERN-LHCC-2006-001 • Detector performance, reconstruction • CMS Physics TDR Volume 2 (2006), CERN-LHCC-2006-021, • Update of HLT rates and trigger tables (Appendix E) The CMS High Level Trigger