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. Nicola Amapane2 The CMS High Level Trigger MUON BARREL CALORIMETERS Silicon Microstrips Pixels ECAL Scintillating PbWO 4 Crystals Cathode Strip Chambers (CSC) Resistive Plate Chambers (RPC) Drift Tube Chambers (DT) Resistive Plate Chambers (RPC) SUPERCONDUCTING COIL IRON YOKE TRACKER MUON ENDCAPS Total weight : 12,500 t Overall diameter : 15 m Overall length : 21.6 m Magnetic field : 4 Tesla HCAL Plastic scintillator brass sandwich The Compact Muon Solenoid

3. Nicola Amapane3 The CMS High Level Trigger LHC Event Rates Acceptable storage rate: 100 Hz Max DAQ 100 kHz Machine Rate: 40 MHz pp interactions Particle mass (GeV/c 2 )  rate @ nominal LHC luminosity Pile-up On-line trigger selection Select 1:4x10 5 Decide every 25 ns! Off-line analysis Signals

4. Nicola Amapane4 The CMS High Level Trigger Trigger Architecture 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 kHz 100 Hz 40 MHz 100 GB/s!! 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

5. Nicola Amapane5 The CMS High Level Trigger Level-1 Trigger Custom programmable processors –To minimise latency Synchronous decision every 25 ns –delayed by 3.2  s = 128 BX (Max depth of pipeline memories) Max output  max DAQ input –Design: 100 kHz; at startup: 50 kHz Only  detectors and calorimeters –e/ , ,  jets, jets, E T miss,  E T Selection by the “Global Trigger” –128 simultaneous, programmable algorithms, each allowing: Thresholds on single and multiple objects of different type Correlations, topological conditions Prescaling

6. Nicola Amapane6 The CMS High Level Trigger Trigger detectors ECAL up to |  |<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

7. Nicola Amapane7 The CMS High Level Trigger L1 Trigger Table For L= 2x10 33 cm -2 s -1 (CMS Physics TDR v.2) Assume 50 KHz DAQ available at low luminosity + factor 3 safety

8. Nicola Amapane8 The CMS High Level Trigger DAQ Event building HLT farm (O(2000 CPU) L1 Modular, 8 “slices” 4 to be installed at startup

9. Nicola Amapane9 The CMS High Level Trigger 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…)

10. Nicola Amapane10 The CMS High Level Trigger Example: Muon HLT Key is to achieve the best p T resolution (and suppress non- prompt muons and b,c decays) Integral rate ( ℒ  = 10 34 cm -2 s -1 ) KLKL   /K  c,b  W Z/  * Threshold on generated p T (GeV/c) 100 Hz Rate (Hz)

11. Nicola Amapane11 The CMS High Level Trigger 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 –p T resolution: 10% to 16% depending on  (muons from W decays) Level-3: Inclusion of Tracker Hits –Regional tracker reconstruction seeded by L2 muons –p T resolution: achieve full CMS resolution of 1% to 1.7% depending on  (muons from W decays) Isolation in calorimeters (at L2) and tracker (L3) to suppress b,c decays and non-prompt muons

12. Nicola Amapane12 The CMS High Level Trigger 1/p T Resolution barrel overlap endcaps  = 0.12  = 0.14  = 0.17  = 0.013  = 0.015  = 0.018 Level-2: Improve L1 barr.ovr.end. 0.170.220.20 Level-3: Full resolution 10x scale

13. Nicola Amapane13 The CMS High Level Trigger Single Muon Rates ℒ  = 10 34 cm -2 s -1 100 Hz L2,L3 reduce the rate by improving the p T resolution L2 is justified as it reduces the rate to allow more time for processing data from the tracker

14. Nicola Amapane14 The CMS High Level Trigger HLT Reconstruction  –L2: cluster ECAL deposits into “superclusters” and apply E T threshold –L3: isolation in HCAL and tracker e –L2 common with  –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

15. Nicola Amapane15 The CMS High Level Trigger 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: 2x10 33 cm -2 s -1 –Evolution of selection with luminosity is a delicate issue, up to now studied in detail only for jet (with prescales)

16. Nicola Amapane16 The CMS High Level Trigger HLT Trigger Table L= 2x10 33 cm -2 s -1 (CMS Physics TDR v.2) contd…

17. Nicola Amapane17 The CMS High Level Trigger HLT Trigger Table (cont). 120 Hz L= 2x10 33 cm -2 s -1 (CMS Physics TDR v.2)

18. Nicola Amapane18 The CMS High Level Trigger Some HLT Efficiencies At low luminosity, relative to events in detector acceptance: W  e 68% W  69% Z  92% Z  ee90% tt  +X72% H(115 GeV)  77% H(150)  ZZ  4  98% H(120)  ZZ  4e90% A/H(200 GeV)  2  45% H + (200-400)   58%

19. Nicola Amapane19 The CMS High Level Trigger 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?

20. Nicola Amapane20 The CMS High Level Trigger 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 p T /E T thresholds are normally sufficient to get good efficiency on most signal More sophisticated selections are possible if necessary

21. Nicola Amapane21 The CMS High Level Trigger 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)