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The role of the CMS electron/photon trigger in the Higgs searches

The role of the CMS electron/photon trigger in the Higgs searches. Nadir Daci. Lake Louise Winter Institute – February 2013. Outline. Higgs searches @ LHC Triggering on the Higgs signal : a real challenge Level-1 electron/photon trigger

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The role of the CMS electron/photon trigger in the Higgs searches

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  1. The role of the CMS electron/photon trigger in the Higgs searches Nadir Daci Lake Louise Winter Institute – February 2013

  2. Outline • Higgs searches @ LHC • Triggering on the Higgs signal : a real challenge • Level-1 electron/photon trigger • High-Level trigger : online selection of electrons and photons • Online selections in 2 contexts : HZZ*4 electrons, H2 photons Lake Louise Winter Institute – February 2013 – Nadir Daci

  3. Higgs searches @LHC • Most sensitive channels : HZZ*4leptons (e,m) and Hgg • High pT isolated electrons/photons, low background in HZZ* • Isolated leptons and photons are easy to distinguish in the huge pp collisions’ hadronic environment • Precise measurement : reconstruct all decay products • ⇒ excellent lepton/photons reconstruction, identification, resolution are needed • The sum of the expected significances of the ZZ* and gg channels is above 5s • ⇒ The potential discovery is driven by these channels Lake Louise Winter Institute – February 2013 – Nadir Daci

  4. Higgs searches @LHC • Triggering efficiently on the Higgs signal is a real challenge • s(H) ~10-4 x s(W,Z) and ~10-8 x s(b) • Extremely rare signal : produced in 1/1010 8TeV collisions • We need a very high collision rate (inst. lumi.) to get a chance to observe Higgs boson decays • @ 1033 cm-2s-1 : 1 Higgs/100s • The trigger system defines the phase space available for any physics analysis • We need to optimize a signal region, while keeping some control regions for background evaluation + efficiency measurements b W , Z H ⇒ A highly efficient Higgs trigger is a critical step : it has played a major role in the recent new boson observation ! Lake Louise Winter Institute – February 2013 – Nadir Daci

  5. The CMS detector • The electron/photon reconstruction relies on the CMS Tracker and the ECAL sub-detectors. • The Tracker uses Si pixels (100x150 mm2) & strips (10cmx80mm) • The ECAL is made of 75848 lead tungstate crystals in a Barrel and two Endcaps. CMS BARREL (36 supermodules) ~ (RMoliere)2 ~ (22mm)2 26 X0 ~ 23 cm ENDCAPS (4 dees) • ECAL crystals : PbWO4 ⇒ dense, short radiation length • ⇒ compact (80% of the e/g energy in 1 crystal) • Fast scintillation : 80% light emitted in 25 ns Lake Louise Winter Institute – February 2013 – Nadir Daci

  6. Electron/photon reconstruction • The CMS ECAL is compact, hermetic and highly segmented. • Its excellent resolution plays a key role in electron/photon identification and energy measurements • The tracker resolution ensures very good measurements of low pT e± • The tracking performance is crucial to check the trigger isolation. ecal energy resolution • Electron / photon reconstruction : • Electron’s trajectory is curved by the B field (in the f direction). • Material budget* in front of the ECAL ⇒ electrons radiate Bremsstrahlung / photons convert • Crystals in the ECAL are clustered to recover Bremsstrahlung g/ conversion e+e- • Compatible tracks (Dh,Df) are reconstructed from tracker hits ⇒ electrons (*) + B field ⇒ ⇒ spread of the EM shower in f Lake Louise Winter Institute – February 2013 – Nadir Daci

  7. The Level-1 trigger • ECAL L1 trigger : electron/photon selection • Use coarse information (1 tower = 5x5 crystals) • Build 4 L1 EM candidates (most energetic pair of towers) per region (4x4 towers) • Keep the 4 candidates with highest ET in the entire ECAL 1 tower (5x5 crystals) L1 decision 3.5 ms 100 kHz max • VETOES • Fine Grain : 90% tower ET contained within 2 adjacent strips (tow ET>6 GeV) • H/E : ratio of ET in the corresponding HCAL and ECAL towers < 5% (L1 ET > 2 GeV) • STREAMS • Isolated stream : • - at least one « quiet corner »  ∑(5 adjacent towers) < 3.5 GeV • - 8 neighbour towers must pass FG and H/E selections • Non-Isolated stream hit • EM shower : • - narrow in h • spread in f Lake Louise Winter Institute – February 2013 – Nadir Daci

  8. Optimizations of the Level-1 trigger • ECAL spikes removal (barrel) : anomalous isolated high energy deposits • secondary particles ionizing the silicon of the photo-detectors • They dominate the L1 bandwidth at high triggering thresholds and high luminosity ! h T1 ⇒ better spikes identification / more electron mis-id. Electrons : - low energy ⇒ corresponding towers < T2 ⇒ not rejected - high energy ⇒ at least 2 crystals > T1 ⇒ not rejected f Spike ET(tower) > T2 ⇒ tower set to 0 Real EM shower if 2 crystals in a strip > T1 (EM shower ⇒f spread) • Optimization : simulation of 3x10 settings using 2010 data Setting : 258 MeV ; 8 GeV ⇒ Bandwidth optimization : Rejects 96% spikes Keeps 98% electrons Lake Louise Winter Institute – February 2013 – Nadir Daci

  9. Optimizations of the Level-1 trigger • Improvements to address issues from the constantly rising Pile-Up SPIKE CONTAMINATION vs PU • Spike removal : new setting used in 2012 • ⇒ simulated using late 2011 data and special high pile-up runs • ⇒ lower spike contamination (divided by ~1.6) • RCT calibration : improved for 2012 data taking • ⇒ corrections for the energy loss in the pre-shower (Endcaps) • Transparency : the irradiation degrades the ECAL crystals transparency • ⇒ measure this loss with lasers (monitoring/calibration tool) • ⇒ weekly average computed in 11 Endcaps h rings • ⇒ re-calibrate L1 trigger towers for the following week L1 TRIGGER EFFICIENCY : ENDCAPS 15 GeV EE online EE corrected ⇒ The transparency corrections allow to recover trigger efficiency in the EE. Lake Louise Winter Institute – February 2013 – Nadir Daci

  10. Performance of the Level-1 trigger • L1 thresholds : • To keep the L1 rate under control, the lowest thresholds are prescaled (select 1/N events) • The High Level Trigger must use unprescaled « L1 seeds » to maximize the amount of data available in the physics analyses ! • ⇒ We were able to keep 8 GeV(2010), 15 GeV(2011) and finally 20 GeV (2012) unprescaled ! • L1 electron/photon efficiency (2012 vs 2011) : • Tag & Probe method : use the Z mass constraint to find real e+e- pairs • ⇒ the tag triggers the event ; the efficiency is the fraction of probes triggering a given threshold Barrel, Endcaps (20 GeV) ⇒ Excellent efficiency in 2011 ⇒ Sharper curves in 2012 ⇒ The 2012 endcaps curve are closer to EB (transparency + PS corrections) 2011 2012 Lake Louise Winter Institute – February 2013 – Nadir Daci

  11. The High-Level trigger • HLT processing : • The HLT receives an input of 100kHz from the L1 • ⇒ the allowed output is 1kHz • It uses a CPU farm of 13k cores • ⇒ 20k event processors (hyperthreading) • ⇒ 160 ms / event • Strategy : split the algorithms into several sub-levels (L2 / L2.5 / L3) • Stream-lined offline-like reconstruction : • ⇒ first reconstruct ECAL super-clusters in regions centered on L1 candidates • ⇒ selection ⇒ launch tracking ⇒ match track to SC ⇒ e candidate Lake Louise Winter Institute – February 2013 – Nadir Daci

  12. The High-Level trigger • HLT tracking efficiency, HLT stability vs pile-up Ntracks vs PU Tracking efficiency vs h • Total HLT • “core” • “parking” online offline PU=14 PU=30 Recorded cross-section vs inst. luminosity • The online tracking efficiency is similar to the offline one at central h, slowly degrades at higher h • The number of HLT tracks increases linearly with PU • ⇒ the HLT tracking is able to handle high PU • The recorded cross-section is almost flat vs PU • ⇒ the HLT physics is stable even for high PU CMS Preliminary 2012 Lake Louise Winter Institute – February 2013 – Nadir Daci

  13. High-Level Trigger : HZZ* • Golden mode : HZZ*4 leptons (cleanest signature in the pp environment) • Here : focus on trigger in the 4 electrons mode. • Analysis requires 4 isolated electrons : pT > 20, 10, 7, 7 GeV • Triggering on low pTleptons is a major challenge : • ⇒ high rate and potentially more fakes ⇒ The initial trigger strategy is an or of the following trigger paths : - 1 tight electron pT>27 GeV - 2 loose electrons pT>17, 8 GeV - 3 loose electrons pT>10 GeV ⇒ Expected efficiency on the Higgs signal : e(H120) = 97.6% e(H130) = 97.7% e(H150) = 98.5% Lake Louise Winter Institute – February 2013 – Nadir Daci

  14. High-Level Trigger : HZZ* • L1 and HLT optimizations : • Trigger strategy in 2011 and 2012 : • 2011 data, Tag&Probe : • pT>17, 8 GeV + tight calorimeter ID + loose ISO & track ID⇒ 5 HzL1 : (12,5) GeV • pT>17, eT(SC)>8, m>30 + very tight selection⇒ 0.2 HzL1 : 12 GeV • 2012 data, Tag&Probe : • pT>17, 8 GeV + tight calorimeter ID, very loose ISO & track ID ⇒ 5 HzL1 : (13,7) GeV • pT>20, eT(SC)>4, m>50 + very tight selection ⇒ 2 Hz L1 : 18 GeV Iso Effect of the HLT endcaps corrections • Typical rates of L1 seeds + 95% efficiencies (barrel/endcaps) • Single electron 2011 (3.5e33): 12 GeV ⇒ 0.5 kHz (19/21 GeV) • Single electron 2012 (6e33) : 20 GeV ⇒ 13 kHz (28/32 GeV) • Double electron 2011 (3.5e33): 12 & 5 GeV ⇒ 7.2 kHz • Double electron 2012 (6e33) : 13 & 7 GeV ⇒ 8 kHz Lake Louise Winter Institute – February 2013 – Nadir Daci

  15. High-Level Trigger : HZZ* • Initial trigger : di-electrons • Efficiency on the Higgs signal : mH>120 GeV ⇒ 96% • Additional trigger : tri-electrons • L1 seed : 12 GeV, 7 GeV, 5 GeV ⇒ 3 kHz • HLT : pT> 15 GeV, 8 GeV, 5 GeV + loose calorimeter/tracker identification ⇒ 3.5 Hz • 3 electrons ⇒ lower fake rate ⇒ looser identification cuts than di-electrons trigger • Gain efficiency : +3% @ 125 GeV ! • Trigger efficiency evaluated on the signal (MC Higgs) • Fraction of Higgs events selected (analysis cuts) passing the trigger selections • mH>120 GeV ⇒ e > 98% Lake Louise Winter Institute – February 2013 – Nadir Daci

  16. High-Level Trigger : Hgg • Signature : narrow mass peak over a large irreducible background (QCD di-photons) • 2 isolated high ET photons ⇒ fully benefits from the ECAL high resolution • Di-photons trigger : [path 1] OR [path 2] • path 1 : L1 = 22 GeV eT1 > 36 GeV eT2 > 22 GeV • This path requires one HLT cluster, then searches for a second cluster in the rest of the ECAL. • path 2 : L1 = (13 GeV, 7 GeV) eT1 > 26 GeV eT2 > 18 GeV m>60 GeV • This path requires directly two HLT clusters. • For both paths, the clusters must pass isolation, identification (shape ) and invariant mass cuts. • Efficiency measurement : • Tag&Probe Zee : events triggering [pTele>32, eTSC>17, m>50] • match 1 offline electron + 1 offline photon to the HLT electron [tag] • match 1 offline photon to the HLT super-cluster [probe] • ⇒ L1 efficiencies : e(13 GeV, 7 GeV) = 99.7% e(22 GeV) = 97.7% • ⇒ HLT efficiencies : e(path 1 OR path 2) = 99.5% Lake Louise Winter Institute – February 2013 – Nadir Daci

  17. Conclusion • Maintaining a highly efficient triggering on electrons and photons is critical in the Higgs searches. • In the hard LHC environment (rising irradiation, increasing pile-up), it has been a real challenge ! • The ECAL Level-1 optimizations allowed to keep L1 thresholds low enough : • The High Level Trigger was optimized as well : tracking, processing speed… • Prospective studies based on the Higgs events kinematics helped defining a trigger strategy. • The overall trigger efficiency on Higgs signal (mH>120) reaches 96% in HZZ*4electrons Lake Louise Winter Institute – February 2013 – Nadir Daci

  18. Conclusion : physics results • Thanks to the trigger and analysis optimizations, major physics results were obtained ! Discrimination 0+/0- Mass ZZ*4L ZZ* channel Mass gg

  19. BACKUP Lake Louise Winter Institute – February 2013 – Nadir Daci

  20. The High-Level trigger • HLT processing : • The HLT receives an input of 100kHz from the L1, the allowed output is 1kHz • It uses a CPU farm of 13k cores ⇒ 20k event processors (hyperthreading) • ⇒ 160 ms / event • Strategy : split the algorithms into several sub-levels (L2 / L2.5 / L3) • L2 : reconstruct EM super-clusters ⇒ apply low ET and h<2.5 • L2.5 e± : match EM super-clusters to a pair of pixel hits : extrapolate from ECAL to tracker through B field • L3 e±: reduce the background rate using calo. & tracker isolation (tracks compatible with e± vtx) • L3 g: background = high ET particles decaying into photons ⇒ calo. & tracking isolation around the g candidate Lake Louise Winter Institute – February 2013 – Nadir Daci

  21. Tracking @ HLT Tracking efficiency Tracking efficiency vs pT online offline online offline Lake Louise Winter Institute – February 2013 – Nadir Daci

  22. Particle Flow @ HLT Jet vs PF Jet @ HLT Lake Louise Winter Institute – February 2013 – Nadir Daci

  23. HLT Stability vs Pile-Up Lake Louise Winter Institute – February 2013 – Nadir Daci

  24. HLT data parking • Data recorded for future offline processing • Vector Boson Fusion: Mjj>650 GeV , Δηjj>3.5 • MultiJet: 4 Jet with pT>50 GeV • HT and MHT: For susy searches • MuOnia: low Mμμ (Jpsi, Psi`, ..) • DoubleMu: Mu13_Mu8 • TauParked: ττ (with 3prong decays) • ⇒ 5% of parked data are promptly reconstructed for monitoring purpose • ⇒ On average 350 Hz of "core physics" is promptly reconstructed and 300 Hz of data is parked for future reconstruction Lake Louise Winter Institute – February 2013 – Nadir Daci

  25. HLT data scouting • Data not recorded in main trigger stream • Additional un-prescaled trigger to survey if unexpected physics shows up (exotica dijet searches) • HT>250 GeV ⇒ 1kHz • ⇒ reduced event content (only objects, no raw data) • ⇒ bandwidth ~ few MB/s Lake Louise Winter Institute – February 2013 – Nadir Daci

  26. HLT menu example : 6 Hz/nb Lake Louise Winter Institute – February 2013 – Nadir Daci

  27. HLT streams 10 kHz Pi0/Eta Alignment, Calibration and Luminosity (Event Size ~kB) 500Hz LumiPixel 1.5 kHz EcalPhiSym 1.5 kHz RPCMonitoring 100 Hz Calibration 1 kHz Stream A Physics(Full Event Content),Scouting (Event Size ~kB) 40 Hz Express 1.2kHz Data Scouting 10 kHz NanoDST Trigger Studies (Event Size ~kB) Data Quality and Monitoring (Various Event Content) 25 Hz Stream B 75 Hz DQM 1 kHz HLTDQMResults 150 Hz HLTDQM 20 Hz HLTMonitoring

  28. L2.5 e± : match EM super-clusters to pixel hits : extrapolate from ECAL to tracker through B field • ⇒ recompute vertex position, then extrapolate to the next pixel layer • ⇒ if a matched hit is found : identified as electron else e± candidate is rejected

  29. Ele27_CaloIdT_CaloIsoT_TrkidT_TrkIsoT || Ele17_CaloIdL_CaloIsoVL_Ele8_CaloIdL_CaloIsoVL || TripleEle10_3CaloIdL_3TrkIdVL • 2011 : Ele17_Ele8_CaloIdT_CaloIsoVL_TkIdVL_TkIsoVL [5.6 Hz]L1 : EG12_5 [7.2 kHz] • Tag&Probe : Ele17_CaloIdVT_CaloIsoVT_TrkIdT_TrkIsoVT_SC8_Mass30L1 : EG12 • 2012 : Ele17_CaloIdT_CaloIsoVL_TrkIdVL_TrkIsoVL_Ele8_CaloIdT_CaloIsoVL_TrkIdVL_TrkIsoVL L1:EG13_7 • T&P : Ele20_CaloIdVT_CaloIsoVT_TrkIdT_TrkIsoVT_SC4_Mass50 • The tag&probe method is used to evaluate the reconstruction and trigger efficiencies • ⇒ Use the Z mass constraint to find events containing a pair of real electrons (OS) • ⇒ Di-electron events : • - the “tag” passes a tight selection & is matched to the single ele trigger • - the “probe” passes the analysis selection • ⇒ efficiency = fraction of probes passing studied criteria (here : each leg of the double ele trigger) 40<mZ1<120 GeV & 4<mZ2<120 GeV

  30. HZZ*4 electrons trigger single threshold double threshold triple threshold pT3 > 10 GeV

  31. H2 photons trigger

  32. The Level-1 trigger • Level 1 structure (from ECAL to L1 decision) HCAL GmT 1 tower (5x5 crystals) RCT GCT L1 decision 3.5 ms ⇒ 100 kHz 800 Mb/s 1.2 Gb/s GT TCC x 68/48 (EB/EE) RCT : Build 4 L1 EM candidates (most energetic pair of towers) GCT : Keep the 4 candidates with highest ET in the entire ECAL GT : Apply ET thresholds and simple isolation cuts • EG object selection @ L1 max • Fine Grain : 90% tower ET contained within 2 adjacent strips (tow ET>6 GeV) • H/E : ratio of ET in the corresponding HCAL and ECAL towers < 5% • (candidate ET > 2 GeV) • Isolated stream : • - at least one « quiet corner »  ∑(5 adjacent towers) < 3.5 GeV • - 8 neighbour towers must pass FG and H/E selections • Non-Isolated stream • VETOES • STREAMS hit Lake Louise Winter Institute – February 2013 – Nadir Daci

  33. L1 menu example : 6 Hz/nb Lake Louise Winter Institute – February 2013 – Nadir Daci

  34. Electron/photon reconstruction • Electron / photon reconstruction : • Crystals are clustered within (h,f) windows • ⇒ gather clusters into Super-Clusters to recover Bremsstrahlung deposits • Photons are reconstructed from these Super-Clusters • Electrons are built from a track and a Super-Cluster : • - the track is extrapolated from the tracker innermost layer (p + B field + brem.) • - the matching between the track and the Super-Cluster is checked (Dh, Df) • The compacity of the crystals allows to build a compact, hermetic, highly segmented EM calorimeter • The excellent energy resolution of the ECAL (<0.5% @100 GeV) induces a better identification, for both triggering and offline analysis purposes. • The tracker resolution allows to recover a very good performance for low pT electrons and in general, is crucial to check the isolation at the trigger level. ecal energy resolution Lake Louise Winter Institute – February 2013 – Nadir Daci

  35. Physics results : HZZ* Lake Louise Winter Institute – February 2013 – Nadir Daci

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