1. Valeria Perez Reale University of Bern On behalf of the ATLAS Physics and Event Selection Architecture Group 1 ATLAS Physics Workshop Athens, May 21 2003 Electron/gamma Selection in the ATLAS Trigger

2. Valeria Perez Reale ATLAS Physics Workshop 2003 2 Introduction  Physics Motivation  Electron Triggers  Electron/gamma selection  Reconstruction Method: LVL1 LVL2 EF  Conclusions

3. Valeria Perez Reale ATLAS Physics Workshop 2003 3 Motivation LHC extreme demands on Detectors and high rates and event sizes for TDAQ systems Large Event Size  1.6 Mbyte High Rate  40 MHz Provide an early and efficient rejection of high rate background online (40MHz 200 Hz) ATLAS Physics Reconstruction and Analysis performed in 3 Levels: LVL1 and HLT Rate reduction rate of O(10 6 )

4. Valeria Perez Reale ATLAS Physics Workshop 2003 4 High Level Trigger 2  s latency O(10) ms latency seconds latency LVL1 Hardware based (FPGA) Coarse calorimeter granularity Trigger muon detectors LVL2 Region of Interest (RoI) Specialized algorithms Fast selection with early rejection EF Full Event available Offline derived algorithms Seeded by LVL2 Best calibration/alignment Latency less demanding see C. Padilla’s talk

5. Valeria Perez Reale ATLAS Physics Workshop 2003 5 Channels of interest for e- Trigger  H  ZZ*  4l The most promising channel for Higgs if m H >130 GeV  H  WW,ZZ  l jj, ll, lljj Interesting for heavy Higgs m H <1 TeV  tt production Rare decay modes of one top-quark  Z  ee Important for e.m. calorimeter calibration Physics Coverage: Higgs(SM, MSSM), new gauge bosons, extra dimensions, SUSY, W, top (see M. Wielers talk)

6. Valeria Perez Reale ATLAS Physics Workshop 2003 6 Electron Triggers Inclusive Trigger: e25i, 2e15i low lumi 2 *10 33 cm -2 s -1 e30i, 2e20i high lumi 10 34 cm -2 s -1 Pre-scaled Trigger: Cross-section measurements extending to smaller pT values Understanding of background/selection cuts … Calibration Trigger: Select samples of e.g. Z->ee, , bb, W-> jj events Pre-scaled Trigger: Calculate vertex position in x, y and z luminosity monitoring see S. Tapprogge’s talk

7. Valeria Perez Reale ATLAS Physics Workshop 2003 7 Electron Selection  Calorimeter E T and shower shape variables to identify e.m. clusters  Tracks in the Inner Detector: Matching track to calorimeter cluster (electron) Conversion recovery (photon)  Electron-identification with Transient Radiation information  Bremsstrahlung recovery Variation in amount of detail for LVL2 and EF

8. Valeria Perez Reale ATLAS Physics Workshop 2003 8 Reconstruction method ReconstructionLVL1 LVL2EF EM ClusterT2CaloLArClusterRec Track IDScan SiTrack TRTkalman xKalman++ iPatRec Combined cluster-track matching egammaRec

9. Valeria Perez Reale ATLAS Physics Workshop 2003 9 LVL1 Calorimeter Trigger for: EM objects (e/  ) jets tau’s E T miss  Aprox. 7200 trigger towers with a granularity of  x  =0.1x0.1 Distinguish EM and HAD tower  Energy Steps of 1 GeV,analogue summation of calorimeter electronics  Signal extends over several bunch crossings use bunch crossing identification electronics Simulate the L1 calorimeter Trigger and reproduce data produced by hardware e.g. from cluster, energy summing, jets.

10. Valeria Perez Reale ATLAS Physics Workshop 2003 10 LVL1 preliminary rates Low Luminosity 2 *10 33 cm -2 s -1 EM2048 kHz EM20I17 kHz EM2521 kHz EM25I5.3 kHz Good agreement with LVL1 TDR TDR 22 Hz High Luminosity 10 34 cm -2 s -1 EM3063 kHz EM30I23 kHz TDR 22 Hz without isolation with isolation

11. Valeria Perez Reale ATLAS Physics Workshop 2003 11 T2Calo  Select isolated EM objects from jets using cluster energy and certain shower shape variables  Refines the LVL1 position from the cell with highest energy in the 2nd sampling of EM calorimeter  Calculates total transverse energy deposited in the EM calo and leaks into the hadron calo Clustering algorithm for EM showers, seeded by LVL1 EM Region of Interest positions. Algorithm Steps: Single 20 GeV electrons Good agreement between online T2Calo and offline calorimeter algorithm

12. Valeria Perez Reale ATLAS Physics Workshop 2003 12 IdScan Track reconstruction algorithm takes as input pixel and SCT clusters  Z Vertex Finder Determine z-position of the interaction  Hit Filter Select only groups of space points consistent with the above z  Group Cleaner Remove noise hits from group  Track Fitter Fit parameters of accepted track Algorithm Steps:

13. Valeria Perez Reale ATLAS Physics Workshop 2003 13 IdScan: Physics performance Single 20 GeV electrons  Good agreement between idScan and offline tracking algorithm xKalman  Performance can be improved by tuning parameters, specially for z-finder at low p T Z Finder Efficiency: low lumi 97% high lumi 89% HitFilter and Track Fitter Eff.: low lumi 97% high lumi 98% ___ IdScan ___ xKalman

14. Valeria Perez Reale ATLAS Physics Workshop 2003 14 SiTrack and TRT SiTrack TRT  b-tagging algorithm  Build Track seeds formed by 2 space points and fits them in a straight line  Track seeding and extension performed using pixel and SCT layers  Offline and online implementation  TRT part of the xkalman offline tracking algorithm  Based on Hough histogram  Online implementation Identify primary vertex using LVL1 . Reconstruct track segments in the SCT and pixel

15. Valeria Perez Reale ATLAS Physics Workshop 2003 15 LVL2 Timing Performance Data Preparation T2Calo, single 20 GeV electrons with noise:  Data prep. in LAr: 186.3 +/- 4.2 ms.  Tilecal: 15.17 +/- 0.03 ms. Execution of Algorithms T2Calo:  1ms IdScan: SiTrack:  single 20 GeV electrons: 1.8 ms  dijets ET>17 GeV low lumi: 6 ms Time measurements on a 2.5 GHz machine

16. Valeria Perez Reale ATLAS Physics Workshop 2003 16 EF dijets pT >17 Gev and 25 GeV single electrons (DC1 data samples) No trigger (neither LVL1 or LVL2) simulated. Track Reconstruction algorithms: xKalman++ and iPatRec Very preliminary study…difficult to compare with results of the TDR since no trigger is available EF algorithms are emulated using offline algorithm selection with looser cuts. e/jet separation analysis:

17. Valeria Perez Reale ATLAS Physics Workshop 2003 17 EF: e/jet selection cuts Calorimeter Cuts: Energy of cluster > 22 GeV and eg_isem=0 Inner detector: Standard cuts on SCT (9 hits), pixels (2 hits with one on B- layer) and impact parameter (<1mm) No more cuts on the straw-hits ID-calo Track matching Track matching between calorimeter and inner detector depends on  (stronger cuts in the barrel region compared to end-cap), especially concerning E/p cuts Transition Radiation Very preliminary study…difficult to compare with results of the TDR since no trigger is available

18. Valeria Perez Reale ATLAS Physics Workshop 2003 18 EF: Efficiency/Rejection xKalman++iPatRec EfficiencyRejection(x10 3 )EfficiencyRejection(x10 3 ) Et cuts97%99.1% Calo cuts92.1% Track found88.2%88.7% Good track found79%77.1% Tighter track- match cuts 72.6%72.5% TR cuts69.7%37.2% Evolution of efficiency and rejection as a function of the different selection cuts For offline comparison see F. Derue’s talk

19. Valeria Perez Reale ATLAS Physics Workshop 2003 19 Conclusions Full e/gamma slice running Good agreement of LVL2 with respect to offline Work in progress to access rates at all Trigger levels Watch out for the Trigger TDR end of June!