Ricardo Gon ç alo, Royal Holloway University of London On behalf of the ATLAS HLT group

1. Overview of the High-Level Trigger Electron and Photon Selection for the ATLAS Experiment at the LHC Ricardo Gonçalo, Royal Holloway University of London On behalf of the ATLAS HLT group NSS 2005 – Puerto Rico, 23-29 October 2005

2. Outline • ATLAS and the Large Hadron Collider • The ATLAS High-Level Trigger • Electron and Photon selection • Performance studies • Summary and outlook Ricardo Goncalo, Royal Holloway University of London

3. ATLAS and the LHC The Large Hadron Collider The ATLAS experiment Trigger requirements

4. The LHC • The LHC will start operation in 2007 and will represent the high-energy frontier in collider physics • Much is expected of the LHC and its experiments: • Study the origin of the electroweak symmetry breaking • Test models of physics beyond the Standard Model • Perform precision Standard Model measurements • … and still be able to detect unexpected new physics Ricardo Goncalo, Royal Holloway University of London

5. ATLAS • Large angular coverage (||<5; tracking coverage up to ~2.5) • Liquid Argon electromagnetic calorimeter with accordion geometry • Iron-scintillating tile hadronic calorimeter; tiles placed radially and staggered in depth • Toroidal magnetic field in muon spectrometer (supercondutor air-core toroids) Ricardo Goncalo, Royal Holloway University of London

6. Challenges faced by the ATLAS trigger • “Interesting” cross sections at least ~108 times smaller than total cross section • 25ns bunch crossing interval (40 MHz) • Up to 25 proton-proton interactions per bunch crossing (depending on luminosity) • Offline processing capability: ~200 Hz • ~5 events selected per million bunch crossings • High-pT events smeared by soft pile-up events Ricardo Goncalo, Royal Holloway University of London

7. The ATLAS High-Level Trigger (HLT) The ATLAS trigger HLT e/ selection Selection method

8. High-Level Trigger The ATLAS trigger • Level 1: • Hardware based (FPGA/ASIC) • Coarse granularity detector data • Average execution time 2.5 s • Output rate ~75 kHz • Level 2: • Software based • Only detector sub-regions processed (Regions of Interest - RoI) seeded by level 1 • Full detector granularity in RoIs • Fast-rejection steering • Average execution time ~10 ms • Output rate ~1 kHz • Event Filter: • Seeded by level 2 • Full detector granularity • Potential full event access • Offline-like algorithms • Average execution time ~1 s • Output rate ~200 Hz Ricardo Goncalo, Royal Holloway University of London

9. HLT e/ selection • High transverse momentum electrons and photons are an important part of several physics signatures • Fake signals produced by narrow jets and by 0 Ricardo Goncalo, Royal Holloway University of London

10. Selection method L1 region of interest: , , ET threshold, isolation in EM calorimeter Coarse granularity EMROI L1 Pass? Event rejection possible at each step L2CALO   cluster? L2 seeded by Level 1 Full detector granularity Fast calorimeter cluster reconstruction (only cluster for  triggers) Fast tracking algorithms L2 L2Tracking Track? Match? EF seeded by Level 2 Full detector granularity Offline-type reconstruction algorithms for calorimeter clusters and inner detector tracks Refined alignment and caibration EFCALO Cluster? EF EFTracking EF Pass? Ricardo Goncalo, Royal Holloway University of London

11. HLT Performance studies Single electron signatures Photon signatures Trigger optimization Physics applications Trigger efficiency from data Timing studies Test beam studies

12. e25i isolated pT>25 GeV electron Signature example: single e • Monte-Carlo samples: • Single electrons • QCD di-jet sample with ET>17 GeV • Pileup and noise included • Using fully simulated data for the initial luminosity scenario we get: • Note: uncertainty on QCD jet cross section is a factor of 2-3 • Trigger cuts optimized as function of  Ricardo Goncalo, Royal Holloway University of London

13. 220i isolated pT>20 GeV photon double trigger Photon menus • Using fully-simulated Monte-Carlo data we get: • For 220i and 60 pT = 20 GeV • converted • not conv. efficiency • Converted photon reconstruction at the Event Filter could be used to reduce the rate || Ricardo Goncalo, Royal Holloway University of London

14. L2 Tracking Efficiency optimization • Efficiency must be balanced against the trigger output rate to optimize available bandwidth • Tools in place to do automatic optimization by scanning selection cuts parameter space • Efficiency vs. rate/jet rejection curve provides continuous set of working points • Every point in the plot corresponds to a set of selection cut values • Envelope is optimum rejection for each efficiency value Ricardo Goncalo, Royal Holloway University of London

15. Physics applications • Ze+e- and We important channels for: • Precision SM physics • Detector commissioning • Detector calibration • Luminosity measurement • Efficiency numbers wrt the following kinematical cuts: • Ze+e-: • 2 electrons with ET>15 GeV, ||<2.5 • We: • 1 electron with ET>25 GeV, ||<2.5 • H (mH=120 GeV) • 1 photon with ET>20 GeV, ||<2.5 • 1 photon with ET>40 GeV, ||<2.5 Ricardo Goncalo, Royal Holloway University of London

16. Trigger efficiency from data MZ (GeV) • Electron trigger efficiency from real Ze+e- data • Tag Z events with single electron trigger (e.g. e25i): N1 • Count events with a second electron (2e25i): N2 • Fit Z mass peak + linear fit to background (B) • Efficiency is function of N1, N2, B1 and B2 • No dependence found on background level (5%, 20%, 50% tried) • Estimated systematic uncertainty small • ~3% statistical uncertainty after 30 mins at initial luminosity Ricardo Goncalo, Royal Holloway University of London

17. Timing studies • Timing of the trigger algorithms essential for performance • Times estimated for a 8 GHz CPU and 1 RoI/event • Level 2 latency is 10 ms; still work to do here but much progress made recently • Most of the Level 2 time taken by unpacking of data • (transit from detector/buffer included???) • Event Filter time small wrt allowed latency (~1s) Ricardo Goncalo, Royal Holloway University of London

18. Test beam studies • Objective was to study e/ separation and electron efficiency in realistic detector • A good opportunity to test the tools • Tracking algorithms used without modifications • Tracking efficiency measured always above 95% Ricardo Goncalo, Royal Holloway University of London

19. Conclusions and outlook

20. Conclusions • The LHC will turn on in 2 years time (not such a long time to go) • The short available time and high pileup rate in the LHC pose serious challenges that the trigger must ovecome • The e/ trigger signatures cover a wide range of physics channels essential to the ATLAS programme • Much work still needed to guarantee we’ll be ready for data taking • But: much ground already covered, e.g. timing of data preparation, bremstrahlung recovery in offline tracking • HLT e/ signatures are well developed and seem able to cope with the harsh LHC environment • Signatures exercised on fully simulated physics channels, both relevant for physics measurements and for detector calibration • Efficiency measurements also done in realistic environment of testbeam • Many tools in place to assist trigger development, tuning and study Ricardo Goncalo, Royal Holloway University of London