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. Ricardo Goncalo, Royal Holloway University of London2 ATLAS HLT e/gamma selection Outline ATLAS and the Large Hadron Collider The ATLAS High-Level Trigger Electron and Photon selection Performance studies Summary and outlook

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

4. Ricardo Goncalo, Royal Holloway University of London4 ATLAS HLT e/gamma selection 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 CM energy14 TeV Time between collisions25 ns Interactions/bunch crossing~5-25 Initial luminosity10 33 cm -2 s -1 Design luminosity10 34 cm -2 s -1

5. Ricardo Goncalo, Royal Holloway University of London5 ATLAS HLT e/gamma selection 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)

6. Ricardo Goncalo, Royal Holloway University of London6 ATLAS HLT e/gamma selection Challenges faced by the ATLAS trigger “Interesting” cross sections at least ~10 8 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-p T events smeared by soft pile-up events

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

8. Ricardo Goncalo, Royal Holloway University of London8 ATLAS HLT e/gamma selection 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 High-Level Trigger

9. Ricardo Goncalo, Royal Holloway University of London9 ATLAS HLT e/gamma selection 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  Physics coverageSignatures (initial lumi)Rate Electrons Higgs, susy, W, top, heavy gauge bosons, extra dimensions e25i, 2e15i, e60~40 Hz PhotonsHiggs, susy, extra dimensions  60, 2  20i ~40 Hz Muons (high-p T ) Higgs, susy, W, top, heavy gauge bosons, extra dimensions, B physics  20i, 2  10, 2  6 ~40 Hz JetsSusy, resonances, compositenessj400, 3j165, 4j110~20 Hz Jets+E T miss Susy, leptoquarksj70 + xE70~5 Hz Tau+E T miss MSSM Higgs, susy  35 + xE45 ~10 Hz OthersPrescaled + calibration + monitoring~20 Hz

10. Ricardo Goncalo, Royal Holloway University of London10 ATLAS HLT e/gamma selection Match? Selection method EMROI L2CALO Pass? L2Tracking cluster? EFCALO Track? EFTracking Cluster? EF Pass? L2 seeded by Level 1 Full detector granularity Fast calorimeter cluster reconstruction ( only cluster for  triggers ) Fast tracking algorithms L1 region of interest: , , E T threshold, isolation in EM calorimeter Coarse granularity EF seeded by Level 2 Full detector granularity Offline-type reconstruction algorithms for calorimeter clusters and inner detector tracks Refined alignment and caibration L1 L2 EF   Event rejection possible at each step

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

12. Ricardo Goncalo, Royal Holloway University of London12 ATLAS HLT e/gamma selection Signature example: single e  e25iEfficiencyRate Level 1 % kHz L2 Calo % kHz L2 Trk % Hz L2 match % Hz EF Calo % Hz EF Trk % Hz EF match % Hz Barrel-endcap crack excluded Passed level 2 background (approx) W  e 20% Z  ee 6% e from b and c decays8%  (quark brem & prompt) 14% Other (  0 , jets, etc) 52% Monte-Carlo samples:  Single electrons  QCD di-jet sample with E T >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  e25i electron p T >25 GeV isolated

13. Ricardo Goncalo, Royal Holloway University of London13 ATLAS HLT e/gamma selection Photon menus Using fully-simulated Monte-Carlo data we get:  For 2  20i and  60 2  20i photon p T >20 GeV isolated double trigger Efficiency 2  20i  602  20i   60 Level 194%85%98% Level 284%81%94% Event filter78%69%89% Rate2 Hz10Hz Barrel-endcap crack excluded p T = 20 GeV converted not conv. efficiency |||| Converted photon reconstruction at the Event Filter could be used to reduce the rate

14. Ricardo Goncalo, Royal Holloway University of London14 ATLAS HLT e/gamma selection 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 L2 Tracking Every point in the plot corresponds to a set of selection cut values Envelope is optimum rejection for each efficiency value

15. Ricardo Goncalo, Royal Holloway University of London15 ATLAS HLT e/gamma selection 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 E T >15 GeV, |  |<2.5  W  e : 1 electron with E T >25 GeV, |  |<2.5  H  (m H =120 GeV) 1 photon with E T >20 GeV, |  |<2.5 1 photon with E T >40 GeV, |  |<2.5 Efficiency Ze+e-Ze+e- W  e 2e15i67.2%  e25i92.9%79.6% e6020.4%6.9% all94.8%80.3% Barrel-endcap crack excluded Efficiency 2  20i  602  20i   60 Level 194%85%98% Level 284%81%94% Event filter78%69%89% Rate2 Hz10Hz H  (m H =120 GeV) Barrel-endcap crack excluded

16. Ricardo Goncalo, Royal Holloway University of London16 ATLAS HLT e/gamma selection Trigger efficiency from data Electron trigger efficiency from real Z  e + e - data 1. Tag Z events with single electron trigger (e.g. e25i): N 1 2. Count events with a second electron (2e25i): N 2 3. Fit Z mass peak + linear fit to background (B) 4. Efficiency is function of N 1, N 2, B 1 and B 2 No dependence found on background level (5%, 20%, 50% tried) Estimated systematic uncertainty small ~3% statistical uncertainty after 30 mins at initial luminosity Method Ze+e-Ze+e- counting L 2 efficiency (%) 87.0  0.287.0  0.6 M Z (GeV)

17. Ricardo Goncalo, Royal Holloway University of London17 ATLAS HLT e/gamma selection 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)

18. Ricardo Goncalo, Royal Holloway University of London18 ATLAS HLT e/gamma selection 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% Data sample Electron eff. (%)  fake rate (Hz) 20 GeV 95.3  0.41.6  0.2 50 GeV 94.9  0.30.7  0.2

19. Conclusions and outlook

20. Ricardo Goncalo, Royal Holloway University of London20 ATLAS HLT e/gamma selection 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