The ATLAS Missing E T trigger

1. The ATLAS Missing ET trigger Pierre-Hugues Beauchemin University of Oxford On behalf of the ATLAS Collaboration The XIV International Conference on Calorimetry in High Energy Physics Beijing, China, May 10th -14th 2010

2. Outline • Motivation of a Missing ET trigger • Overview of the ATLAS Missing ET at Level 1 • Overview of the ATLAS HLT Missing ET algorithms • Results from 7 TeV ATLAS data • Online to offline comparison • Data to MC comparison • Missing ET trigger and ATLAS commissioning • Summary and future plans

3. Motivation for a Missing ET trigger • Discover new physics processes involving undetectable particles. • Ex: Dark matter candidates • Can be used in combination with some other trigger signatures • Ex: Allow to study non-boosted Wtn • Estimate efficiency of other uncorrelated triggers • First data-driven estimate of electron efficiency for Wen events n  An Missing ET trigger plays an important role in the ATLAS physics program

4. Missing ET at Level 1: an overview Calorimeter Trigger Towers (TT) are built: • sum of calo cells in an - range of ~0.1x0.1, over full depth of each calo Pre-processor (PPr) Jet Elements (JE) are formed • Digitalize TT ET signal • Apply noise subtraction • Sum 4 TT, HAD and EM Jet/Energy processor Missing ET is computed • Ex and Ey are computed from JEs • A Look Up Table tells, from EX and EY, which Missing ET thresholds passed • Communicate results to the CTP

5. HLT Missing ET algorithms • L2 Feature Extraction (FEX) algorithm: only corrects the L1 Missing ET for muon contribution • Bandwidth limitation • EF FEX algorithms: full granularity and resolution of calorimeter to refine L1/L2 • Apply 1-sided, 3-sigma cut noise suppression on each cells • Take muon corrections from EF muon measurement • Apply calibrations to improve resolution • Hypotheses testing algorithms: use different elements from FEX for chains specific purpose • Possibility of many different trigger chains with no extra cost in timing

6. 7 TeV data: L1 to offline correlation • L1 Missing ET smaller than offline due to hard noise suppression Before offline clean-up After offline clean-up XE10 XE10 • Lowest L1 Missing threshold (10 GeV) don’t pick up events with Missing ET due to bads jets  No efficiency problem • ATLAS standard jet clean-up cuts remove noisy and badly timed jets • Noise averaged out in Jet Elements (JEs)

7. 7 TeV data: EF to offline correlation • Strong correlation between EF and offline measurements. • Both for Missing ET and Sum ET • Standard ATLAS jet clean-up cuts applied • Offset between EF and offline is an expected feature: • Artefact of our 1-sided 3-sigma noise suppression cut Missing ET Sum ET

8. 7 TeV data: data to MC comparison • The data EF Missing ET distribution agrees well to MC distribution for collision events  EF Missing ET measurement is well understood! Standard ATLAS Jet clean-up cut applied

9. Data to MC: EF Missing ET turn-on curves • Missing ET turn-on curve: efficiency of a trigger Missing ET selection as a function of an offline Missing ET reference. • Sharp turn-on curves  minimal distortion of the offline Missing ET • Excellent agreement between data and MC The EF Missing ET trigger perform as expected on physics events EF Missing ET > 5 GeV EF Missing ET > 20 GeV e = 95%

10. Data to MC: EF Sum ET turn-on curves • Sum ET turn-on curve: computed similarly as Missing ET turn-on  minimal distortion of the offline Sum ET measurement by trigger The EF Sum ET trigger perform as expected on physics events • Plateau reached before the 30 GeV due to EF offset (shown above) EF Sum ET > 30 GeV EF Sum ET > 100 GeV e = 95%

11. Missing ET and detector commissioning • Missing ET trigger rate is the most sensitive to detector problems: • It is a global quantity not limited to a narrow - region. • Rates are dominated by steeply falling QCD Missing ET distribution • Many problems have already been found thank to Missing ET trigger: • LAr cabling (HEC and barrel) • LAr FEB headers (bugs in DSP code) • Tile: bug in collecting data and noise description • Muon EF time-out and different muon FEXes have been debugged  Missing ET trigger is a precious tool to debug overall detector

12. Summary • The Missing ET trigger performed well on first LHC 7 TeV collision data • Strong correlation with offline quantities • Good agreement with Monte Carlo simulations • Steep turn-on curves directly measured in data • Missing ET trigger can be safely used to: • Study the performance of different object • Combined with other signature for a wider kinematic reach • Hopefully find new physics! • Help bringing ATLAS to a normal mode of running  Demonstrate the feasibility of a trigger based on global object

13. Back-up Slides

14. L1 processing

15. Setup for Missing ET trigger commissioning • Run a simplified version of L1/HLT algorithms: • No calibration is applied at any level  EM scale Missing ET measurement • Only trigger chains with no muon corrections are activated • Muon corrections are computed by the FEX  can be studied offline • Special Missing ET trigger chains for commissioning: • Force FEX to run over all L1 accept events, without accepting events • For debugging purpose: we run 3 EF FEX algorithms in parallel: • Standard cell-based noise suppression • Cell base, no noise suppression • FEB-based • use the collective info of the up to 128 cells attached to Front-End Boards to gain speed Extra algorithms will disappear after commissioning, but will be enabled in cosmic menu (run in empty bunch crossing)