1. 1 The ATLAS Missing E T trigger Pierre-Hugues Beauchemin University of Oxford On behalf of the ATLAS Collaboration 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 10 th -14 th 2010

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

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

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

5. 5 HLT Missing E T algorithms L2 Feature Extraction (FEX) algorithm: only corrects the L1 Missing E T 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. 6 7 TeV data: L1 to offline correlation L1 Missing E T smaller than offline due to hard noise suppression After offline clean-up XE10 Before offline clean-up XE10 Lowest L1 Missing threshold (10 GeV) don’t pick up events with Missing E T 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 7 TeV data: EF to offline correlation Strong correlation between EF and offline measurements. Both for Missing E T and Sum E T 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 Sum E T Missing E T

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

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

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

11. 11 Missing E T and detector commissioning Missing E T 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 E T distribution Many problems have already been found thank to Missing E T 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. 12 Summary The Missing E T 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. 13 Back-up Slides

14. 14 L1 processing

15. 15 Setup for Missing E T trigger commissioning Run a simplified version of L1/HLT algorithms: No calibration is applied at any level  EM scale Missing E T measurement Only trigger chains with no muon corrections are activated ▬ Muon corrections are computed by the FEX  can be studied offline Special Missing E T 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)