1

The ATLAS Missing ET triggerThe ATLAS Missing ET 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 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

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 We events

Discover new physics processes involving undetectable particles.

• Ex: Dark matter candidates

An Missing ET trigger plays an important role in the ATLAS physics program

4

Missing ET 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 ET signal

Apply noise subtraction

Sum 4 TT, HAD and EM

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

After offline clean-up

XE10

Before offline clean-up

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

Sum ETMissing ET

8

7 TeV data: data to MC comparison

The data EF Missing ET distribution agrees well to MC distribution for collision events

StandardATLAS

Jet clean-up

cut applied

EF Missing ET measurement is well understood!

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

EF Missing ET > 20 GeV

= 95%

EF Missing ET > 5 GeV

The EF Missing ET trigger perform as expected on physics events

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 > 100 GeV

= 95%

EF Sum ET > 30 GeV

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

SummarySummary

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 SlidesBack-up Slides

14

L1 processingL1 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)