real time 2010monika wielers (ral)1 atlas e/ / /jet/e t miss high level trigger algorithms...

Real Time 2010 Monika Wielers (RAL) 1

ATLAS e///jet/ETmiss High Level

Trigger Algorithms Performance with first LHC collisions

Monika Wielers (RAL) on behalf of

the ATLAS Collaboration

Real Time 2010


IntroductionOutline

Performance of the ATLAS High Level Trigger (HLT=L2 + EF)

Electrons and photonsTausJets

ETmiss

Next commissioning stepsConclusions

Will show plots from s = 900 GeV (~9 b-1 of stable beam data) and s= 7 TeV collisions (so far >10 nb-1 recorded)

Much more 7 TeV collisions data available compared to 900 GeV data. Access to much higher ET values in 7 TeV data

Commissioning done currently by running in pass-through mode at HLT starting from low-ET L1 triggers

We mainly see ‘fakes’ right now rather than the signals we are ultimately interested in


Electrons and Photons: Overviewe/ selection key for many physics analyses

J/ψ, B physics low-pT e’s [ 5 – 20 GeV]

W, Z, top, Higgs, SUSY, prompt medium pT e’s and ’s [ 20 – 100 GeV]

G, Z’ high pT e’s and ’s [pT >100 GeV]

Processing stepsStarting point: L1 EM region of interests (RoI)clustering: a bit simplified compared to offlinetracking: 3 fast pattern recognition algorithms being evaluated online Use of calo shapes and cluster-track matching variables in selection Use of offline algorithmsRun clustering and tracking algorithms

Due to timing constraints: no conversion finding, no brem recovery

Use of calo shapes and cluster-track matching variables in selectionHLT and offline use same variables for signal identification

L2

EF


We candidate as seen by the trigger

aa

Electron candidate

L1 tower ET in space

As seen by L2 As seen by offline

pT(e+)=34 GeV

(e+) = -0.42

ETmiss = 26 GeV

MT = 57 GeV


Electrons and Photons: PerformanceGood trigger performance can be evaluated in terms of

Small resolution between trigger and offline selection variablesAgreement between data and MC for the selection variable distributions

Example:Shower shape in 2nd EM layer R=E(37)/E(77) (cell units, one unit is =0.025 0.025)Good agreement between trigger and offline found for resolutionData and MC agree reasonably well

As selection cuts were derived from MC for start-up a reasonable agreement gives confidence our selection will work online

R distributions for trigger objects matched to offline e candidates


Tau HLT Performance: Overview

Tau’s are key signature forW, Z SM processesHhh, heavy HiggsSusy searches with a light tau slepton

~65% of ’s decay hadronically in 1- or 3-prongs (, +n0 or 3, 3+n0)Requires dedicated trigger looking for

“Narrow” jet in calorimeter1 or 3 associated tracks in tracking detector

Identification based on jet isolation, jet narrowness and track multiplicityHLT processing steps similar to electrons

Starting point: L1 tau RoI


Tau HLT Performance in 900 GeV collisionsExamples:

L2 ET spectrum

EF EM and hadronic radius (measurement of shower size in -: EcellR2

cell/Ecell in =0.1 x 0.1)

Expect small values for tau’sReasonable agreement between data - MC

Gives us confidence that the selections optimised on MC will work!


Jet Performance: Overview

Physics Motivation

Jet cross section

Susy

Black hole searches

HLT processing

Starts from a L1 jet RoI

Iterative cone algorithm with R<0.4 at L2

Cone jet algorithm with R<0.7 at EF (use of offline algorithm)

Note, other jet algorithms under evaluation


Jets: HLT Performance in 900 GeV collisions

Relative energy resolution between L2 and offline jets at EM scale

Good agreement between data and MC Small shift in peak position arises from different jet finder used in HLT and offl.

Good agreement between data and MC simulations also seen in -resolution at L2 and EF


Missing ET: Overview

Physics motivation

Susy searches and searches for extra-dimensions

ETmiss triggers often combined with jet triggers

HLT processing

Correct L1 ETmiss for muon contribution (can’t read out all

calorimeter cell information due to time constraints)

Apply ETmiss cut in hypothesis step

Compute ETmiss based on full calorimeter cell information

Apply 2 noise cut at cell level

Apply simple layer based calibration

Apply ETmiss cut in hypothesis step

L2

EF


Missing ET: HLT Performance in 7 TeV collisions

Strong linear correlation between EF and offline Missing ET measurements

Some of the high ETmiss values arise from “bad” jets (due to noise

fluctuations and will be removed in offline)

Most of the events with fake ETmiss don’t pass the L1 XE10

selectionET

miss after XE10 (no offline clean-up)


Missing ET: HLT Performance in 7 TeV collisions

Excellent agreement between data and MC bin-by-bin turn-on curves

Sharp turn-on curves minimal distortion of the offline ETmiss

measurement by trigger

Higher threshold is statistically limitedThe EF Missing ET trigger performs as expected on physics events

EF Missing ET > 5 GeV EF Missing ET > 20 GeV


Next commissioning steps for e///jet/ETmiss

1st commissioning stepDeploy the HLT online without active rejection Verify HLT results w.r.t. offline, MC

2nd commissioning stepStart active rejection re-do studies using physics signals

Physics running Measure performance on signal enriched sample

Tag&Probe for Z and J/, ETmiss trigger for Wl

Start optimising triggers for higher luminosities (includes pile-up)

HLT rejectionAlready active for minimum bias triggers since a whileHLT rejection for lowest L1 EM thresholds enabled in night from 24th to the 25th of May for run with peak luminosity of 2.1 1029 cm-2s-1 Lowest muon triggers will be the next ones to go in HLT rejectionJets will use mixture of pre-scale and HLT

Ultimate goal

In progess

Just started!


Summary

Data from s = 900 GeV and 7 TeV LHC collisions have moved the commissioning of the ATLAS HLT one step ahead

L1 calorimeter and muon trigger system working reliably

HLT algorithms are running routinely online in pass-through mode (no active rejection, but results are created and available for analysis)

Lowest threshold e/ triggers just went into rejection!Comparison of trigger quantities with reference offline objects show in general reasonable agreement and performance is reasonably well reproduced by MC simulations

We increased our confidence that the selections we set-up will work as expected

Trigger system in very good shape and we can face the challenge to select good quality physics data… interesting times lie ahead of us


Backup


The ATLAS Detector


A 3 level trigger system:

Region of Interest ( RoI ) concept: only detector information contained in an angular region x = 0.2x0.2 around L1 cluster position are processed by next Δη Δφ

level (increase speed and reduce network load)

~40 MHz

even

t rat

e

~75 KHz

~2 KHz

~200 Hz

2 sμ

40 ms

~4 s

leve

l lat

ency

L1

L1Trigger (LVL1):hardware basedonly muon and calo informationreduced granularity

EF (HLT): software based full event information available ‘quasi’ offline algorithms

Level2 (HLT):software basedall detectors available (RoI approach) dedicated algorithms and calibration

L2

EF

on detector

The ATLAS Trigger System


Towards the Physics menu at 1031 cm-2s-1

Example: Jet plans

Keep running HLT in pass-through (including multijets)

Then enable multi jet signature only

Then enable HLT

Example: Tau plans

From HLT pass-through chains (starts from L1 tau RoI > 5 GeV) to…


Electrons and Photons: Performance in 7 TeV collisions

Example: Electron identification variable Δη: difference in between cluster and extrapolated track

L2 distribution well described by MC simulations (also observed at EF)Good EF resolution w.r.t. offlineL2 resolution is ~ factor 3 worse: due to the completely different tracking algorithm (IdScan) used

To be approved

To be approved

real time 2010monika wielers (ral)1 atlas e/ / /jet/e t miss high level trigger algorithms...

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