lhcc review, cern, 19/10/99paul bright-thomas, for alan watson 1 lvl1 calorimeter algorithm updates...

LHCC Review, CERN, 19/10/99Paul Bright-Thomas, for Alan Watson 1

LVL1 Calorimeter Algorithm Updates

Changes since the TDR:Greater “integration” of e/ and /h triggers:

Numbers of em and tau thresholds now adjustableCommon RoI algorithm for e/ and /h algorithmsSplit e/ hadronic isolation into two regions

“Evolution” rather than “Revolution”:Main effects are to:

Increase flexibilitySimplify implementation

Changes approved by the ATLAS T/DAQ Steering Group in July ‘99


TDR e/ Algorithm

Object accepted if: 1 trigger cluster cluster

thr

em isolation ET em isol thr

had isolation ET had isol thr

RoI cluster ET = local maximum

Tau algorithm based on same 44

tower window (next slide)

Em Calorimeter Had Calorimeter

had isolationem isolation

RoI clustertrigger cluster


Current e/ Algorithm

Object accepted if: 1 trigger cluster cluster thr

em “ring” ET em isol thr

had “ring” ET outer had isol thr

had “core” ET inner had isol thr

RoI cluster ET = local maximum

Tau algorithm same except: trigger clusters = 2 em + 22

had towers

only the 2 “ring” sums are used for isolation

(Tau algorithm unchanged from TDR)

Em Calorimeter Had Calorimeter

had isolationem isolation

RoI clustertrigger cluster


Adjustable Numbers of e/ and /h Triggers

TDR design had fixed numbers of em and tau selections

Proposed Nem = N = 8

Optimum allocations hard to predictWill be luminosity-dependentMay depend on what we find

Better if we could make these numbers adjustableHard to vary total ( Ntot = Nem N)Can adjust Nem & N within this totalRequires small increase in functionality within CP chip


Adjusting Numbers of e/ and /h Triggers

Em & Tau algorithms very similar See Table

To switch between the two: Use multiplexors to select either

em or tau trigger clusters

Do not set “hadronic core” isolation for tau triggers

Don’t need total flexibility:We propose:

8 sets of em thresholds

8 sets adjustable em/tau

(Even this not fixed if FPGAs used)

Element e/ Trigger /h Trigger

TriggerCluster

21 / 12em towers

21 / 12 em 22 hadronic

EmI solation

"Ring" of 12towers

"Ring" of 12towers

HadI solation 1

"Ring" of 12towers

"Ring" of 12towers

HadI solation 2

22 towersinside "ring"

N/ A

RoI Cluster 22 towers,em+had

22 towers,em+had


Hadronic Isolation: “Core” & “Ring”

Performance IssuesImbalance between the two regions

“core” provides greater part of jet rejection

“ring” is more vunerable to noise

Separation allows different weighting of the two

same (or slightly better) overall performance

more flexible response to conditions & requirements

ET in 2 regions (signal & background)

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Core & Ring Isolation: Signal vs Background

Signal:Look at electrons + 48 mbias

Pessimistic pileup modelParameterize BCID

More ET in “ring”

Two sums largely uncorrelated

Background:“Core” ET typically larger

Correlations weak

“Ring” contributes to rejection

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Effect of two-region hadronic isolation

Compare rates for same isolation efficiency look at electrons with

“pessimistic” pileup

compare jet background rates for cuts giving 95%, 98% & 99% isolation efficiency

find some improvement when very high efficiency required

look also with “more realistic” pulse-shape modelling

improvement smaller (few %)no situation in which rate is worse

0

100

200

300

400

500

600

Rat

e (a

rbit

rary

uni

ts)

e >

95%

e >

98%

e >

99%

4x4 had

core+ring

Cluster > 15 GeV, em & had isolation


A Common RoI Algorithm?

Motivations Simplify implementation

only 1 set of RoI clusters to form & testmakes “selectable” em/tau triggers easier to build

Simplify RoI data

single object produces unique RoI coordinate for both em and tau algorithms

Which algorithm? Em RoI for taus?

Much of tau ET may be in HCAL

May degrade shower containment

Not ideal Tau RoI for e/?

Isolated e/ should deposit little ET in HCAL

Objects for which it makes a difference will fail hadronic isolation anyway

Worth investigating


Possible Effect: Most Extreme Case

Possible Effects:RoI coordinate may shift by 0.1

Rare (< 1% of e/)

Clusters for which this happens will mostly fail isolation anyway

Possible effects of this:

Moves hadronic ET between “ring” and “core” sums

Very rarely may increase em isolation or reduce trigger cluster

Size of effects: Changes in em sums can be no

larger than hadronic ET deposit

em RoI em + had RoI

em ET deposit hadronic ET deposit

t r igger cluster RoI and windowboundaries


Effect on Efficiency: 30 GeV e- + pileup

What effects do we see:(with pessimistic pileup model)

No effect in 99% of e RoIs

Small differences in isolation ET have no effect on efficiency.

See same story for: Different electron pT

“ noise/BCID assumptions

“ pileup levels

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em RoI em+had RoI


Effect on Jet Rejection

More effect seen in background: O(10%) of background clusters

with ET > 15 GeV have RoIs shifted (cf < 1% e/)

Does it affect jet rejection? Use pessimistic pileup (as

should maxmize size of any effects)

Choose isolation cuts to give 95%, 98% and 99% isolation efficiency

Compare jet rates for 2 RoI algorithms

small reduction in rate seen

0

20

40

60

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Rat

e (a

rbit

rary

uni

ts)

e > 95% e > 98% e > 99%

em RoI

em+hadRoI

Cluster > 15 GeV, em & had isolation


Efficiency in Physics Events

Look at effect in physics events In case more sensitive than

single e/ events

Look at a range of processes containing e/ plus jets

What do we see? For most processes, no effect

Greatest effect in t eb, jjb

1% events have shifted RoIsSmall changes in Had Isoln.No difference in efficiency

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Summary of Changes

Nem and Ntau variableIncreases flexibility of systemNo possible performance drawback

Two-region Hadronic IsolationMay increase flexibility/robustness of performanceSlight improvement in jet rejection.

Common RoI AlgorithmTechnical simplificationNo adverse effect on performance (maybe very small

gain)

lhcc review, cern, 19/10/99paul bright-thomas, for alan watson 1 lvl1 calorimeter algorithm updates...

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