The ATLAS Liquid Argon calorimeter alignment in time for exotic search

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The ATLAS Liquid Argon calorimeter alignment in time for exotic search

L. Aperio Bella

To cite this version:

L. Aperio Bella. The ATLAS Liquid Argon calorimeter alignment in time for exotic search. Journées

Jeunes Chercheurs 2010, Nov 2010, Angers, France. pp.91-94. �in2p3-00551856�

Ludovia APERIO BELLA

Laboratoire d' Anney-le-Vieux de Physique des

Partiules

Résumé

Le alorimétre ArgonLiquide(LAr) del'expériene

ATLAS doit pourvoir à la reonstrution d'énergie

des les partiules qui sont produites au ollisionneur

LHC. L'alignementautemps destouteslesellulesdu

alorimétreestessentielpourgarantireunebonneréso-

lution autemps,eparamètrejoueunrleimportante

dansbeauoupdesthéoriesaudelàdeModeléStandard

quiprédisentl'existenedespartiulesproduiteaveun

tempsdeviemesurabledansledéteteur.Danserap-

port on se propose de dérire l'alignement en temps

desleellulesduLArqueaétéfaitaveledonnesdes

ollisionsprotonprotondansl'année2010.

1.1 Introdution

The Liquid Argon Calorimeter (LAr) system is an

importantpartoftheATLASdetetor[1℄.Itallowsto

trigger, to identify and to measure eletrons, photons

and jets of energies ranging from a fewGeV to a few

TeV. To insure optimal energy reonstrution all the

alorimeter readouthannelsshould bewellalignedin

time. Previousstudiesresultedin thealignmentupto

afew ns [2℄[3℄. Test-beamstudieshaveshown thatit

is possible to ahieve time alignment on the level of

100

Amorerenedtimemeasurementofthealorimeter

signal ould have several appliations in the ATLAS

environment.

It anhelp in the identiation of longlived parti-

les and in themeasurementof theirdeaytime. One

typialexampleours in theGaugeMediated Super-

symmetriModelsinwhihtheneutralinotravelsasig-

niantdistanebeforedeayingintoaninvisiblegrav-

itino and a non-pointing and out-of-time photon [5℄.

Thisfeatureanbeusedtomeasuretheneutralinolife-

timeusingtiminginformationfromtheeletromagneti

alorimeterofthereonstrutedphotondiretion.Ade-

layed luster timeis expetedbeauseof the negative

valueof

β =

< 1

Timing information ould also be used in searhes

for exoti partiles with anomalously high ionization

energylossestodistinguishthemfromStandardModel

bakgrounds[6℄.

Finallyathighluminositythetimeinformationmay

alsobeusedtorejeteventswithlargepile-upativity

from previousbunh rossings.

This note desribes the LAr hannel timing align-

ment as it was performed during the 2010 proton-

proton data-takingoftheLHC.

1.2 The ATLAS Liquid Ar-

gon alorimeter and its signal

read-out system

Figure1.1Cut-awayviewoftheATLASLiquidArgon

alorimeter

As shown in Fig. 1.1, The ATLAS LAr alorimeter

onsistsoffoursubdetetors:theeletromagnetibar-

rel(EMBarrel),theeletromagnetiendap(EMEC),

thehadroniendapandtheforwardalorimeter.

The subdetetors over an

η

|η| = 4.9

η

η = − ln(tan(

))

η

φ

are separatedintothree dierentlongitudinal setions

orlayers(Fig.1.2).Thefrontlayerisnelysegmented

intostripsalong

η

φ

η

φ

BarrelandvieversafortheEMEC.Furthermoreinthe

entralregion(

|η| < 1.8

TheLAralorimeterisasamplingalorimetermade

of liquid argon as the ative material and lead, op-

perortungstenasthepassiveabsorber.Whenharged

partilesrosstheliquidargongapbetweeneletrodes

andabsorbers,theyionizeit.Undertheinueneofthe

eletrield, theionizationeletronsdrifttowardsthe

eletrodeinduingaurrent.

At the hannel level, the treatment of the analog

∆ϕ = 0.0245

∆ η = 0.025 37.5mm∆ η = 0.0/8 = 4.69031 mmm

∆ϕ=0.0245x4 36.8mmx

Trigger Tower

∆ϕ = 0.0982

∆ η = 0.1

16X₀

4.3X₀

2X₀

1500 mm

470 m m

η ϕ

η = 0

Stri p cel l s i n L ay er 1

Square cel l s i n L ay er 2 1.7X₀

Cells in Layer 3

∆ϕ×∆η = 0.0245× 0.05

Cells in PS

∆ϕ×∆η = 0.025 × 0.1

Trigger Tower

=147.3mm4

Figure1.2Theaordionandsamplingstrutureofthe

EMBarrelalorimetershowingthediereneingranularity

betweenthedierentsamplings.Itslongitudinalsegmenta-

tionallowsustolookattheshowerdevelopment[1℄

signal is performed in the front-end eletronis. On

the front-end boards (FEB) loated in the Front-End

Cratesoutsidetheryostatthesignalisrstamplied

by a preamplier. In order to aommodate the large

dynamirangeofpulse(fromtensofMeVtoafewTeV)

and minimize thetotalnoisefrom the eletronis,the

signalis shaped andsplit into three linearsaleswith

a typial multipliative fatorsof

1

9.2

92

low,medium andhighgain.

Foragivenhannel thesethreesignalsarestoredin

an analogpipeline [7℄ untilthetriggerdeision.When

thedeisionismadethesignalseitherfromeahgainor

fromthemostsuitedoneaordingtoaseletionbased

on theamplitudeof thesignalwith themedium gain,

are digitized by a 12 bit Analog-to-Digital Converter

(ADC)in vesamplesspaedby

25

ToeahFront-End Crateisassoiatedaratewhih

ontainstheread-outdriver.Thereonstrutionofthe

timeandenergyofthephysissignalforeahhannel,

performedintheread-outdriverbytheOnlineDiglital

SignalProessor,isbasedonanOptimalFiltering(OF)

algorithm [8℄.

1.2.1 The Optimal Filtering Method

TheOptimalFilteringmethod providesoptimalval-

ues of timeand energyfor eah given hannel in eah

givenevent.Thismethodrequiresthattheexatshape

oftheexpetedphysissignalaftertheeletroniread-

outhainisknown.Toobtainthisshapefromtheorig-

inal ionization pulse atransfer funtion is needed for

every hannel. This funtion is obtained from a ded-

iated alibration runs in whih both the input and

outputpulse arewell-known.

TheOptimalFilteringCoeients(OFCs)

a

b

are omputed per hannel from the predited physis

pulse-shapeintimeusingthefollowingformulas:

A = X

i=1

a

s

Aτ = X

i=1

b

s

where

A

τ

the physispulse and

s

equationsareonlyvalidifthevaluesof

τ

perfetly timed detetor and in-time partiles

τ

be lose to zero, while the larger values indiate the

need for better timing or the presene of out-of-time

partiles in the event.If ADC samples are dominated

by noise, Eq. 1.2 does not give reliable results. Than

τ

ADC sample largerthan fourtimes thenoiseRMS in

that readouthannel.

As

τ

in thebeginingof data taking, there are 8sets of the

OFCs with 3ns phase-shift between then used in the

physisrunningand24setswith1nsphase-shiftusedin

the alibrationruns toonstrutthe referenephysis

pulse-shape 1

. Figure1.3 showsthedierene between

apreditedsignalshapeandarealionizationsignalin

the seond layerof the EM Barrel,given by aosmi

muon.

time (ns) 0 100 200 300 400 500 600 700 800

ADC counts

-200 0 200 400 600 800 1000 1200

-0.04 -0.02 0 0.02 0.04 Data

(Data-Prediction)/Max(Data) Prediction

MIDDLE LAYER EM BARREL 2008 ATLAS cosmic muons

ATLAS Preliminary

Graph

Graph

Graph

Figure 1.3 A typialpulsefor thedierentEMBarrel

middlelayers. Theblue dotsorrespondto thepredition,

theredonestothedataandthegreenpointsorrespondto

thediereneinperentagebetweendataandpredition.

With thepresentsettings (8 phaseswith 3ns shift)

the setsarehosenin awaythatthefourth setof the

OFCsorrespondsonaveragetothesignalpeakbeing

in thethirdsample.

Atthe beginning oftheLHC data taking,the OFC

sets loaded in the LAr Online Diglital Signal Proes-

sor arehosenaordingtothetiminginformationex-

trated from the alibration data and the omplete

1. The predited pulse is dened with a time dierene of

25ns

24 ^{ns between two onseutive samples and isbinned in} 24

delaystepsspaedby∼1 .04ns

knowledge of the readout path [2℄. When the physis

pulse is reorded

A

τ

Eqs.(1.1)and(1.2).If

τ

edure isperformediterativelyusingsets oftheOFCs

with appropriatephasesuntil thevaluesof

τ

arelessthan502ps.

Notethatatimemisalignmentof

∼ 5

physispulseandthereferenepulseinduesabiason

thereonstrutedenergyof

0.5%

1.3 Timing analysis

Tohoose good physispulses for the timing align-

mentweuseseletionasoutlinedbelow.

Only hannels in the high gain and in whih are

properlyalibrated and for whih the OFCs iteration

onvergesareused in thefollowinganalysis.A

χ

qualityfator

Q

pulseshapewiththerefereneshapeanbeusedtose-

let onlygoodpulses.

Toalulate onlyLArhannelsthatmeasure anen-

ergy above1GeV are seleted for this study. A mea-

surementabovethisthresholdisextremelyunlikelyto

betheresultofeletroninoisealone.Figure1.4shows

this utinthetimevsenergydistributionfor thefour

dierentlayersintheEMBarrel.Thisutisneessary

beauseforagivenhannelinaollisioneventthetime

spreadwithtorespettotheenergymeasuredisaon-

volution of the notalignment in time of the hannels

andtheintrinsitimespreadofthehannelduetothe

energydependeneofthetimeresolution(

σ

=

⊗c

With this energy ut for eah layer the time spread

within aFEB islessthan

∼ 3

ofcTime [ns]

-100 -80 -60 -40 -20 0 20 40 60 80 100

Energy [MeV]

0 2000 4000 6000 8000 10000 12000 14000 16000 18000 20000

1 10 102 ATLASWork in Progress

EM Barrel PS

ofcTime [ns]

-100 -80 -60 -40 -20 0 20 40 60 80 100

Energy [MeV]

0 2000 4000 6000 8000 10000 12000 14000 16000 18000 20000

1 10 102 103 ATLASWork in Progress

EM Barrel strips

ofcTime [ns]

-100 -80 -60 -40 -20 0 20 40 60 80 100

Energy [MeV]

0 2000 4000 6000 8000 10000 12000 14000 16000 18000 20000

1 10 102 ATLASWork in Progress

EM Barrel middle

ofcTime [ns]

-100 -80 -60 -40 -20 0 20 40 60 80 100

Energy [MeV]

0 2000 4000 6000 8000 10000 12000 14000 16000 18000 20000

10-1 1 10 ATLASWork in Progress

EM Barrel back

Figure 1.4 The distributionof time vsenergyfor the

fourdierentlayersintheEMBarrel.

Any distortion ofthe signalshapean also produe

a bias of the time reonstrution,for that reason an-

other important eet whih ould ompromise the

timealignmentoftheLArhannelsistheeletriross-

talk.Theeletroniross-talkisaphenomenainwhih

neighboring ells. This signal loss in the neighboring

ellsauseadistortionin thepulseshape.

Toavoidthatthiseetouldbiasthetimemeasure-

mentweonsideredforeaheventthehighestenergeti

hannelineahlayer(

E

ounttherst twoneighboringells(

E

thestatistisweonsideredalsothethehighestenergy

ellaftertheseondneighborsifitsenergy

E

E

Toalignintimethe

≈ 180

ter twoadjustmentsareused:

a global hardware alignment at the level of the

FEB timeoset

ahannelbyhanneladjustmentofthesetofOp-

timalFiltering Coeients(OFC).

The latterhasaniteminimal orretionof

∼ 500

beausethereisurrentlynoextrapolationbetweentwo

onurrentpointsin thereferenepulsewhihare

1.04

nsapart.

1.4 Timing alignement results

BytheendofApril2010weolleted

∼ 0.3

ollision data whih gaveenough statististo haveat

least two eventsin

94, 4%

data wealulatedtheFEBtime osetastheaverage

of themeantimeof eah hannel forasingleFEB for

eah LAr subdetetor. This orretion was applied to

alignthealorimetertimeatthehardwarelevel.

At theend of July 2010 with aintegrated luminos-

ityof

∼ 1.1

97, 3%

hannelbyhannelorretionwetakethesinglehan-

nel averagetimeand wesubtrat theglobalrun time,

thisavoidsadependenyofthisorretionontheshift

ausedbytheLHClokdriftfoundinMay2010data.

FEB time offset [ns]

-10 -5 0 5 10

Number of FEBs per 0.5 ns

1 10 10

10

s = 7 TeV collisions

Work in Progress

EM BARREL

RMS before correction = 1.0 ns RMS after correction = 0.4 ns ATLAS

Figure 1.5ForEMBarrel:theblakhistogramshows

the FEB time osets before the orretion, and the red

histogramshowsthesamedistributionaftertheorretion

wereapplied.

Weappliedthesinglehannelorretionasashiftto

thereferenepulse shape.BeausetheOFCsetsome

in units of 1.04ns, time orretion preisionis driven

bythisbinningonstraint.

EndCap_tch_TOT_FEB_slot_6_pfx

Entries 128

Mean 64.06

Mean y 0.0002555

RMS 36.94

RMS y 0.3419

channel

0 20 40 60 80 100 120

ofcTime [ns]

-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1

EndCap_tch_TOT_FEB_slot_6_pfx

Entries 128

Mean 64.06

Mean y 0.0002555

RMS 36.94

RMS y 0.3419

ATLASWork in Progress

RMS 0.3419

ofcTime [ns]

-15 -10 -5 0 5 10 15

# channel Slot6

1 10 102

103

RMS 0.3419 ATLASWork in Progress

Figure1.6ForEMEndap:intheupperplotonaverage

the timeversusthe hannelnumberinall theFEB inthe

middlelayerwithaφ^{simmetryintheatualtimesituation;}

inthe bottomplotthe hannel timespreadwithin aFEB

inthemiddlelayer,intheatualtimesituation.

The eet of the timing adjustment at the level of

theFEBs isshownin Figure1.5. For example,forthe

EM Barrel of the LAr the blak histogram shows the

FEB time osets before the orretion, and the red

histogram shows the same distribution after the im-

plementation of both the orretions (FEBs time o-

set and the hannel by hannel one) with Otober-

November2010data orresponding to aluminosityof

∼ 8

theFEBsare wellalignedwithagoodRMSof

0.4

Figure1.6showsthedistributionofthehannelstime

spread versus the hannel number. From the results

shownhereitislearthatwereahatimespreadofthe

singlehannelsinside aFEBanthe orderof

∼ 400

foralmostallthehannels.

1.5 Outlook

Inthispaperwehaveshowntheresultsofourwork

on the time alignment of the hannels of the LAr

alorimeteruptoafewhundredpslevel.Timingmea-

surement beomes more preise with higher statistis

aquired. It is not possible to ombine runs from the

wholeyear2010forthismeasurementbeausetheLHC

lokdriftof

2

eststatistirunsavailable.In2011thedatawillbea-

quired more eiently thus this problem is removed.

We will implement a new round of FEBs orretions

a new set of hannel by hannel orretions with the

early2011data.Thenwewill onentrateonstudying

small systemati eets, suh asa shift in the beam-

spot position, the analog pipeline storing delays and

the implementation of extrapolation proedure in the

OF phase ode whih would allow themodiationof

the preisionofthehannelby hannel orretionim-

plementation.Thosemodiationwouldn'tallowusto

reahultimatetimingalignmentonthelevelof

100

Aknowledgements

ThisworkisbeenpossiblethanksNiolasBergerwho

has produedthentuples usedfor thisanalysis,Paolo

Iengo and Maro Del Mastro for the implementation

of the time orretion whih we have produed and

IsabelleWingerter-Seez and Tetiana Hryn'ova for the

usefuldisussions.

Référenes

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[2℄ Conf.ATLAS-COM-LARG-2009-001T.Guillemin,

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[3℄ Conf. ATLAS-COM-LARG-2009-048 A. Penson

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