Jet and jet substructure physics at ALICE · Leticia Cunqueiro, Oak Ridge National Laboratory for...

JetandjetsubstructurephysicsatALICE

LeticiaCunqueiro,OakRidgeNationalLaboratoryfortheALICECollaboration

CERNseminar,24th September2019

1

Jetsinprotonprotonandheavy-ioncollisions

2

JetandjetsubstructureplayamajorroleintheLHCphysicsprogramme

Usejetsaswell-callibrated probesofthehotanddensemediumcreatedinheavy-ioncollisions

ThemodificationofthejetyieldsandandselectedhardsubstructurecanconstraindynamicalpropertiesofthehotanddenseQCDmedium.

Maingoalistostudythehigh-temperaturephaseofQCD toinvestigatehowmacroscopicpropertiesofthismatterarisefromQCDinteractions.

3

Quantifyingthemedium-inducedmodificationsofjets

• Inclusive/semi-inclusiveobservableslikejetpT spectraandratiosfordifferentresolutionR

• Quantificationofjetshapesbyfunctionsthatdependon4-momentaofconstituents(angularity,pTD,jetmass)…

• Usetheclusteringhistory:

theideaistounwindtheclusteringhistory(decluster thejet)toaccessthesubjet branchesorprongsateachdeclustering step

Thesethreetypeofmeasurementsaredynamicallyconnected!

4

plotfromG.Salam,QM18

UnwindtheCambridge-Aachenclustering(orotherconvenientalgorithm)

Ateachstep,registerthekT,∆R coordinates

Followthehardestbranchateachstep

d2P = 2↵s(k?)CR

⇡ dln(z✓)dln( 1✓ )

Invacuum,flat2DdensityexceptforvariationofthecouplingwithscalekTGeneralobservable,otherscanbederivedfromit

kT

𝛥R

Invacuum:theLundplane

5

Mapofsplittings inmedium

K.Tywoniuk etal,Noveltoolsandobservablesforjetphysicsinheavyioncollisons,https://arxiv.org/pdf/1808.03689.pdf

Multiplescalesinmedium:

MediumlengthL andtemperatureT

Splittings canbeproduced/decohered inside/outsidethemediumInterplaybetweenL,formationtimetf.anddecoherence timetd

Coherenceangle𝜽c=1/ 𝑞$𝐿3�

structureatanglessmallerthan𝜽cisnotresolvedbythemediumwhere𝒒)=<kT2>/𝜆 (GeV2/fm),thetransportcoefficientofthemediumand𝜆,thedensityofscatteringcentres.

Medium-inducedradiationfollowsdifferent“rules”thanvacuumradiation:Thedensityofsplittings inheavyioncollisionsisnotexpectedtobeconstantintheLundplane

resolved unresolved

6

TheLundplaneand(groomed)observables

MostofthejetfeaturesandsubstructurealgorithmscanbederivedfromtheLundplane.

Forinstance:

Fixedformationtimeisadiagonallinewithpostive slopeintheplanetf~2zE/kT2

Constantmomentumfractionzcanbedrawnasalinewithnegativeslope

ThenumberofentriesintheLundplaneisthenumberofdeclustering steps,n.

presentationbyF.Dreyer

ThedifferentSoftDrop(SD)groomingcutscanbedrawnasexclusionregionsintheplane

zg isthemomentumimbalanceofthefirstsplittingsatisfyingtheSDcutProcedureremovessoftandlargeangleemissionsuntilasufficientlysymmetricsplittingisfound

7

SoftDrop/massDrop groominginheavyioncollisions

Inpp:limitcontaminationofQCDbackground(andpileup) inacontrolledwaywhileretainingbulkofperturbativeradiation

InPbPb:select“hard”componentofquenchedjets.Toisolatemediumeffects,easiertocalculate

ALICEJetperformanceandcapabilities

8

Track-basedjets

FulljetsCentralbarrelplusEMCal clustersEMCal𝞿 coverage:1080|𝜂|<0.7AcceptedclusterspT,cluster>300MeV

Central-barreltracking|𝜂tracks|<0.9High-precision trackingdowntopT,track=0.15GeVJetsarecorrectedtochargedparticlelevel

ALICE,arXiv:1905.02536

9

0 50 100 150 200 250 300 350)c (GeV/

Tp

0

0.2

0.4

0.6

0.8

1

1.2

1.4hd Tpd=X

R s2 d /

hd Tpd=0

.2R s2 d

ALICE preliminary-1 = 4 pbintL = 13 TeV, spp,

TkJets, Anti- > 0.3 GeVclusterE, c > 0.15 GeV/track

Tp

R| < 0.7 - jeth| < 0.7, |clusterh| < 0.7, |trackh|

=0.3R=0.2/R

=0.4R=0.2/R

=0.5R=0.2/R

Correlated uncertaintiesShape uncertainties

POWHEG + PYTHIA8

ALI−PREL−315725

0 50 100 150 200 250 300 350)c (GeV/

Tp

8-10

7-10

6-10

5-10

4-10

3-10

2-10

1-10

1

10

210))c (m

b/(G

eV/

hd Tpds2 d

ALICE preliminary-1 = 4 pbintL = 13 TeV, spp,

TkJets, Anti- > 0.3 GeVclusterE, c > 0.15 GeV/track

Tp

R| < 0.7 - jeth| < 0.7, |clusterh| < 0.7, |trackh|=0.2R

=0.3 x 3R

=0.4 x 10R

=0.5 x 30R

=0.6 x 100R

Correlated uncertainties

Shape uncertainties

ALI−PREL−315682

WidekinematicreachattheLHC,fromfewGeVtotheTeV scale

RatiosofcrosssectionsfordifferentR aresensitivetothejettransverseenergyprofile

CrosssectionsandtheirratiosfordifferentjetresolutionR

10

SmallR dominatedbysplash-outhadronization losses(~1/R atLO)Large RdominatedbyUEatlowjetpT

NPcorrectionsvsR

CrosssectionsandtheirratiosfordifferentjetresolutionR

RatiosofcrosssectionsingoodagreementwithPOWHEG andfixedorderNLOcalculations


Theangularitiesinppcollisions

11

ALICE,JHEP1810(2018)139

Girth:λ11

Long.momentum dispersion~ λ01

12

ALI-PREL-310033

The2-pronggroomedmomentumimbalance,zg

Thezg invacuumisdirectlylinkedtotheAltarelli-Parisisplittingfunction

NojetpT dependenceobserved,asexpectedifmeasurementoftheQCDzkernel

Larkoskietal,Phys.Rev.Lett.119(2017)no.13,132003

The2-pronggroomedmomentumimbalance:zg andR dependence

13

ThejetR dependenceisexploredinawidekinematicrangeAtlowpT (left)plot,non-perturbativeeffectsobscuretheconnectiontotheQCDsplittingfunction

pT,jet

Thenumberofprongspassingthemass-dropcut

14


PYTHIA-basedsimulationsunderestimatethenumberofprongsnotpassingthemassdropcut

Thehardsplittingapertureangle

15

Rg

Thesplittingopeninganglefortwodifferentdeclustering strategies:exclusivekT andCA+mass dropgroomingthatselectdifferentsplittings inthejet

N=2exclusivekT declustering,apertureangle CA+SDgrooming,apertureangle

TheHeavyioncollisioncase

16

UncorrelatedbackgroundinHeavy-Ioncollisions

17ALICE,JHEP1203(2012)053

Largeregion-to-regionfluctuationsinthebackgroundinducelargejetmomentumirresolutionLow-pT /largeRcombinatorial:needtosuppresscombinatorialjetsSeveraltechniquesareappliedinALICE:data-drivenh-jetcorrelation,applyingaleadinghadronbiasetc.

Breakdownofcomponentsofthejetirresolutionduetobackground:numberofparticlesinthejetarea andflow

Averagepedestalbackground Backgroundfluctuationsvsmultiplicity

𝜹(pT)=pT,raw-𝞀Area– pT,probe

Thesuppressionofjetyieldsinheavyioncollisions

StrongsuppressionofjetyieldsinPb-Pb relativetopp

Nojet Rdependencewithinuncertainties,significantfractionofenergytransportedoutsidethejetcone18


19

Newapproachestosuppresscombinatorialbackground

Backgroundfluctuationssuppressed:-Combinatorialjetsreduced-Smallerunfoldingcorrection-ExtendtolowjetpT andlargeR

Caveat:backgroundsubtractionmightbecomemodeldependent

ALI-PERF-324612

Methodpaper:Haake,Loizides Phys.Rev.C99(2019)no.6,064904

Area-basedisanunbiasedmethod,knowsnothingaboutthejetstructure

pTsub=pTraw -𝜌Area

NewMLapproach:aregressionmodeltosubtractbackground.Inputparameters:area-basedsubtractedjetpT,angularity,numberofconstituents,pT ofthe8leadingjetconstituents

Backgroundfluctuations WidthofthefluctuationsvsjetR

20

Newapproachestosuppresscombinatorialbackground

ALI-PREL-324739ALI-PREL-325710

LowerpT reachcomparedtopreviousmeasurements

R =0.6measuredforthefirsttimeinALICEinPb-Pb collisions

TheradialprofilemeasuredviatheratioofcrosssectionswithdifferentR issimilartopp

Nuclearmodificationfactor RatioofcrosssectionsforR=0.2adR=0.6

21

Largepedestalbackground,impactonsubstructure

PedestalbackgroundcontaminationsubtractedusingArea-derivatives[1]andConstituentSubtraction[2]methods.Residualdifferencesbetweentheprobeandthesubtractedembeddedprobeareunfolded

[1]Soyez etal,Phys.Rev.Lett.110(2013)no.16,162001 [2]Bertaetal,JHEP1908(2019)175

SubtractionPerformanceangularity Subtractionperformance2-subjettiness

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trueSDn

0 2 4 6 8 10 12

em

bS

Dn

0

2

4

6

8

10

12

3−

10

2−10

1−10

ALICE Preliminary

= 2.76 TeVNN

s0-10% Pb-Pb Embedded PYTHIA

=0.4R Charged Jets, TkAnti-

c < 120 GeV/ch,part

T,jetp80 <

ALI-SIMUL-155669

truegR

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

em

bg

R0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

3−

10

2−10

1−10

ALICE Preliminary

= 2.76 TeVNN



c < 120 GeV/ch,part

T,jetp80 <

ALI-SIMUL-155665

truegz

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

em

bg

z

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

3−

10

2−10

1−10

ALICE Preliminary

= 2.76 TeVNN



c < 120 GeV/ch,part

T,jetp80 <

ALI-SIMUL-155673

Detectorandbackgroundresponseforjetsubstructure

Off-diagonalities aredominatedbycaseswherethesubleadingprongisreplacedbyahighermomentumprongsmearedbythebackgroundorpurelycombinatorial

Moreaggressivegroomingoralternativebackgroundsubtractiontobeexplored,inparticularforRg

TheangularitiesinPbPb collisions

Small-R jetsshowmodificationsinthefragmentation:

Smallerangularity(morecollimated)

LargerpTD (harderfragmentation)

Small-R jetsareinterestingtoconstraintheorymodelsbecausetheyarelesssensitivetocertainmodelingredientslikethewakeorcorrelatedbackground

ALICE,JHEP1810(2018)13923

TheangularitiesinPb-Pb collisions

Themodifiedfragmentationismoresimilartothatofquark-jettemplatesPicturecompatiblewithsoftfragmentstransportedatlargeanglesrelativetothejetaxis

ALICE,JHEP1810(2018)139

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ThegroomedmomentumimbalanceinPb-Pb collisions

ALICE,arXiv:1905.02512 25

Apertureangledependenceofthesplittings

Note:normalized tototalnumber ofjetsinthegivenpT bin


26

Apperture angledependenceofzg inmediumcomparedtovacuum(smeared)referenceAtlargeanglesΔR>0.2,suppressionofsymmetricsplittingsAtsmallanglesΔR<0.1,enhancementofsplittingsNoobservedenhancementofsplittings passingtheSDcutinmediumrelativetovacuumIncreasednumberofuntaggedjetsinmedium


27ALICE,arXiv:1905.02512



ThenumberofSDprongsinPb-Pb collisions

ThenSD countsthenumberofprongsthatpassthemassdropconditionasthejetisiterativelydeclustered

Smallernumberofprongsinmediumthaninvacuum(alsoinagreementwiththemorequark-likefragmentationshownbytheangularities)


Iscolorcoherencedrivingthesuppressionofthe2-prongsplittings?

30

Coherentlimit𝜃 <𝜃Csmall-angleprongsareresolvedasasinglecolorcharge

Incoherentlimit 𝜃 >𝜃Clargeangleprongsareresolvedbythemedium independentlyandthusmorequenched

However:thedatatrendsofthezg aredescribedbytwomodels(JEWELandHYBRID)thatdonothavecolorcoherencebuiltin;thissuggeststhatthedrivingmechanismissomethingsimplerlikeformationtimeorkinematicbiases.

Resolvedsubstructure Unresolvedsubstructure

Casalderrey-Solana,Mehtar-Tani,Salgado,Tywoniuk Phys.Lett.B725(2013)357–360

31

Ifitisnotcolorcoherencewhatdictatesthesuppressionofsplittingsatlargeangle,isitdrivenbyformationtimeeffects?

Atlargeangles/energies:

Smallpopulationofunmodifiedvacuumsplittings:earlyvacuumsplittings arequenchedastheytraversethemedium

Orisitakinematiceffect?ByselectinglargeanglesplittingswebiastowardshigherQ2 structuresthataremorequenched?

tf~2zE/kT2 ~1/(z𝜽2)

LilianaApolinario,https://indico.cern.ch/event/577856/contributions/3424218/

Formation timevsdeclustering steps,jetchronography?

5- 4- 3- 2- 1- 0 1 2 3 4 5

)hardest,graphT

log(k

5-4-3-2-1-012345)

hard

est,

decl

uste

ring

Tlo

g(k

1

10

210

310

410 = 5.02 TeVsPythia8 pp > 80 GeV/c jet

Tp

5- 4- 3- 2- 1- 0 1 2 3 4 5

)hardest,graphT

log(k5-4-3-2-1-012345)

hard

est&

&SD

, dec

lust

erin

gT

log(

k

1

10

210

310 = 5.02 TeV + HIC toy backgroundsPythia8 pp

> 80 GeV/c, Constituent Subtraction jetT

p

Correlationbetweentheparton showerandthedeclustering

https://indico.gsi.de/event/8938/

ThesekindoftestsproposedanddiscussedatEmmiworkshop:thespace-timestructureofjetquenching,theoryandexperiment

Canwefindaregionofthephasespaceforwhichthecorrelationbetweenthegraphandthedeclusteredbranchesisrobust?ThehardestemissionfromthegraphseemswellcorrelatedtothehardestbranchafterembeddingintoheavyionbackgroundconstituentsubtractionandSDgrooming.

32

Newideasonjetsubstructureandheavyflavours

33

Thedeadcone

Probability foraheavyquarkQtoradiateasoftgluon:

Inthelimitofsmallanglesbutstill

Inthecasewhere

Y.Dokshitzer,V.A.Khoze andS.I.Troyan “OnspecificQCDpropertiesofheavyquarkfragmentation”

34

where𝚹0 =mQ/E

Battaglia etal,http://cds.cern.ch/record/989441/files/cer-002652356.pdf

Thedeadcone:implicationsandexperimentaldifficulties

35

Thedecaysoftheheavyflavour particleshappenatsimilarangularscalesandpollutethedeadcone

Battaglia,Orava,Sami “Astudyofdepletionoffragmentationparticlesatsmallanglesinb-jetswiththeDELPHIdetectoratLEP”Thefirstandonlyattemptofdirectdeadconemeasurementatcolliders

Choiceofareferenceaxis:jetaxis,thrust,thedirectionofthevertex?

Thereareessentially2indirectconsequencesofthedeadcone:

-RestrictionofhardgluonswithsmallkT ,whichisthemostprobablecomponentoftheradiation(P~𝞪S(kT2))->reductionofz,FFpeakedatlargervaluesofz

-Smallerintrajetmultiplicities

kT

zpt,jet

TheLundplaneforheavyflavours

QQ

Q

Wefollowtheprongcontainingtheheavyflavorateachdeclustering step

Prongwithheavyflavour almostalwaysthehardest:noambiguitywhencomparingtolightquarks

Ateachstepofthejettree:parentsubjet,Eradiator.

For𝛳 <mQ/Eradiator weexpecttoseeasuppressionofsplittings.

ForlowEradiator ,whichmeanspenetratingthejettodeeplevels,thephasespaceforthedeadconeobservationgrows. 36

Eradiator,1Eradiator,2

Eradiator,3

𝛳1

𝛳2

𝛳3

TheLundplaneforheavyflavoursLundplaneredrawnintermsofthesplittingangleandtheparent(radiator)energy

Relativedifference toinclusivejets:blueregions indicatesuppressionofradiationofftheheavy-flavourprong

Forgivenradiatorenergy, suppressioneffectsappearatsmalleranglesforD-jetsthanB-jets,asexpected

Akeyaspectistoselecthardsplittingsbycuttingln(kT)>0, tosuppresshadronization effects

Cunqueiro,Ploskon,Phys.Rev.D99(2019)no.7,074027 37

D-jets,parton-level B-jets,parton-level

D-jets,hadron-level B-jets,hadron-level

Relativ

edifferen

cestoinclusivejets

Thedistributionofsplittingangles

Pythiasimulationspredictsuppressioneffectsof theorderof20%(100%)forD-jet(B-jets)relativetoinclusivejetsatanglesof0.1radianstypicallyresolvablewithatrackingdevice.

NoconstrainonthejetpT ,itissufficient todecluster thejetdeepenough toreachsubjet energyscalessenstivie tomassscaleCunqueiro,Ploskon,Phys.Rev.D99(2019)no.7,074027 38

Relativediffference ofD-jetsandinclusivejets Relativediffference ofB-jetsandinclusivejets

HeavyFlavour jetswithfullkinematicreconstructionoftheheavyflavour hadron

D0invariantmassdistributionfortheKpi channel FullyreconstructedD0-jetspectrum

D0 mesonDecaychannelD0->K-𝞹+,BR=3.89%TPCdE/dx+TOFPIDforK/𝞹 discriminationTopological selections(secondaryvertex)pT,D>3GeV/c ALICE,JHEP1908(2019)133 39

Upcomingnewresultsat13TeV!

Isitpossibletomeasurethedeadconedirectlyat13TeV usingD0-jets?staytunedforQMinonemonth:https://indico.cern.ch/event/792436/

Otherfundamentalpropertiesofheavyflavour jetswithinreach:

HeavyFlavour jetswithfullkinematicreconstructionoftheheavyflavour hadron

ALICE,JHEP1908(2019)133

P.IltenetalPhys.Rev.D96(2017)

Measurethegroomed splittingmomentum imbalancezg,whentheleadingprong isheavy-flavourTagged.->Q->Qg

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D0-jetfragmentationfunction

Summary

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• Strongsuppressionofjetyieldsandlargeoutofconeradiationinheavyioncollisionsrelativetopp

• Small-RjetshapesindicateaharderandmorecollimatedfragmentationofthejetcoreinHIC

• Substructuremeasurementsusingclusteringhistoryverypromissing:

Accessregionsoftheintra-jetphasespacethataretheoretically/experimentallymorerobust

Suppressionofsymmetricsplittings atlargeapertureanglesinHICrelativetopp.earlysplittings thatarefullyembeddeden mediumandthusmorequenched?

Accesssplittingtimeviathebranchkinematics?

• NewbackgroundsubtractiontechniquesallowtoexpandtheexploredphasespacetolowjetpT andlargeR,nex:extensiontojetsubstructure

• Interestingprospectsforheavyflavour jetsandsubstructure,fundamentalpropertieslikedeadcone

StaytunedforQM19inNovember!

ALI-SIMUL-320949

HeavyFlavour jets

OntheimportanceoftheReconstructionofthedecays

43PlotsfromKatharinaGarner, showing theeffectofturningonthedecaysatthelevelofsubstructureofb-jets

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ThesplittingmapinPbPb

Noveltoolsandobservables forjetphysicsinheavy-ioncollisionsAndrewsetal,https://arxiv.org/pdf/1808.03689.pdf

Powerfulvisualtoolsforsearchesofmedium-inducedsignal

10 20 30 40 50 60 70 80 90 100

/GeV

)c

) (m

b hd

ch T,je

tp

/(ds2 d

5-10

4-10

3-10

2-10

1-10 ALICE Preliminary

= 5.02 TeVNNsPb -p

= 0.4R, Tkcharged b-jets, anti-

)c (GeV/chT,jetp

10 20 30 40 50 60 70 80 90 100

Ratio

to d

ata

0.5

1

1.5

2

2.5Datasystematic uncertaintyPOWHEG HVQPOWHEG systematic uncertainty

ALI−PREL−323633

HeavyFlavour jets,workinprogressFullyreconstructedb-jetspectrum

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Thebranchingformationtime

Canweusetf ofthedeclustered branchestomapthetimeintheheavyioncollision?

Futurephysicsopportunities forhigh-density QCDattheLHCwithheavy-ionandprotonbeamsZ.Citron etal,https://arxiv.org/abs/1812.06772

Fixedbranchingtimetf =2𝟂/kT2.arediagonallinesintheLundplane

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Jet and jet substructure physics at ALICE · Leticia Cunqueiro, Oak Ridge National Laboratory for...

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