Lepton identification at particle flow oriented detector for the future $e^{+}e^{-}$ Higgs factories

1 MB·

by  Dan Yu

Eur.Phys.J.CmanuscriptNo.
(willbeinsertedbytheeditor)
Lepton identification at particle flow oriented detector for the future
e+e− Higgs factories
DanYu1,2,ManqiRuana,1,VincentBoudry2,HenriVideau2
1IHEP,China
2LLR,EcolePolytechnique,France
7
1
0
2
Received:date/Accepted:date

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F
Abstract Theleptonidentificationisessentialforthephysics In terms of Higgs measurements, the electron-positron

7 programs at high-energy frontier, especially for the precise colliders play a role complementary to the hadron collid-
1 measurementoftheHiggsboson.Forthispurpose,aToolkit erswithdistinguishableadvantages.Manyelectron-positron

for Multivariate Data Analysis (TMVA) based lepton iden- Higgs factories have been proposed, including the Interna-
]
t tification (LICH1) has been developed for detectors using tional Linear Collider (ILC), the Compact LInear Collider
e
d highgranularitycalorimeters.Usingtheconceptualdetector (CLIC),theFuturee+e-CircularCollider(FCC-ee)andthe
- geometryfortheCircularElectron-PositronCollider(CEPC) CEPC[1][5][6].Theseproposedelectron-positronHiggsfac-
s
n andsinglechargedparticlesampleswithenergylargerthan tories pick and reconstruct Higgs events with an efficiency
i 2 GeV, LICH identifies electrons/muons with efficiencies closeto100%,anddeterminetheabsolutevalueoftheHiggs
.
s
higherthan99.5%andcontrolsthemis-identificationrateof couplings.ComparedtotheLHC,thesefacilitieshavemuch
c
i hadron to muons/electrons to better than 1%/0.5%. Reduc- betteraccuracyontheHiggstotalwidthmeasurementsand
s
y ingthecalorimetergranularityby1-2ordersofmagnitude, Higgs exotic decay searches, in addition the accuracies of
h the lepton identification performance is stable for particles Higgs measurements are dominated by statistic errors. For
p
withE>2GeV.AppliedtofullysimulatedeeH/µµHevents, example, the circular electron-positron collider (CEPC) is
[
the lepton identification performance is consistent with the expectedtodeliver1millionHiggsbosonsinitsHiggsop-

2 singleparticlecase:theefficiencyofidentifyingallthehigh eration,withwhichtheHiggscouplingswillbemeasuredto
v
energyleptonsinanevent,is95.5-98.5%. percentorevenpermillelevelaccuracy[6].
2
TheleptonidentificationisessentialtothepreciseHiggs
4
5 measurements.TheStandardModelHiggsbosonhasroughly
7 1 Introduction 10%chancetodecayintofinalstateswithleptons,forexam-
0
ple, H→ WW* →llvv/lvqq, H→ZZ*→llqq, H→ττ, H→
. After the Higgs discovery, the precise determination of the
1 µµ,etc.TheSMHiggsalsohasabranchingratioBr(H→bb)
0 Higgsbosonpropertiesbecomesthefocusofparticlephysics
= 58%, while the lepton identification provides an impor-
7 experiments.Phenomenologicalstudiesshowthatthephysics
tant input for the jet flavor tagging and the jet charge mea-
1
atTeVscalewouldberevealediftheHiggscouplingscould
: surement.Ontopofthat,theHiggsbosonhasasignificant
v reachthepercentlevelmeasurementaccuracy[1][2].
chancetobegeneratedtogetherwithleptons.Forexample,
i
X TheLHCisapowerfulHiggsfactory.However,thepre- in the ZH events, the leading Higgs generation process at
cisionofHiggsmeasurementsattheLHCislimitedbythe
r 240-250 GeV electron-positron collisions, about 7% of the
a hugeQCDbackground,thelargetheoreticalandsystemati-
Higgs bosons are generated together with a pair of leptons
caluncertainties.Inaddition,theHiggssignalattheLHCis
( Br(Z→ee) and Br(Z→ µµ) = 3.36% ). At the electron-
usually tagged by the Higgs decay products, making those
positron collider, ZH events with Z decaying into a pair of
measurementsalwaysmodeldependent.Therefore,thepre-
leptonsisregardedasthegoldenchannelfortheHZZcou-
cisionofHiggscouplingsattheHL-LHCistypicallylimited
pling and Higgs mass measurement[7]. Furthermore, lep-
to5-10%leveldependingontheoreticalassumptions[3][4].
tons are intensively used as a trigger signal for the proton
ae-mail:[email protected] colliderstopickupthephysicseventsfromthehugeQCD
1LeptonIdentificationforCalorimeterwithHighgranularity backgrounds. The Particle Flow Algorithm (PFA) becomes
2
theparadigmofdetectordesignforthehighenergyfrontier[8, tracking system and a calorimeter systems with extremely
9,6,12].Thekeyideaistoreconstructeveryfinalstatepar- highgranularity.
ticle in the most suited sub-detectors, and reconstruct all InthisCEPCconceptualdetectordesign,theforwardre-
the physics objects on top of the final state particles. The gion,andtheyokethicknesshavebeenadjustedtotheCEPC
PFA oriented detectors have high efficiency in reconstruct- collisionenvironmentwithrespecttotheILDdetector.The
ingphysicsobjectssuchasleptons,jets,andmissingenergy. core part of this detector is a large solenoid of 3.5 Tesla.
The PFA also significantly improves the jet energy resolu- Thesolenoidsystemhasaninnerradiusof3.4metersanda
tion, since the charged particles, which contribute the ma- lengthof8.05meters,insidewhichbothtrackerandcalori-
jority of jet energy, are usually measured with much better metersystemareinstalled.Thetrackingsystemiscomposed
accuraciesinthetrackersthaninthecalorimeters[14,9,10, ofaTPCasthemaintracker,avertexsystem,andthesili-
11,13]. contrackingdevices.Theamountofmaterialinfrontofthe
Toreconstructeveryfinalstateparticle,thePFArequires calorimeter is kept to ∼ 5% radiation length. Both ECAL
excellentseparationbyemployinghighly-granularcalorime- andHCALusesamplingstructuresandhaveextremelyhigh
ters. In the detector designs of the International Large De- granularity. The ECAL uses tungsten as the absorber and
tector (ILD) or the Silicon Detector (SiD) [1,15], the total silicon for the sensor. In depth, the ECAL is divided into
numberofreadoutchannelsincalorimetersreachesthe108 30 layers and in the transverse direction, each layer is di-
level. In addition to cluster separation, detailed spatial, en- videdinto5by5mm2 cells.TheHCALusesstainlesssteel
ergyandeventimeinformationontheshowerdevelopments absorberandGRPC(GlassResistivePlateChamber)sensor
is provided. An accurate interpretation of this recorded in- layers.Ituses10by10mm2cellsandhas48layersintotal.
formationwillenhancethephysicsperformanceofthefull As a Higgs factory, the CEPC will be operated at 240-
detector[16]. 250GeVcenterofmassenergy.Tostudytheadequatelep-
Using the information recorded in the high granularity tonidentificationperformance,wesimulatedsingleparticle
calorimeterandthedE/dxinformationrecordedinthetracker, samples(pion+,muon-,andelectron-)overanenergyrange
LICH(LeptonIdentificationinCalorimeterwithHighgran- of1-120GeV(1,2,3,5,7,10,20,30,40,50,70,120GeV).
ularity),adedicatedleptonidentificationalgorithmforHiggs Ateachenergypoint,100keventsaresimulatedforeachpar-
factories has been developed. Using CEPC conceptual de- ticle type. These samples follow a flat distribution in theta
tectorgeometry[6](basedonILD)andtheArbor[14]recon- andphioverthe4π solidangle.
structionpackage,itsperformanceistestedonsingleparti- ThesesamplesarereconstructedwithArbor(version3.3).
clesandphysicsevents.Forthesingleparticleswithenergy To disentangle the lepton identification performance from
higher than 2 GeV, LICH reaches an efficiency better than the effect of PFA reconstruction and geometry defects, we
99.5%inidentifyingthemuonsandtheelectrons,and98% select those events where only one charged particle is re-
forpions.Itsperformanceonphysicsevents(eeH/µµH)and constructed. The total number of these events is recorded
the final efficiency agrees with the efficiency at the single asN ,andthenumberoftheseeventsidentifiedwith
1Particle
particlelevel. correctparticletypesisrecordedasN .Theperfor-
1Particle,T
This paper is organized as follows. The detector geom- manceofleptonidentificationisthenexpressedasamigra-
etry and the samples are presented in section 2. In section tion matrix in Table 2, its diagonal elements εi refer to the
i
3,thediscriminantvariablesmeasuredfromchargedrecon- identification efficiencies (defined as N /N ),
1Particle,T 1Particle
structed particles are summarized and the algorithm archi- andtheoffdiagonalelementPi representtheprobabilityof
j
tectureispresented.Insection4,theLICHperformanceon atypeiparticletobemis-identifiedastype j.
single particle events is presented. In section 5, the corre-
lationsbetweenLICHperformanceandthecalorimeterge-
Table1 MigrationMatrix
ometryareexplored.Insection6,theLICHperformanceon
ZHeventswhereZdecaysintoeeorµµ pairsisstudied,the e−like µ−like π+like undefined
resultsarethencomparedwiththatofsingleparticleevents.
e− εe Pe Pe Pe
e µ π und
In section 7, the results are summarized and the impact of µ− Peµ εµµ Pπµ Puµnd
calorimetergranularityisdiscussed. π+ Pπ Pπ επ Pπ
e µ π und
2 Detectorgeometryandsample
Inthispaper,thereferencegeometryistheCEPCconceptual 3 Discriminantvariablesandtheoutputlikelihoods
detector[6],whichisdevelopedfromtheILDgeometry[1].
ILDisaPFAorienteddetectormeanttobeusedforcentreof LICHtakesindividualreconstructedchargedparticlesasin-
massenergiesupto1TeV.Itisequippedwithalowmaterial put, extracts 24 discriminant variables for the lepton iden-
3
tification,andcalculatesthecorrespondinglikelihoodtobe
anelectronoramuon.Thesediscriminantvariablescanbe
characterizedintofivedifferentclasses:
Electron
800
– dE/dx Muon
ForatrackintheTPC,thedistributionofenergylossper 600
Pion
unit distance follows a Landau distribution. The dE/dx
estimatorusedhereistheaverageofthisvaluebutafter 400
cuttingtailsatthetwoedgesoftheLandaudistribution
(first7%andlast30%).ThedE/dxhasastrongdiscrim-
200
inantpowertodistinguishelectrontracksfromothersat
lowenergy(under10GeV)(Figure1).
0
0 0.2 0.4 0.6 0.8 1
FD_all
· 10-6
x 0.5 Fig.2 FractaldimensionusingbothECALandHCALfore−,µ−and
d 90
E/ π+at40GeV
d 80
0.4 70
60
energydepositedinacylinderaroundtheincidentdirec-
0.3 50
tionwitharadiusof1and1.5Moliereradius.
40
30
– HitsInformation
0.2
20
Hits information refers to the number of hits in ECAL
10
and HCAL and some other information obtained from
0.1 0
0 0.2 0.4 0.6 0.8 1 1.2 hits,suchasthenumberofECAL(HCAL)layershitby
log10(TrkEn)
theshower,numberofhitsinthefirst10layersofECAL.
Fig.1 dE/dxfore−,µ−andπ+,forelectronsitisstablearound2.4×
10−7,formuonandpionitissmalleratenergylowerthan10GeVand – ShowerShape,SpatialInformation
afterthattheystartmixingwithelectron
Thespatialvariablesincludethemaximumdistancebe-
tweenahitandtheextrapolatedtrack,themaximumdis-
tanceandaveragedistancebetweenshowerhitsandthe
– FractalDimension axis of the shower (defined by the innermost point and
The fractal dimension (FD) of a shower is used to de- the center of gravity of the shower), the depth (perpen-
scribe the self-similar behavior of shower spatial con- diculartothedetectorlayers)ofthecenterofgravity,and
figurations,followingtheoriginaldefinitionin[16],the thedepthoftheshowerdefinedasthedepthbetweenthe
fractal dimension is directly linked to the compactness innermosthitandtheoutermosthit.
oftheparticleshower.
Atafixedenergy,theEMshowersaremuchmorecom-
Thecorrelationofthosevariablesatenergy40GeVare
pactthanthemuonorhadronshower,leadingtoalarge
summarized in Figure 3, the definitions of all the variables
FD.Themuonshowerusuallytakestheconfigurationof
are listed in Appendix A. It is clear that the dE/dx, mea-
a1-dimensionalMIP(MinimumIonizingParticle)track,
sured from tracks, does not correlate with any other vari-
thereforehasaFDclosetozero.TheFDofthehadronic
ables which are measured from calorimeters. Some of the
shower usually lays between the EM and MIP tracks,
variablesarehighlycorrelated,suchasFD_ECAL(FDcal-
sinceitcontainsbothEMandMIPcomponents.Atyp-
culated from ECAL hits) and EcalNHit (number of ECAL
icaldistributionofFDfor40GeVshowersispresented
hits).Howeverallthesevariablesarekeptbecausetheircor-
inFigure2,
relationschangewithenergyandpolarangle.
Foranycalorimetercluster,LICHcalculates5different
LICH uses TMVA[17] methods to summarize these in-
FDvalues:fromitsECALhits,HCALhits,hitsin10or
put variables into two likelihoods, corresponding to elec-
20firstlayersofECAL,andallthecalorimeterhits.
tronsandmuons.MultipleTMVAmethodshavebeentested
– EnergyDistribution andtheBoostedDecisionTreeswithGradientboosting(BDTG)
LICH builds variables out of the shower energy infor- methodischosenforitsbetterperformance.Thee-likeness
mation,includingtheproportionofenergydepositedin (L )and µ-likeness(L )fordifferentparticlesina40GeV
e µ
the first 10 layers in ECAL to the entire ECAL, or the sampleareshowninFigure4.
4
– barrel1:middleofbarrel(|cosθ|<0.3),
– barrel2:edgeofbarrel(0.3<|cosθ|<0.7),
100
dEdx
NH_ECALF10 – overlap:overlapregionofbarrelandendcap(0.7<|cosθ|
FD_ECALL20 80
<0.8),
FD_ECALF10
aAvL__NEHcHal 60 – endcap:(0.8<|cosθ|<0.98).
rms_Hcal
EEClu_r 40
EEClu_R Take the sample of 40 GeV charged particle as an ex-
EEClu_L10 20
FD_HCAL ample,themigrationmatrixisshowninTable2.Comparing
FD_FEDC_AaLll 0 thistabletotheresultofALEPHforenergetictaus[18],the
MaxDisHel
minDepth -20 efficiencies are improved, and the mis-identification rates
cluDepth
graDepth -40 fromhadronstoleptonsaresignificantlyreduced.
EcalEn
avDisHtoL
-60
maxDisHtoL
NLHcal
NLEcal -80 Table2 MigrationMatrixat40GeV(%)
HcalNHit
EcalNHit -100
Type e−like µ−like π+like
Fig.3 Thecorrelationmatrixofallthevariables e− 99.71±0.08 <0.07 0.21±0.07
µ− <0.07 99.87±0.08 0.05±0.05
π+ 0.14±0.05 0.35±0.08 99.26±0.12
Theleptonidentificationefficiencies(diagonaltermsof
the migration matrix) at different energies are presented in
Figure5forthedifferentregions.Theidentificationefficien-
ciessaturateat99.9%forparticleswithenergyhigherthan
2GeV.Forthosewithenergylowerthan2GeV,theperfor-
mancedropssignificantly,especiallyinbarrel2andoverlap
regions.Fortheoverlapregion,thecomplexgeometrylim-
its the performance; while for the barrel2 region, charged
particles with Pt < 0.97 GeV cannot reach the barrel, they
willeventuallyhittheendcapsatlargeincidentangle,hence
theirsignalismoredifficulttocatalog.
Concerningtheoff-diagonaltermsofthemigrationma-
Fig.4 Thee-likelinessandµ-likenessofe−,µ− andπ+ at40GeV,
trix,thechancesofelectronstobemis-identifiedasmuons
greylinesarethecutsfordifferentcatalogsinnextsection
and pions are negligible (Pe,Pe <10−3), the crosstalk rate
µ π
Pµ is observed at even lower level. However, the chances
e
4 Performanceonsingleparticleevents of pions to be mis-identified as leptons (Pπ, Pπ) are of the
e µ
order of 1% and are energy dependent. In fact, these mis-
Thephasespacespannedbythelepton-likelihoods(L and
e identificationsaremainlyinducedbytheirreduciblephysics
Lµ)canbeseparatedintodifferentdomains,corresponding effects: pion decay and π0 generation via π-nucleon colli-
to different catalogs of particles. The domains for particles
sion.Meanwhile,themuonsalsohaveasmallchancetobe
of different types can be adjusted according to physics re-
mis-identifiedaspionsatenergysmallerthan2GeV.Figure
quirements. In this paper, we demonstrate the lepton iden- 6 shows the significant crosstalk items (Pπ, Pπand Pµ) as
e µ π
tification performance on single particle samples using the
a function of the particle energy in the endcap region. The
followingcatalogs:
green shaded band indicates the probability of pion decay
– Muon:L >0.5 beforereachingthecalorimeter,whichisroughlycompara-
µ
– Electron:Le>0.5 blewithPµπ.
– Pion:1-(L +L )>0.5
µ e
– Undefined:L <0.5&L <0.5&1-(L +L )<0.5
µ e µ e
5 Leptonidentificationperformanceonsingleparticle
Theprobabilitiesofundefinedparticlesareverylow(<10−3)eventsfordifferentgeometries
atsingleparticlesampleswiththeabovecatalog.
polaSrinanceglteheoEfcdHalticNNhsaHtlLeNNriEitLHmibcHniatuaalcixvtatDDEiiloaicisgsanlHHrlEacttpoDloonuLaLmefDpriMentttphihDatcFheexDlpDFse_tDiehasF_(HlDlEθvEe_ClaE)HEArC,CEiLEltauCAEh_brlLCuLmel_al1esuRv0T_s_A_HrNMLdcFH_aDeElVFH_pcDEAaNe_lCHEndAC_iEdsLEdAsFCxLt1AorL0aLn2i0Fn1t0ehde Tetheerspyoswteemrcsocnasluemwpitthiotnheannduemlebcetrroonfircecaodsotuotfcthhaenncaellos.riImt’s-
independentlyonfoursubsets: importanttoevaluatethephysicsperformancefordifferent
5
barrel1 barrel2 number less than 20, marginal performance degradation is
%)102 %)102 observed: the efficiency of identifying muons degrades by
eff(100 eff(100 1-2% for low energy particles (E ≤ 2 GeV), and the iden-
98 98 tification efficiency of pion degrades slightly over the full
96 electron 96 electron energyrange,seeFigure7.
muon muon
94 94
pion pion
92 92 hcal cellsize 60mm hcal 20 layers
90 90
1 10 102 1 10 102 %)102 %)102
overlap Energy endcap Energy tagged eff(19080 tagged eff(19080
%)102 %)102 96 muon 96 muon
eff(100 eff(100 94 pion 94 pion
98 98
92 92
96 electron 96 electron
90 90
muon muon 1 10 Ener1g02y 1 10 Ener1g02y
94 94
pion pion
92 92 Fig.7 Theefficiencyofleptonidentificationfortwodifferentgeome-
90 90 tries
1 10 102 1 10 102
Energy Energy
Fig. 5 The efficiency of lepton identification for e−, µ− and π+ as
functionofparticleenergyinthefourregions
6 Performanceonphysicsevents
endcap
The Higgs boson is mainly generated through the Higgsst-
%)6
eff( Ppm rbaohslounngfupsirooncepsrsoc(ZesHse)saantdemlecotrreonm-parogsiintraolnlyHthigrogusgfahcvtoerciteosr.
Pp
4 em AsignificantpartoftheHiggsbosonswillbegeneratedto-
Pp
pion decay rate gether with a pair of leptons (electrons and muons). These
leptonsaregeneratedfromtheZbosondecayoftheZHpro-
2 cess.Forthe electrons,theycanalsobegenerated together
withHiggsbosonintheZbosonfusionsevents,seeFigure
8. At the CEPC, 3.6×104 µµH events and 3.9×104 eeH
0 eventsareexpectedatanintegratedluminosityof5ab−1.In
1 10 Ene1r0g2y theseevents,theparticlesareratherisolated.
Fig.6 Themis-identificationratesofleptonidentificationfor µ and
π in ∼ 5000 events for the endcap region; Pion decay rate band (to
accountforthepolaranglespread)isindicatedforcomparison
calorimetergranularities,atwhichtheLICHperformanceis
analyzed.
The performance is scanned over certain ranges of the
followingparameters: Fig.8 FeynmandiagramsofmajorHiggsproductionwithleptonsat
CEPC:theHiggsstrahlungandZZfusionprocesses.
– the number of layers in ECAL, taking the value of 20,
26,30;
TheeeHandµµHeventsprovideanexcellentaccessto
– thenumberoflayersinHCAL:20,30,40,48;
themodel-independentmeasurementtotheHiggsbosonus-
– the ECAL cell size = 5×5 mm2, 10×10 mm2, 20×20
ingtherecoilmassmethod[7].Therecoilmassspectrumof
mm2,40×40mm2
eeHandµµHeventsisshowninFigure9,whichexhibitsa
– HCAL cell size = 10×10 mm2, 20×20 mm2, 40×40
highenergytailinducedbytheradiationeffects(ISR,FSR,
mm2,60×60mm2,80×80mm2
bremsstrahlung,beamstrahlung,etc),whileinCEPCthebe-
In general, the lepton identification performance is ex- amstrahlungeffectisnegligible.Thebremsstrahlungeffects
tremely stable over the scanned parameter space. Only for forthemuonsaresignificantlysmallerthanthatfortheelec-
HCAL cell size larger than 60×60 mm2 or HCAL layer trons,therefore,ithasahighermaximumandasmallertail.
6
thelastrowofTable3.Theeventidentificationefficiencies
is roughly the square of the identification efficiency of the
initialleptons.Comparingtheperformanceofbothgeome-
tries,itisshownthatwhenthenumberofreadoutchannelsis
reducedby4,theeventreconstructionefficiencyisdegraded
by1.3%and1.7%,forµµHandeeHeventsrespectively.
Fig. 9 The recoil mass spectrum of ee/µµ, low energy peak in eeH
correspondstotheZfusionevents
7 Conclusion
Figure 10 shows the energy spectrum for all the recon- Thehighgranularitycalorimeterisapromisingtechnology
structedchargedparticlesin10keeH/µµHevents.Thelep- fordetectorsincolliderfacilitiesoftheHighEnergyFron-
tons could be classified into 2 classes, the initial leptons tiers.Itprovidesgoodseparationbetweendifferentfinalstate
(those generated together with the Higgs boson) and those particles, which is essential for the PFA reconstructions. It
generatedfromtheHiggsbosondecaycascade.FortheeeH alsorecordstheshowerspatialdevelopmentandenergypro-
events, the energy spectrum of the initial electron exhibits filetoanunprecedentedlevelofdetails,whichcanbeused
a small peak at low energy, corresponding to the Z fusion fortheenergymeasurementandparticleidentifications.
events. The precise identification of these initial leptons is To exploit the capability of lepton identification with
the key physics objective for the lepton identification per- high granularity calorimeters and also to provide a viable
formanceofthedetector. toolkitforthefutureHiggsfactories,LICH,aTMVAbased
leptonidentificationpackagededicatedtohighgranularca-
lorimeter,hasbeendeveloped.Usingmostlytheshowerde-
101404104 OElreIMOEincglrtuieirtincogioataninrnll o famefnrrlloe uoefcmmorlteonr c HmoHtnri igoHggngisgsgs 104 IMnuitoianl fmroumon Higgs scription variables extracted from the high granularity ca-
101303103 MHauEHMHdolaareundodcontrrnrooonnn 103 EHlaedctrroonn lorimeter and also the dE/dx information measured from
102
101202 102 tracker, LICH calculates the e-likeness and µ-likeness for
10
1010 10 each individually reconstructed charged particle. Based on
1
11 0 20 40 MC60 Energ8y0 (GeV1)00 1 these output likelihoods, the leptons can be identified ac-
00 2020 4040 6060 8080 101000 0 20 40 60 80 100
MMC CE nEenregryg (yG (GeVe)V) MC Energy (GeV) cordingtodifferentphysicsrequirement.
Fig.10 EnergySpectrumofchargedparticlesineeHeventat250GeV
Applied to single particle samples simulated with the
centerofmassenergy
CEPC_v1 detector geometry, the typical identification effi-
ciencyforelectronandmuonishigherthan99.5%forener-
Since the lepton identification performance depends on gieshigherthan2GeV.Forpions,theefficiencyisreaching
the particle energy, and most of the initial leptons have an 98%.Theseefficienciesarecomparabletotheperformance
energyhigherthan20GeV,wefocusedontheperformance reached by ALEPH, while the mis-identification rates are
studyofleptonidentificationonthesehighenergyparticles significantlyimproved.Ultimately,theperformancesarelim-
atdetectorswithtwodifferentsetsofcalorimetercellsizes. itedbytheirreducibleconfusions,inthesensethatthechance
The µ-likeliness and e-likeliness of electrons, muons, for muon to be mis-identified as electron and vice versa is
and pions, for eeH events and µµH events are shown in negligible, the mis-identification of pion to muon is domi-
Figure11andFigure12.Table3summarizesthedefinition natedbythepiondecay.
of leptons and the corresponding performance at different The tested geometry uses a ultra-high granularity calo-
conditions.Theidentificationefficienciesfortheinitiallep- rimeter: the cell size is 1 by 1 cm2 and the layer number
tons is degraded by 1-2% with respect to the single parti- ofECAL/HCALis30/48.Inordertoreducethetotalchan-
cle case. This degradation is mainly caused by the shower nelnumber,LICHisappliedtoamuchmoremodestgran-
overlap,andit’smuchmoresignificantforelectronsaselec- ularity,itisfoundthattheleptonidentificationperformance
tronshowersaremuchwiderthanthatofmuon,leadingtoa degrades only at particle energies lower than 2 GeV for an
larger chance of overlapping. The electrons in µµH events HCALcellsizebiggerthan60×60mm2 orwithanHCAL
andviceversa,aregeneratedintheHiggsdecay.Theiriden- layernumberlessthan20.
tification efficiency and purity still remains at a reasonable The lepton identification performance of LICH is also
level. For charged leptons with energy lower than 20 GeV, tested on the most important physics events at CEPC. In
theperformancedegradesbyabout10%becauseofthehigh theseevents,multiplefinalstateparticlescouldbeproduced
statistics of background and the cluster overlap. The event inasinglecollision,theparticleidentificationperformance
identification efficiency, which is defined as the chance of willpotentiallybedegradedbytheoverlapbetweennearby
successfullyidentifyingbothinitialleptons,ispresentedin particles.TheleptonidentificationoneeH/µµHeventat250
7
Table3 µµH/eeHeventsleptonidentificationefficiency
Geom1(ECALandHCALCellSize10×10mm2) Geom2(ECALandHCALCellSize20×20mm2)
µµH eeH µµH eeH
µdefinition Lµ>0.1 Lµ>0.1 Lµ>0.1 Lµ>0.1
edefinition Le>0.01Lµ<0.1 Le>0.001Lµ<0.1 Le>0.01Lµ<0.1 Le>0.001Lµ<0.1
εe 93.41±0.92 98.64±0.08 91.60±1.02 97.89±0.11
ηe 92.02±1.00 99.74±0.04 89.89±1.10 99.67±0.04
εµ 99.54±0.05 95.53±0.76 99.19±0.06 86.48±1.26
ηµ 99.60±0.04 96.31±0.70 99.83±0.03 95.38±0.81
εevent 98.53±0.13 97.06±0.19 97.24±0.18 95.40±0.24
Fig.11 e-likelihoodandµ-likelihoodofchargedparticleswithE>20GeVineeHevent
Fig.12 e-likelihoodandµ-likelihoodofchargedparticleswithE>20GeVinµµHevent
GeVcollisionenergyhasbeenchecked.Theefficiencyfora Acknowledgements ThisstudywassupportedbyNationalKeyPro-
singleleptonidentificationisconsistentwiththesinglepar- grammeforS&TResearchandDevelopment(GrantNO.:2016YFA0400400),
the Hundred Talent programs of Chinese Academy of Science No.
ticleresults.Theefficiencyoffindingtwoleptonsdecreases
Y3515540U1,andAIDA2020.
by1∼2%whenthecellsizedoubles,whichmeansthatthe
detectorneeds2∼4%morestatisticsintherunning.IneeH
events,theperformancedegradesbecausetheclusteringal-
AppendixA: Appendixsection
gorithmstillneedstobeoptimized.
ListandmeaningofvariablesusedintheTMVAwhichare
Toconclude,ultra-highgranularitycalorimeterdesigned notmentionedinthetext:
for ILC provides excellent lepton identification ability, for
operationclosetoZHthreshold.Itmaybeaslightoverkill – NH_ECALF10:Numberofhitsinthefirst10layersof
forCEPCandaslightlyreducedgranularitycanreachabet- ECAL
tercompromise.AndLICH,thededicatedleptonidentifica- – FD_ECALL20: FD calculated using hits in the last 20
tionforfuturee+e-Higgsfactory,isprepared. layersofECAL
8
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