higgs self coupling djamel boumediene, pascal gay lpc clermont-ferrand

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01/06/2007 D. BOUMEDIENE | Higgs self coupling | LCWS3 Introduction Study realized for a center of mass energy of 500 GeV Additional backgrounds w.r.t published analysis m H = 120 GeV, Br(H → bb) = 68% Signal cross section 0.18 pb  hhh / hhh ~ 1.75  hhZ /  hhZ Presence of 6 jets, 8 jets events → overlap → importance of jet reconstruction (typical final state for ILC physics)

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  • Higgs self couplingDjamel BOUMEDIENE, Pascal GAYLPC Clermont-Ferrand

    D. BOUMEDIENE | Higgs self coupling | LCWS

  • Outline1) Introduction2) lhhh measurement3) Detector performance impact on lhhh measurement

    particle flow b-tag choice c contamination4) Conclusion

    D. BOUMEDIENE | Higgs self coupling | LCWS

  • Introduction

    Study realized for a center of mass energy of 500 GeVAdditional backgrounds w.r.t published analysismH = 120 GeV, Br(Hbb) = 68%Signal cross section 0.18 pbDlhhh/lhhh ~ 1.75 DshhZ/shhZPresence of 6 jets, 8 jets events overlap importance of jet reconstruction (typical final state for ILC physics)

    D. BOUMEDIENE | Higgs self coupling | LCWS

  • Typical analysis scheme

    Energy depositsParticlesJets di-jet massesParticle FlowalgorithmsJet algorithms(Durham)Jet pairing(based on di-jet masses)jet1jet2jet3jet4jet5jet6

    D. BOUMEDIENE | Higgs self coupling | LCWS

  • Fast simulation choiceParametric fast simulation

    pflow DE/E = 30%/Etypical b-tag efficiency = 90% (c contamination 35%)

    Test of different detector performance through fast simulation with different parameters

    different pflow different b-tag

    It is too difficult to regenerate all the MC for different detector resolutions (geometries)

    how to simulate different particle flow in the full simulation

    many parameters :ECALHCALID PartConfusionvarious sets of parameters may lead to the same particle flow resolution

    D. BOUMEDIENE | Higgs self coupling | LCWS

  • Parametric fast simulation

    D. BOUMEDIENE | Higgs self coupling | LCWS

  • Monte Carlo simulation

    To obtain a realistic result, the generated luminosity should be greater than the expected luminosity (drawback for a full simulation)

    Processes(pb)N GeneratedGenerated luminosity (pb-1)N expected (L = 500 pb-1)hhZ0,1844115k81340,4992,2Backgrounds6991820k332167tt526,4740k1880,7263200ZZZ1,05140k38059,0525tbtb0,720k28571,4350ZZ45,1250k1108,222560nntt0,14132720k141515,870wwz35,3130k3682,717650wtb16,8200k2976,28400eezz0,28710k34843,2143nnww3,62730k8271,31813evzw10,09460k5944,15047nnzz1,0825720k18474,6541ttZ0,697520k28673,8541

    D. BOUMEDIENE | Higgs self coupling | LCWS

  • Event reconstruction Signal : 3 channels hhqq6 jetsmh & mZ

    hhnn4 jetsmissing energyMh

    hhll4 jets2 energetic leptonsmZ & mhEach event is reconstructed

    in 6 jetsin 4 jetsin 2 jets

    Jet pairing based on di-jet masses associated to bosons

    Jets combined in pairs in order to test different final stateshhZ, hh(nn), ZZZ, ZZ, WWZFor each eventhhZ=(m12-mh)/sh+(m34-mh)/sh +(m45-mZ)/sZZZ=(m12-mh)/sZ+(m34-mZ)/sZ. . .

    D. BOUMEDIENE | Higgs self coupling | LCWS

  • b - tagging

    Use the b flavor signature to reduce the background

    Global variable (crude approach)

    Estimator of # of b jets per event taking into account de c and uds contamination (parametrisation of a given VDET)No b-like jet / event4 b-like jets / event

    D. BOUMEDIENE | Higgs self coupling | LCWS

  • Event preselectionMinimal b flavored content

    Event by event basis

    Visible energy

    2 isolated leptons in a mass window of 25 GeV around mZ

    ZZZWWZhhllbackground

    SelectionEvisb content2 Isoltd lepthhqq>370 GeV> 3.6Nohhnn1.8Nohhll>370 GeV>1.8Yes

    D. BOUMEDIENE | Higgs self coupling | LCWS

  • Event selectionMultivariable Method

    Global inputs :Visible energySphericity Reconstructed inputs :Number of jets (ycut)hhZ hh 2-bosons 3-bosons : hypothesis based on reconstructed masses

    D. BOUMEDIENE | Higgs self coupling | LCWS

  • NN outputhhllhhnnhhqq

    D. BOUMEDIENE | Higgs self coupling | LCWS

  • Expected number of events at L = 2 ab-1Channel hhqq

    Signal : 67.3 Background : 93tt 9.2tbtb 37.ZZZ 16.4ttZ 18.8Channel hhnn

    Signal : 20 Background : 14.6ZZ 2.4tbtb 6.8 Channel hhll

    Signal 10.3Background : 26.8Significance = 6.6 s

    D. BOUMEDIENE | Higgs self coupling | LCWS

  • Measurement of the cross section and of the hhh couplingshhZ is measured using a maximum likelihood method and assuming a Poisson law distribution for the NN output x btag

    The 2 dimensional distribution is fitted :

    NN Output X b-tag

    The pseudo experiments method is used to evaluate the expected statistical error

    For eb=90% and pflow 30% : Dlhhh/lhhh = 16%

    Dshhh/shhh = 9 %The observed contribution of the various channels to the measurement :

    hhqq 40%hhnn 25%hhll 34%(@ 500GeV L = 2ab-1)

    D. BOUMEDIENE | Higgs self coupling | LCWS

  • Jet charge can help Definition of the jet charge (Chj):

    For a jet j : Chj= qi wi / wiwhere qi is the charge of the particle iwi=(pi.ej)pi is the particles momentum, ej the jet direction

    Boson charge = sum of the charges of the two jets

    For a given event, definition of a

    = (Chh1)/s +(Chh2)/s+ (ChZ)/s

    It should improve the hhZ selection

    tthhZ

    D. BOUMEDIENE | Higgs self coupling | LCWS

  • Impact of the detector performanceMove from a realistic fast sim to a smearing of the visible energy of partons

    Simulation of different Particle flow resolutions

    Information at parton levelMerge all the daughter (except ns) of the quark in one objectBUT it cannot be done formally because of the string modelMethod : clusterise of the daughters to form the parton directionSmear the reconstructed parton to simulate the jetsAn optimistic approach for bad particle flow resolutionsSmearing of parton energy

    Range : 0%/E 130% /E perfect particle flow

    Jet pairing is changed (based on boson masses)

    lhhh measurement

    For each detector hypothesis :New NN trainedCuts fully optimized

    D. BOUMEDIENE | Higgs self coupling | LCWS

  • Dlhhh/lhhh versus DE/EDirect pflow contributionPrecision loss should be more important for high DE/E (pflow contribution is underestimated)Perfect clusterisationsDl/l increases with DE/E

    Gain when pflow are added = 22.5% 16%

    This reduction corresponds to a reduction in the required luminosity by a factor 2 : without the particle flow information 4 ab-1 are needed to reach 16% (optimistic)

    This result valid for a given b-tag/c-tag and for an non-optimized eb choice

    D. BOUMEDIENE | Higgs self coupling | LCWS

  • b-tag choiceFor a given b-tag/c-tag parametrisation

    b-tag efficiency (eb) choice = analysis choice

    efficiency purity (c contamination)

    Which eb optimizes lhhh measurement ?

    Previous working point : eb=90% ec =35% (45GeV jet)Very pure tagingLow efficiencyc-contaminatedHigh efficiencyEjet = 45 GeV(Richard Hawkings parametrisation)Tuned for back-to-back jets

    D. BOUMEDIENE | Higgs self coupling | LCWS

  • Study of the detector performanceSimulation of different b-tag efficiencies

    b-tag is defined statistically (from true jet flavor) c-tag b-tag efficiencies respect the Hawkings parametrisation

    Tested range : 40% 95%For different pflow resolutions

    lhhh measurement

    For each pflow resolution :NN trainedCuts optimized 77 analyses are tested (for each combination pflow x btag)

    D. BOUMEDIENE | Higgs self coupling | LCWS

  • Dlhhh/lhhh versus b-tag efficiency

    Optimum @ eb ~ 65%

    ~ same resolutions for different pflow

    For pflow = 30%/E

    A precision of 15% is reachable on lhhh with the optimal ebpflow=0%pflow=120%

    D. BOUMEDIENE | Higgs self coupling | LCWS

  • Background sensitivity to the b-tag choicetthhZtbtbZZZ & ttZWhich background is most sensitive to b-tag choice ?

    tt

    tt WbWb bbccss

    c-jets tagged as b-jets bbbbqq like final state hhZ like final state

    bbbbqq is the hhZ flavor signature used in the selection

    D. BOUMEDIENE | Higgs self coupling | LCWS

  • Dlhhh/lhhh versus b-tag & pflowExpected statistical error (in %) for a luminosity of 2ab-1For a given c-tag / b-tag parametrisation15%Best eb corresponds to

    ec < 5%euds negligible

    What is the dependence of Dlhhh/lhhh b-tag purity for a given efficiency (different b-tag/c-tag)

    ec= 35%ec~ 5%

    D. BOUMEDIENE | Higgs self coupling | LCWS

  • Dlhhh/lhhh versus b-tag efficiency

    D. BOUMEDIENE | Higgs self coupling | LCWS

  • b-tag purity ( c tagging)Preliminaryhhqqec= 25%Previous working pointec= 35%Tested on hhqqUsing eb = 90%

    For a reduction of 30% of the c contamination (typically from 35% to 25% @ Ejet=45GeV)

    Dlhhh/lhhh is reduced by 12%

    ec= 25%ec= 35%

    D. BOUMEDIENE | Higgs self coupling | LCWS

  • Summary & OutlookThe expected statistical precision on lhhh is evaluated to 15% with a typical detector configuration and for a luminosity of 2ab-1

    A particle flow of 30%/E reduces the necessary luminosity by at least a factor 2

    The b/c tag performance has an important effect and may be convoluted with Calorimeters

    The relation between clusterisation of the jets and pflow could be important and not completely treated yet.

    Obviously the analysis itself may be improved (e.g. by taking into account the jet charge)

    D. BOUMEDIENE | Higgs self coupling | LCWS