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Higgs self coupling measurement at ILC
Djamel BOUMEDIENE, Pascal GAY LPC Clermont-Ferrand
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Outline Introduction hhZ final state measurement at ILC
SM Higgs boson couplings MSSM Higgs bosons couplings hhZ final state measurement at ILC Which experimental environment to measure the hhZ final state ? Conclusion
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Introduction – SM case hhh/hhh ~ 1.75 hhZ/hhZ
hhh involved in double Higgs-strahlung processes : e+e- Zhh Study realized for a center of mass energy of 500 GeV t-channel (W-fusion) negligeable mh = 120 GeV, Br(h→bb) = 68% Signal cross section 0.15 pb : low high luminosity required hhh/hhh ~ 1.75 hhZ/hhZ Final state with 6 jets, 8 jets : typical for ILC physics overlap → importance of jet reconstruction
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MSSM Higgs couplings More trilinear couplings :
lhhh, lhhH, lHHH, lhAA, lhH+H-, lHH+H-, lhAA, lHAA involved in many processes : A. Djouadi, W. Killian, M. Muhlleitner and P. Zerwas. Eur. Phys. J. C10
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MSSM Higgs couplings For low tanb : H hh resonance increases
the signal cross section w.r.t SM A. Djouadi, W. Killian, M. Muhlleitner and P. Zerwas. Eur. Phys. J. C10
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P. Osland, P. N. Pandita, Phys. Review D 59 - 055013
MSSM Higgs couplings Dependence of the couplings on mh, tan P. Osland, P. N. Pandita, Phys. Review D
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Event reconstruction Signal : 3 channels
hhqq 6 jets mh & mZ hhnn 4 jets missing energy Mh hhll 2 energetic leptons mZ & mh Each event is reconstructed in 6 jets 4 jets 2 jets Jet pairing based on di-jet masses Jets combined in pairs in order to test different final states : hhZ, hh(nn), ZZZ, ZZ, WWZ For each event ²hhZ=(m12-mh)²/s²h+(m34-mh)²/s²h +(m45-mZ)²/s²Z ²ZZ=(m12-mh)²/s²Z+(m34-mZ)²/s²Z . . .
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b - tagging Use the b flavor signature to reduce the background
Global variable (conservative approach) Estimator of the number of « b jets » per event, which takes into account de c and uds contamination (parametrisation for a given VDET) No b-like jet / event 4 b-like jets / event
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b-tag choice Working point : For a given b-tag/c-tag parametrisation
c =35% (45GeV jet) For a given b-tag/c-tag parametrisation Each choice of b-tag efficiency (b) implies a different analysis The efficiency choice sets the purity (c contamination) Which b optimizes hhh measurement ? Ejet = 45 GeV Very pure taging c-contaminated Low efficiency High efficiency (Richard Hawkings parametrisation) Tuned for back-to-back jets
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Event preselection hhll Minimal b flavored content
Event per event basis Visible energy 2 isolated leptons in a mass window of 25 GeV around mZ Selection Evis b content 2 isolated lept hhqq >370 GeV > 3.6 No hhnn <370 GeV >1.8 hhll Yes hhll ZZZ WWZ background
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Event selection Multivariable method Global inputs :
Visible energy Sphericity Reconstructed inputs : Number of jets (ycut) ²hhZ ²hh ²2-bosons ²3-bosons : hypothesis based on reconstructed masses
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NN output hhqq hhll hhnn
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Parametric fast simulation
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Monte Carlo simulation
To obtain a realistic result, the generated luminosity should be greater than the expected luminosity (drawback for a full simulation) Processes σ(pb) N generated Generated luminosity (pb-1) N expected (L = 500 pb-1) hhZ 0,1528 20k 130890 76,4 Backgrounds 699 1820k 332167 tt 526,4 1M 1899,7 263200 ZZZ 1,051 40k 38059,0 525 tbtb 0,7 28571,4 350 ZZ 45,12 110k 2437,9 22560 nntt 0,141327 141515,8 70 wwz 35,3 130k 3682,7 17650 wtb 16,8 240k 14285,7 8400 eezz 0,287 10k 34843,2 143 nnww 3,627 30k 8271,3 1813 evzw 10,094 60k 5944,1 5047 nnzz 1,08257 18474,6 541 ttZ 0,6975 28673,8 349
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hhZ cross section and the SM-hhh coupling
hhZ is measured using a bi-dimensional maximum likelihood method ( NN output btag ) and assuming a Poisson law The expected statistical error is evaluated using the pseudo experiments method For b=90% and a particle flow resolution DEjet/√Ejet = 30% hhh/hhh = 28% hhh/hhh = 16% Number of events : signal : 72 events background : 128 events (mainly : tt, ZZh, tbtb) Significance d = 5.2 ( √s=500GeV L = 2ab-1 )
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The particle flow Jet energy resolution used : sEjet/Ejet= 30%/√Ejet
For a typical jet : 65% charged particles tracking detectors 25% photons EM calorimeters 10% neutral hadrons EM and hadronic calorimeters negligeable ~ (0,6%)² Ejet ~ (1,3%)² Ejet ~ (26%)² Ejet Detector materials + segmentation + algorithm smartness
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(based on di-jet masses)
Impact of the detector performance Can be entirely parameterized by DEjet/Ejet Energy deposits Particles Jets di-jet masses Particle Flow algorithms Jet algorithms (Durham) Jet pairing (based on di-jet masses) jet5 h Z jet6 jet4 jet1 jet3 jet2
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Study of the detector performance
Simulation of different particle flow resolutions Tested range : 0%/√Ejet 130%/√Ejet Simulation of different b-tag efficiencies c-tag ↔ b-tag efficiencies respect the Hawkings parametrisation Tested range : 40% → 95% For different pflow resolutions hhh measurement For each particle flow resolution and each b-tag : Training of NN Optimization of the cuts 77 analyses are tested (for each combination of pflow and btag) The aim : Dlhhh/lhhh (DEjet/√Ejet, eb)
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hhh/hhh versus b-tag efficiency
b ~ 65% Same optimum for different pflow values For pflow = 30%/Ejet, a precision of 19% is reachable on hhh with the optimal b pflow=120% / √Ejet pflow=0% / √Ejet
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Background sensitivity to the b-tag choice
The most sensitive background : e+e- t t and e+e- ZZh tt → WbWb → bbccss if too many c-jets are tagged as b-jets ↔ bbbbqq like final state ↔ hhZ like final state bbbbqq is the hhZ flavor signature used in the selection
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hhh/hhh versus b-tag & pflow
Expected statistical error (in %) for a luminosity of 2ab-1 Optimal b corresponds to c < 5% uds negligible For a given c-tag / b-tag parametrisation What is the dependence of hhhhhh ↔ b-tag purity for a given efficiency (different b- tag/c-tag) c= 35% c~ 5% 19%
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b-tag purity ( c tagging)
Previous working point c= 35% c= 25% For a reduction of 30% of the c contamination (from 35% to 25% @ Ejet=45GeV) hhhhhh is reduced by 12% eb = 90% h hqq c= 25% c= 35%
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Summary & Outlook The Higgs trilinear couplings are accessible at ILC already at √s=500GeV with a luminosity of 2 ab-1. Interpretation of the present work may be done in SUSY framework as well as general 2HDM. (cf. M. Herquet talk) The expected statistical precision on hhh in the SM framework is evaluated to 19% with a typical detector configuration A particle flow resolution of 30%/E reduces the necessary luminosity by at least a factor 1.75, compared to no pflow scenario The b/c tag performance has an important effect and may be convoluted with the performance of the Calorimeters e+e- experiments are the ideal environment to measure the Higgs coupling(s)
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