Higgs Self coupling
Djamel BOUMEDIENE, Pascal GAY
LPC Clermont-Ferrand
Outline
1) Introduction
2) lhhh measurement
3) Detector performance impact on lhhh measurement
 Energy flow
 b-tag choice
 c contamination
4) Conclusion
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D. BOUMEDIENE | Higgs self coupling | ILC Soft Orsay
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Introduction
Study realized for a center of mass energy of 500GeV
Events generated with Whizard - Pythia
Additional backgrounds w.r.t published analysis
mH = 120GeV, Br(H→bb)=68%
Signal cross section 0.18pb
Dlhhh/lhhh ~ 1.75 DshhZ/shhZ
Presence of 6 jets, 8 jets events → overlap → importance of jet
reconstruction (typical final state for ILC physics)
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Fast sim choice
Parametric fast simulation used
 eflow DE/E = 30%/E
 Typical b-tag efficiency = 90% (c contamination 35%)
Test of different geometry of detectors
 different eflow
 different b-tag
 Fast sim allows to vary these parameters
To regenerate all the MC for different geometry detectors is too much
difficult
 How to simulate different energy flow in the full simulation
 Various sets of parameters may lead to the same energy flow resolution
 Many parameters :
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•
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ECAL
HCAL
ID Part
Confusion
…
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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 expected luminosity (handicap for full simulation)
Processes
hhZ
Generated
luminosity
-1
(pb )
N expected
-1
(L = 500 pb )
0,18441
15k
699
1820k
526,4
740k
1880,7
263200
ZZZ
1,051
40k
38059,0
525
tbtb
0,7
20k
28571,4
350
45,12
50k
1108,2
22560
0,141327
20k
141515,8
70
wwz
35,3
130k
3682,7
17650
wtb
16,8
200k
2976,2
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
20k
18474,6
541
0,6975
20k
28673,8
541
Backgrounds
tt
ZZ
nntt
ttZ
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N Generated
σ(pb)
81340,49
92,2
332167
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Event reconstruction
Signal : 3 Channels
 hhqq
• 6 jets
• mh & mZ
 hhnn
• 4 jets
• Missing energy
• Mh
 hhll
• 4 jets
• 2 energetic leptons
• mZ & mh
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Events reconstructed
 in 6 jets
 in 4 jets
 in 2 jets
Jet pairing based on di-jet
masses associated to bosons
 Jets combined in pairs to test
different final states
 hhZ, hh(nn), ZZZ, ZZ, WWZ
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b - tagging
Use of 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 / event
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D. BOUMEDIENE | Higgs self coupling | ILC Soft Orsay
4 b-like jets / event
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Event Preselection
Minimal b flavored content
 Event by event basis
Visible energy
2 isolated leptons in a mass
window of 25GeV arround mZ
Selection
Evis
b content
2 Isoltd
lept
hhqq
>370 GeV
> 3.6
No
hhnn
<370 GeV
>1.8
No
hhll
>370 GeV
>1.8
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 recontructed masses
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NN Output
hhll
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hhnn
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hhqq
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Expected Number of events
L = 2 ab-1
Channel hhqq
 Signal : 67.3
 Background : 93
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tt 9.2
tbtb 37.
ZZZ 16.4
ttZ 18.8
Channel hhnn
Significance = 6.6 s
 Signal : 20
 Background : 14.6
• ZZ 2.4
• tbtb 6.8
Channel hhll
 Signal 10.3
 Background : 26.8
• tt
• tbtb
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Measurement of the Cross Section
and hhh coupling
shhZ is measured using maximum Likelihood method assuming a
Poisson law distribution
2 dimensional distribution is fitted :
NN Output X b-tag
Pseudo experiment method is used to evaluate the expected
statistical error
For (eb=90% and eflow 30%) : Dlhhh/lhhh = 16%
Dshhh/shhh = 9 %
Channel weights
(@500GeV
L=2ab-1)
 hhqq 40%
 hhnn 25%
 hhll 34%
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Impact of detector performance
Move from a realistic fast sim to a smearing of the visible energy of partons
Simulation of different Energy flow
 Information at parton level
•
•
•
•
•
Merge all together every daughter (except n’s) of the quark in one object
BUT it cannot be done formally because of string model
Method : clusterisation of the daughters to form the parton direction
These objects are smeared afterwards → simulated jets
An optimistic approach for bad energy flow resolutions
 Smearing of parton energy
 Different eflow
• Range : 0%/E → 130% /E
 perfect energy flow
• Jet pairing is changed (based on boson masses)
lhhh measurement
 For each detector hypothesis :
• New NN trained
• Cuts fully optimized
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Dlhhh/lhhh versus DE/E
Direct eflow contribution
-
Precision loss should be more
important for high DE/E (eflow
contribution is underestimated)
Perfect clusterisations
Dl/l increases with DE/E
 Gain when eflow are added = 22.5% → 16%
This reduction corresponds to a reduction of required luminosity of a
factor 2 : without the energy flow 4 ab-1 are needed to reach 16%
(optimistic)
This result is for a given b-tag/c-tag and for an un-optimized eb choice
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b-tag choice
For a given b-tag/c-tag
parametrisation
 b-tag efficiency (eb) choice
= analysis choice
 efficiency ↔ purity (c
contamination)
Ejet = 45 GeV
Very pure taging
Low efficiency
c-contaminated
High efficiency
 Which eb optimizes lhhh
measurement ?
 Previous working point :
• eb=90%
• ec =35% (45GeV jet)
(Richard Hawkings parametrisation)
Tuned for back-to-back jets
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Study of Detector performance
Simulation of different b-tag efficiencies
 b-tag is defined statistically (from true jet flavor)
 c-tag ↔ b-tag respects the Hawkings parametrisation
• Tested range : 40% → 95%
• For different eflow
lhhh measurement
 For each eflow resolution :
• NN trained
• Cuts optimized
• 77 analyses are tested (for each combination eflow x btag)
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Dlhhh/lhhh versus b-tag eff.
Optimum @ eb ~ 65%
~ same for different eflow
eflow=120%
For eflow = 30%/E
 A precision of 15% is
reachable on lhhh with the
optimal eb
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eflow=0%
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Which background is sensitive to btag choice
Which background is sensitive to
b-tag choice :
 tt
hhZ
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
tbtb
ZZZ & ttZ
tt
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Dlhhh/lhhh versus b-tag & EFlow
Expected statistical error (in %) for a lumi of 2ab-1
For a given c-tag / b-tag parametrisation
ec= 35%
Best eb corresponds to
 ec < 5%
 euds negligible
ec~ 5%
 15%
What is the dependence
Dlhhh/lhhh ↔ b-tag purity for
a given efficiency (different btag/c-tag)
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Dlhhh/lhhh versus b-tag eff.
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b-tag purity ( c tagging)
Previous working point
ec= 25%
ec= 35%
Tested on hhqq
Using eb = 90%
For a c contamination reduction
of 30% (typically from 35% to
25% @ Ejet=45GeV)
 Dlhhh/lhhh is reduced by 12%
hhqq
ec= 25%
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D. BOUMEDIENE | Higgs self coupling | ILC Soft Orsay
ec= 35%
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Summary & Outlook
Expected statistical precision on lhhh is evaluated to 15% with a
typical detector configuration (L=2ab-1)
Fast simulation approach is useful to define the best detector at
first level
Effect of 30%/E EFlow is a factor 2 on luminosity (maybe more
dur to the optimistic evaluation at 60%/sqrt(E))
b/c tag is of importance and may be convoluate with
Calorimeters
Jets clusterisation is also of importance and not completey
treated yet.
Obviously the analysis it self may be improved (e.g. jet charge)
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