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Michele Faucci Giannelli
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Transcript of Michele Faucci Giannelli
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Michele Faucci Giannelli
TILC 09, Tsukuba, 18 April 2009
Measuring the Higgs self-coupling using the ILD detector
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18 April 2009 Michele Faucci Giannelli, Tsukuba, TILC09 2
Overview
• Introduction: – from ZHH cross section to Higgs self-coupling
• Event simulation and reconstruction
• 6-jet environment
• ZHH selection
• B tagging performance
• Conclusion
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18 April 2009 Michele Faucci Giannelli, Tsukuba, TILC09 3
Theory
The sensitivity on the cross section and the self coupling are not linear.
σ (Z
HH
)
λ/λSM
10% resolution on σ (ZHH)
18% resolution on λ
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18 April 2009 Michele Faucci Giannelli, Tsukuba, TILC09 4
Physics parameters
• This process is best studied at 500 GeV and with a Higgs mass of 120 GeV to maximize the cross section
• Given the Z and Higgs SM decays:– BR (Zqq) 70% – BR (Hbb) 73%
Belanger et al, Ph.L. B 576, p152
• The BR are:– qqbbbb (40%) – bbbb (16%)– qqbbWW (12%)– llbbbb (only 4.5%)
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18 April 2009 Michele Faucci Giannelli, Tsukuba, TILC09 5
Physics parameters
• CM energy 500 GeV and MH = 120 GeV
– Maximizes the cross section
• Main channel is ZHH qqbbbb– BR = 40%
• The signal cross section is 0.18 fb– less than 100 signal events for 500 fb-1
– Only 31 qqbbbb events
• Several backgrounds with 6-jet final state:– tt is the main one with σ = 550 fb
• 160,000 hadronic events
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18 April 2009 Michele Faucci Giannelli, Tsukuba, TILC09 6
Sample used
ILD samples used (both polarizations):
• 40000 ZHHqqHH (~9000 qqbbbb)
• bbqqqq (tt compatible): 500 fb-1
• qqqqH : 4000 for each channel
• qqqqqq (non tt compatible) : 4000 each– bbbbqq: 10000 each
• QQH (Q only b or c): 500 fb-1
• qqqql (semileptonic tt) : 500 fb-1
• ttX (X = H,Z,bb): 10000 each
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18 April 2009 Michele Faucci Giannelli, Tsukuba, TILC09 7
Jet energy resolution
• The reconstructed jets are paired to the Monte Carlo quarks using the combination that maximizes the sum of the six scalar products
• All jets are divided in bins of energy, for each bin the resolution is evaluated
Jets of 40-60 GeV
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18 April 2009 Michele Faucci Giannelli, Tsukuba, TILC09 8
Jet energy resolution
Jet energy resolution(50 ± 1)%/√E
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18 April 2009 Michele Faucci Giannelli, Tsukuba, TILC09 9
Boson mass resolution• Using the same pairing, it is possible to reconstruct
the bosons using the correct jets
Realistic PFA Mass (GeV) Width (GeV)Higgs 117.2 ± 0.2 5.8 ± 0.4Z 90.7 ± 0.2 5.5 ± 0.3
Higgs Z
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18 April 2009 Michele Faucci Giannelli, Tsukuba, TILC09 10
Analysis• A first loose cut on total b tagging was applied
to reduce the number of events to process.
Cut applied at 3.2ZHHbbccss
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18 April 2009 Michele Faucci Giannelli, Tsukuba, TILC09 11
Analysis• Several variables were used in the analysis:
– Thurst– Second Fox-Wolfram moment– Total energy– Y6 (angular distance between jets)– # tracks– # particle in jets– EM energy ratio in jets
• The cuts were optimized using a multi dimensional method.– All variables varied simultaneously within a range, best set
of cuts determined by maximizing the figure of merit S/√(S+B)
• After cuts S/√(S+B) = 0.36
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18 April 2009 Michele Faucci Giannelli, Tsukuba, TILC09 12
Some distributions
ZHHbbqql
ZHHbbqql
ZHHqqH
ZHHqqH
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18 April 2009 Michele Faucci Giannelli, Tsukuba, TILC09 13
Some distributions (2)
ZHHbbqql
ZHHbbqql
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18 April 2009 Michele Faucci Giannelli, Tsukuba, TILC09 14
Final optimization• B tagging is used together with the masses
• Two cases:– if Zbb:
• b tagging cannot be used to separate Z jets from Higgs jets
– if Zno bb:• four most b-like jets are associated with Higgs
• The total B tagging value at which the two cases are separated was one of the parameters of the final optimization – Same procedure as before
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18 April 2009 Michele Faucci Giannelli, Tsukuba, TILC09 15
Final optimization (2)Parameters used in final optimization:• Total B tagging• Threshold value for Zbb
• with combinatorial reduced depending on the value
of Total B tagging and thresholdExample:
– Total b tagging = 4.1 and threshold = 4.0 all combination were used – Total b tagging = 4.1 and threshold = 4.2 combination were reduced to reconstruct the Higgs
2
2
562
2
342
2
122
H
H
H
H
Z
Z MMMMMM
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18 April 2009 Michele Faucci Giannelli, Tsukuba, TILC09 16
Result
The optimization gave these values:
• Total b tag > 3.8
• Threshold for Zbb at 4.2
• below threshold < 7
• above threshold: no cut
The large combinatorial for the above threshold causes a large fraction of the background to look like ZHH events hence the mass information is not useful in that case
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18 April 2009 Michele Faucci Giannelli, Tsukuba, TILC09 17
Result (2)• The resolution to the ZHH cross section,
defined as √(S+B) /S, is 88% for an integrated luminosity of 500 fb-1
• The same analysis was performed on the two sets of fully polarized files, with the two independent analysis combined at the end. The same result was obtained.
• Considering an integrated luminosity of 2 ab-1, as done in all previous fast simulation studies, a resolution of 44% could be achieved on the ZHH cross section.
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18 April 2009 Michele Faucci Giannelli, Tsukuba, TILC09 18
B tagging
Fake rate for light jets is reduced respect old model but is not yet at the level of Z pole. There is still room for improvement.
LDC Zqq 90GeVLDC ZHH 500GeVILD ZHH 500GeV
UDS JETS
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18 April 2009 Michele Faucci Giannelli, Tsukuba, TILC09 19
Conclusions• An LC note is available with more details
• The resolution to ZHH cross section is 88%
• This value is close the resolution obtained from fast simulation studies (scaling to same integrated luminosity)
• If better vertex reconstruction could be achieved, possibly with a better jet finder, measuring the Higgs self-coupling should be possible with an integrated luminosity of 2 ab-1
and using more decay modes of ZHH.
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18 April 2009 Michele Faucci Giannelli, Tsukuba, TILC09 20
Backup Slides
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18 April 2009 Michele Faucci Giannelli, Tsukuba, TILC09 21
B tagging
Zqq at 500 GeV Zqq at 360 GeV
Zqq at 200 GeV Zqq at 90 GeV
UDS
It is energy dependent!
Studies and improvements needed on b-tagging algorithms as a function of: •number of jets •jet energy