Current Status on ZHH Analysis
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Transcript of Current Status on ZHH Analysis
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Current Status on ZHH Analysis
’08 5/31 GLD ZHH-group
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Analysis menu
MH v.s. (ZHH)
500GeV
750GeV
1TeV
ZHH study
ZHH analysis was started to cover wide Higgs-mass region.
• ECM=500GeV is the best for MH=120GeV.
• ECM>750GeV is preferable to study for MH>160GeV.
Our analysis menu• Light Higgs : MH=120GeV, ECM=500GeV
• Heavy Higgs : MH=170GeV, ECM=500GeV~1TeV
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Study of Light Higgs Case
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Introduction
Status at TILC08
• Analysis was started for HH-mode.
• Signal significance of 1.4 was obtained against ZZbbbb. Signal : 12 events ZZbbbb : 64 events
Today’s topic
• Re-optimization of the selection cut.
• Analysis with B.G. of ZZbbbb and tt. (ZZ) = 395.8 fb (tt) = 583.6 fb
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Selection cutsThe new selection cuts are applied to improve B.G. rejection.
Primary cut for reconstructed particles
• Energy cut (E1,2 < 250GeV)
• Momentum cut (P1,2 < 200GeV)
• cut (1,2 < 0.9)
Fine selection cut
• 2 cut (2 < 13)
• Higgs mass cut (95GeV < MH1,2 < 125GeV)
• Missing mass cut (90GeV < Mmiss < 200GeV)
• Angle cut (|cos1,2| < 0.9)
b-tag cut
New selection cuts
Example of new cuts (Momentum and cut) is shown.
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Momentum cutMomentum of the reconstructed particles was checked.
• Higgs-momentum of the signal is below 200 GeV/c.
• The momentum above 200 GeV/c was rejected.
P1(GeV/c)0 100 200 300 400
HH B.G.
P1(GeV/c)0 100 200 300 400
ZZtt
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cut(=p/E) of the reconstructed particles was checked.
• of the signal is below 0.9.
<0.9 was selected.
The fine selection cuts were applied after these primary cuts.
10.3 0.5 0.7 0.9 10.3 0.5 0.7 0.9
HH B.G.
ZZtt
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B-tag cutAfter the fine selection cuts, b-tagging was applied.
• Nb-tag ≧3 was selected. b-tag: 3 tracks with 3 separation from IP.
• 23.5 events of ZZbbbb still contaminates for 2 ab-1. All tt events are rejected.
Nb-tag0 1 2 3 4 5
HH
Nb-tag0 1 2 3 4 5
B.G.
ZZtt
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Signal significance
The signal significance was estimated for 2ab-1
• Signal : 12.1 events
• B.G. : 23.5 events
Significance : 2.0
Rec. (MH1+MH2) for 2ab-1
TILC08
• Signal : 12 events
• B.G. : 64
1.4
ZZttSignal + BG
GeV
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Reduction summary
• No cut
• E1,2 < 250GeV
• P1,2 < 200GeV
• 1,2 < 0.9
• 2 < 13
• 95GeV < MH1,2 <125GeV
• 90GeV < Mmiss < 200GeV
• |cos1,2| < 0.9
• Nb-tag ≧3
: 77.6
: 70.8 (x 0.91)
: 70.8 (x 1.00)
: 60.9 (x 0.86)
: 33.8 (x 0.56)
: 26.7 (x 0.79)
: 25.5 (x 0.96)
: 22.2 (x 0.87)
: 12.1 (x 0.55)
18,100
14,022 (x 0.77)
4,876 (x 0.35)
2,033 (x 0.42)
686 (x 0.34)
223 (x 0.33)
130 (x 0.58)
41.6 (x 0.32)
23.5 (x 0.56)
HH ZZbbbb
• The signal efficiency is 16%.
• 55% of signal is lost at b-tag cut. However, b-tag is necessary to reject tt-B.G..
1,167,200
35,950 (x 0.031)
29,063 (x 0.81)
25,678 (x 0.88)
6886 (x 0.27)
1517 (x 0.22)
817 (x 0.54)
467 (x 0.57)
0 (x 0.0)
tt
Next step is to consider tbtb-B.G. (0.7fb).
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Summary for HH• The selection cut was re-optimized.
• The signal significance of 2.0 was obtained against the ZZbbbb and tt.
This result is comparable to Djamel’s report at LCWS07, after scaling (ZZ) to the correct value (395.8fb).
• The other B.G. modes like tbtb should be taken into account.
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Study of Heavy Higgs Case
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ZHH signal has small xsec ~0.1fb @ 750 GeV, sensitive to HHH coupling
many possible final states with different characteristics:from 10 hadronic jets, to 4 charged leptons + 6 neutrinos,
and almost everything in between!
Many backgrounds with orders of magnitude higher xsec
so far, considered only subset with true Z bosonZWW (this has largest xsec), ZZZ, Ztt, ZZWW
events generated by MadGraph, studied @ QuickSim level
Introduction
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Fit events to ZHH->ZWWWW hypothesismany possible signal final states need to be considered
look for Z->e+e-, mu+mu-if not found, try Z->tau tau, Z->nu nu and Z->jj possibilities
look for additional e, mu, assume from W decayidentify tau-like jets
# charged tracks = 1 or 3invariant mass < m_taujet charge = +-1
try two possibilities: true tau jet, or hadronic jetfor other Ws, try both tau decay and hadronic decay possibilities
for each possible decay topologyforce non-leptonic part of event into required # jetsask if invariant masses of Z, W, H->jets consistent with expectation
Event reconstruction
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require that event fits at least one signal final state hypothesis
use kinematic variables to decide if event is real ZHH event or backgroundvisible energy, momentum vs. # assumed neutrinosYcut of jet clustering required to force # jets
no single variable gives good separation->combine into Neural Network
train network to separate signal from dominant ZWW background
try at 750 GeV & 1 Tev – slightly different event topologies
Signal/background discrimination
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Train neural network to separate signal from ZWW backgrounduse TMultiLayerPerceptron package (in root)
inputs:#jets, #el+#mu, #taus, Z decay type, Ycut of jet forcing, |total momentum|, Z miss. mass
1 hidden layer, with 5 nodes
5k training events (50% ZHH, 50% ZWW)
5k testing events
2k training cycles
reached minimum
no significant overtraining
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Some NN inputs @ 1TeV centre-of-mass energy ZHH, ZWW
Z decay (invis, lept, jet) # taus # hadronic jets
Jet forcing ycut
NN output
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NN output @ 750 GeV
NN output @ 1000 GeV
reminder:total # signal events~100 / ab-1
Both look ~impossible...
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ZHH production sensitive to gHHHsmall xsec, large backgrounds
for mH = 170 GeV, many possible final states
trained NN to separate ZHH & ZWW- 170 GeV higgs- 750 GeV & 1 TeV centre-of-mass
some separation, but signal still swamped by huge
backgrounds
maybe this measurement is not possible...
Summary for heavy Higgs analysis
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Supplement
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Energy cutEnergy of the reconstructed particles was checked.
• Higgs-energy of the signal is below 250 GeV.
• The energy above 250 GeV was selected.
E1(GeV)0 100 200 300 400
HH
E1(GeV)0 100 200 300 400
B.G.
ZZtt
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2 cut2 distribution was checked after momentum and selection.
• The signal has the peak at 2 =0.
• B.G. has the broader shape than the signal.
2<13 was selected.
20 20 40 60 80
HH
20 20 40 60 80
B.G.
ZZtt
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Mass cutThe Higgs mass cut is applied to select well-reconstructed events.
• Selection: 95GeV < MH < 125GeV
MH1(GeV)0 40 80 160 200120
HH
MH1(GeV)0 40 80 160 200120
B.G.
ZZtt
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Missing mass cutThe missing mass was reconstructed.
• The peak is observed at Z-mass in HH events.
• Selection: 90GeV < Mmiss < 200GeV
Mmiss(GeV)0 100 200 400 500300
HH
Mmiss(GeV)0 100 200 400 500300
B.G.
ZZtt
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Angle cutThe angular distributions of the reconstructed particles are checked.
• Selection: |cos|<0.9
cos1
-1 -0.6 -0.2 0.2 0.6 1
HH
cos1
-1 -0.6 -0.2 0.2 0.6 1
B.G.
ZZtt