Current Status on ZHH Analysis
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Transcript of Current Status on ZHH Analysis
Current Status on ZHH Analysis
’08 5/31 GLD ZHH-group
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
Study of Light Higgs Case
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
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.
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
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
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
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
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).
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.
Study of Heavy Higgs Case
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
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
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
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
Some NN inputs @ 1TeV centre-of-mass energy ZHH, ZWW
Z decay (invis, lept, jet) # taus # hadronic jets
Jet forcing ycut
NN output
NN output @ 750 GeV
NN output @ 1000 GeV
reminder:total # signal events~100 / ab-1
Both look ~impossible...
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
Supplement
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
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
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
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
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