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Current Status on ZHH Analysis

Current Status on ZHH Analysis. ’08 5/31 GLD ZHH-group. Analysis menu. ZHH study ZHH analysis was started to cover wide Higgs-mass region. E CM =500GeV is the best for M H =120GeV. E CM >750GeV is preferable to study for M H >160GeV. M H v.s. s (ZHH). 750GeV. 1TeV. 500GeV.

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Current Status on ZHH Analysis

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  1. Current Status on ZHH Analysis ’08 5/31 GLD ZHH-group

  2. Analysis menu • 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. MH v.s. s(ZHH) 750GeV 1TeV 500GeV • Our analysis menu • Light Higgs : MH=120GeV, ECM=500GeV • Heavy Higgs : MH=170GeV, ECM=500GeV~1TeV

  3. Study of Light Higgs Case

  4. Introduction • Status at TILC08 • Analysis was started for HHnn-mode. • Signal significance of 1.4s was obtained against ZZbbbb. • Signal : 12 events • ZZbbbb : 64 events • Today’s topic • Re-optimization of the selection cut. • Analysis with B.G. of ZZbbbb and tt. • s(ZZ) = 395.8 fb • s(tt) = 583.6 fb

  5. Selection cuts • The new selection cuts are applied to improve B.G. rejection. • Primary cutfor reconstructed particles • Energy cut (E1,2 < 250GeV) • Momentum cut (P1,2 < 200GeV) • b cut (b1,2 < 0.9) • Fine selection cut • c2 cut (c2 < 13) • Higgs mass cut (95GeV < MH1,2 < 125GeV) • Missing mass cut (90GeV < Mmiss < 200GeV) • Angle cut (|cosq1,2| < 0.9) • b-tag cut New selection cuts Example of new cuts (Momentum and b cut) is shown.

  6. Momentum cut • Momentum of the reconstructed particles was checked. • Higgs-momentum of the signal is below 200 GeV/c. • The momentum above 200 GeV/c was rejected. HHnn B.G. tt ZZ 400 400 100 300 100 300 0 0 200 200 P1(GeV/c) P1(GeV/c)

  7. b cut • b(=p/E) of the reconstructed particles was checked. • b of the signal is below 0.9. • b<0.9 was selected. HHnn B.G. tt ZZ b1 b1 0.5 0.9 0.3 0.5 0.9 0.7 0.3 0.7 The fine selection cuts were applied after these primary cuts.

  8. B-tag cut • After the fine selection cuts, b-tagging was applied. • Nb-tag ≧3was selected. • b-tag: 3 tracks with 3s separation from IP. • 23.5 events of ZZbbbb still contaminates for 2 ab-1. • All tt events are rejected. B.G. HHnn tt ZZ 5 5 0 1 3 4 Nb-tag 2 Nb-tag 0 1 3 4 2

  9. Signal significance • The signal significance was estimated for 2ab-1 • Signal : 12.1 events • B.G. : 23.5 events Rec. (MH1+MH2) for 2ab-1 • TILC08 • Signal : 12 events • B.G. : 64 •  1.4 s Signal + BG tt ZZ Significance : 2.0 s GeV

  10. Reduction summary tt HH nn ZZbbbb 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) 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) • No cut • E1,2 < 250GeV • P1,2 < 200GeV • b1,2 < 0.9 • c2 < 13 • 95GeV < MH1,2 <125GeV • 90GeV < Mmiss < 200GeV • |cosq1,2| < 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) • The signal efficiency is 16%. • 55% of signal is lost at b-tag cut. • However, b-tag is necessary to reject tt-B.G.. Next step is to consider tbtb-B.G. (0.7fb).

  11. Summary for HHnn • The selection cut was re-optimized. • The signal significance of 2.0 s was obtained against the ZZbbbb and tt. • This result is comparable to Djamel’s report at LCWS07, after scaling s(ZZ) to the correct value (395.8fb). • The other B.G. modes like tbtb should be taken into account.

  12. Study of Heavy Higgs Case

  13. Introduction 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 boson ZWW (this has largest xsec), ZZZ, Ztt, ZZWW events generated by MadGraph, studied @ QuickSim level

  14. Event reconstruction Fit events to ZHH->ZWWWW hypothesis many 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 decay identify tau-like jets # charged tracks = 1 or 3 invariant mass < m_tau jet charge = +-1 try two possibilities: true tau jet, or hadronic jet for other Ws, try both tau decay and hadronic decay possibilities for each possible decay topology force non-leptonic part of event into required # jets ask if invariant masses of Z, W, H->jets consistent with expectation

  15. Signal/background discrimination require that event fits at least one signal final state hypothesis use kinematic variables to decide if event is real ZHH event or background visible energy, momentum vs. # assumed neutrinos Ycut 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

  16. Train neural network to separate signal from ZWW background use 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

  17. Some NN inputs @ 1TeV centre-of-mass energy ZHH, ZWW Z decay (invis, lept, jet) # taus # hadronic jets Jet forcing ycut NN output

  18. NN output @ 750 GeV NN output @ 1000 GeV reminder: total # signal events ~100 / ab-1 Both look ~impossible...

  19. Summary for heavy Higgs analysis ZHH production sensitive to gHHH small xsec, large backgroundsfor mH = 170 GeV, many possible final statestrained NN to separate ZHH & ZWW - 170 GeV higgs - 750 GeV & 1 TeV centre-of-masssome separation, but signal still swamped by huge backgroundsmaybe this measurement is not possible...

  20. Supplement

  21. Energy cut • Energy of the reconstructed particles was checked. • Higgs-energy of the signal is below 250 GeV. • The energy above 250 GeV was selected. HHnn B.G. tt ZZ 400 400 100 300 100 300 0 0 200 200 E1(GeV) E1(GeV)

  22. c2 cut • c2 distribution was checked after momentum and b selection. • The signal has the peak at c2 =0. • B.G. has the broader shape than the signal. • c2<13 was selected. HHnn B.G. tt ZZ 80 40 60 c2 20 0 80 40 60 c2 20 0

  23. Mass cut • The Higgs mass cut is applied to select well-reconstructed events. • Selection: 95GeV < MH < 125GeV HHnn B.G. tt ZZ 160 200 40 0 80 160 200 120 40 0 80 120 MH1(GeV) MH1(GeV)

  24. Missing mass cut • The missing mass was reconstructed. • The peak is observed at Z-mass in HHnn events. • Selection: 90GeV < Mmiss < 200GeV HHnn B.G. tt ZZ 400 500 100 0 200 300 400 500 100 0 200 300 Mmiss(GeV) Mmiss(GeV)

  25. Angle cut • The angular distributions of the reconstructed particles are checked. • Selection: |cosq|<0.9 HHnn B.G. tt ZZ -1 1 -0.2 0.2 -0.6 -1 1 -0.2 0.2 0.6 -0.6 0.6 cosq1 cosq1

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