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Akimasa Ishikawa (Tohoku University)
Full Simulation Study of Search for Invisible Higgs Decays with the ILD Detector at the ILC Akimasa Ishikawa (Tohoku University) Just started on Monday.
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Invisible Higgs Decay In the SM, an invisible Higgs decay is H ZZ* 4n process and its BF is small ~0.1% If we found sizable invisible Higgs decays, it is clear new physics signal. The decay products are dark matter candidates. At the LHC, one can search for invisible Higgs decays by using recoil mass from Z or summing up BFs of observed decay modes with some assumptions. The upper limit is O(10%). At the ILC, we can search for invisible Higgs decays using a recoil mass technique with model independent way! e+e- ZH known measured
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Signal and Backgrounds
Pseudo signal : ZH, Zqq, HZZ*4n Also we have Zee, mm, tt, H4n samples Backgrounds I found qqll, qqln and qqnn final states are the dominant backgrounds. (other backgrounds also studies but the results mostly omitted from the slides) Wen, Wqq Znn, Zqq WW semileptonic, Wqq, Wln ZZ semileptonic, Zqq, Zll nnH, generic H decays qqH, generic H decays
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MC setup and Samples Generator : Wizard Samples
for both signal and backgrounds ECM = 250GeV Higgs mass 125GeV Polarization of P(e+,e-)=(+30%,-80%), (-30%, +80%) Throughout the slides, “Left” and “Right” polarization Samples Official DBD samples Interference is considered, ex WWqqen and enW enqq Half of the samples are used for cut determination. The other to be used for efficiency calculation. [fb] Wen sl Znn sl WW sl ZZ sl nnH qqH qqH H4n eeH H4n mmH ttH “Left” 161.2 271.8 12436 856.9 489.5 210.2 0.224 0.0116 0.0111 “Right” 102.2 92.5 869 467.2 275.6 142.0 0.151 0.0079 0.0075
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Overview of the Selections
Durham jet algorithm Two-jet reconstruction No isolated leptons No forward going electrons It is found ineffective with full simulator Z mass reconstructed from di-jet Also used for Likelihood ratio cut cos(qZ) Production angle of Z Just apply < 0.99 cut to eliminate peaky eeZ background before making likelihood ratio Likelihood ratio of Z mass, cos(qZ), cos(qhel) cos(qhel) : Helicity angle of Z Recoil mass The final plot Fitted to set upper limit (not yet). The figures to be shown are only eLpR pol. config., not scaled to 250fb-1 Optimization is needed !
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No Isolated Lepton Selection Identification efficiency is not high.
signal Selection Common P > 10 GeV Cone cosq >0.98 IP3D < 0.01 Electron 1>EEcal / ( EECal + EHCal) >0.9 1.4 > E/p > 0.7 Muon EEcal / ( EECal + EHCal) < 0.4 E/p < 0.3 Identification efficiency is not high. We need to optimize the selection especially leptons going closely to jets. ZZ sl One Z mm or tt The other qq
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Number of PFOs and Tracks
To eliminate low multiplicity events like Znn, Ztt NPFO > 16 Ntrack > 6 signal Znn Zll NPFO Ntrack
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Z mass 80GeV < MZ < 100GeV Sigma Mean = 90.7GeV, sigma = 10.6GeV
Fast sim signal signal 80GeV < MZ < 100GeV Sigma Mean = 90.7GeV, sigma = 10.6GeV nnZ nnH enW qqH ZZ sl WW sl
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Z mass for enW The distributions for fast and full simulations are quite different, I guess due to the lepton ID selections. Fast sim Full sim WW sl
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Polar angle of Z We know eeZ is small but can be eliminated by polar angle of Z. And some backgrounds can be reduced. To be included to Likelihood ratio cut eeZ nnZ sl signal
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Recoil Mass 120 < Mrecoil < 140GeV We found signal nnZ nnH enW
qqH is new peaking background. nnH is new background peaking close to signal region nnZ nnH enW qqH ZZ sl WW sl
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Cut Summary Number of events scaled to 250fb-1 “Left” Wen sl Znn sl
WW sl ZZ sl nnH qqH qqH H4n Pseudo signal No cut 40297 67951 214232 19383 52546 (1.000) No lepton 13512 64247 155472 17297 44609 (0.948) Trk and PFO 13431 62401 152834 16092 44605 (0.941) MZ 1567 45186 162967 70813 1201 64 (0.758) cosqZ 1548 44656 159303 65418 1195 (0.755) MRecoil 526 7423 12436 6578 490 43 (0.613) “Right” Wen sl Znn sl WW sl ZZ sl nnH qqH qqH, H4n Pseudo signal No cut 25546 23124 189596 116797 10646 35488 (1.000) No lepton 9475 21792 94327 83528 9491 30184 (0.946) Trk and PFO 9409 21188 93993 81666 8815 30181 (0.940) MZ 1414 15036 11615 35010 639 48 28.89 (0.763) cosqZ 1349 14850 11388 31476 653 47 (0.759) MRecoil 453 1504 869 3309 276 36 (0.609)
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Comparison with fast simulator
Upper is s in ab from old study, lower is #events with 250fb-1 with new one. Sorry. Multiply 4 for new one to compare with old one. Total background s=22fb for old one, s=18fb for new one. Cut / s[ab] Signal [%] ZZ nnZ WW enW eeZ No cut 100.00 982000 5000 684000 Isolated lepton 95.55 835853 4767 545711 Forward electron veto 586409 3971 455945 Z mass 48.92 156627 1966 34445 26718 752847 Cos(qZ) 48.54 153362 1920 33041 25806 8651 LR 48.04 150800 1893 30629 24231 6542 Recoil mass 42.10 20073 705 3695 7733 430 “Right” Events/250fb-1 qqH, H4n Pseudo signal ZZ sl Znn sl WW sl Wen sl nnH qqH No cut (1.000) 116797 23124 189596 25546 10646 35488 No lepton (0.946) 83528 21792 94327 9475 9491 30184 Trk and PFO (0.940) 81666 21188 93993 9409 8815 30181 MZ 28.89 (0.763) 35010 15036 11615 1414 639 48 cosqZ (0.759) 31476 14850 11388 1349 653 47 MRecoil (0.609) 3309 1504 869 453 276 36
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Summary and Plan Just started. Found new peaking background, qqH
No significant difference from the result with fast simulation Optimize the lepton ID Likelihood ratio cut Toy MC study for an upper limit Temporal result to be Snowmass, hopefully Add Z leptonic channel
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