Full simulation study of search for invisible higgs decays with the ild detector at the ilc
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Full Simulation Study of Search for Invisible Higgs Decays with the ILD Detector at the ILC. Akimasa Ishikawa (Tohoku University). Just started on Monday. Invisible Higgs Decay. In the SM, an invisible Higgs decay is H  ZZ* 4 n process and its BF is small ~0.1%

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Full Simulation Study of Search for Invisible Higgs Decays with the ILD Detector at the ILC

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Full Simulation Study ofSearch for Invisible Higgs Decayswith the ILD Detector at the ILC

Akimasa Ishikawa

(Tohoku University)

Just started on Monday.

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



Signal and Backgrounds

  • Signal

    • Pseudo signal : ZH, Zqq, HZZ*4n

      • Also we have Zee, mm, tt, H4n 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, Wqq

    • Znn, Zqq

    • WW semileptonic, Wqq, Wln

    • ZZ semileptonic, Zqq, Zll

    • nnH, generic H decays

    • qqH, generic H decays

MC setup and Samples

  • Generator : Wizard

    • 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 WWqqen and enW enqq

    • Half of the samples are used for cut determination. The other to be used for efficiency calculation.

Overview of the Selections

Optimization is needed !

  • 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

No Isolated Lepton


  • 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

Number of PFOs and Tracks

  • To eliminate low multiplicity events like Znn, Ztt

    • NPFO > 16

    • Ntrack > 6





Z mass

Fast sim signal


  • 80GeV < MZ < 100GeV

    • Sigma Mean = 90.7GeV, sigma = 10.6GeV





ZZ sl

WW sl

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

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

nnZ sl



Recoil Mass


  • 120 < Mrecoil < 140GeV

    • We found

      • qqH is new peaking background.

      • nnH is new background peaking close to signal region





ZZ sl

WW sl

Cut Summary

  • Number of events scaled to 250fb-1

Comparison with fast simulator

  • Upper is s in abfrom 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.

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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