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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

known

measured

Signal and Backgrounds

- Signal
- 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

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 WWqqen 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

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

Number of PFOs and Tracks

- To eliminate low multiplicity events like Znn, Ztt
- NPFO > 16
- Ntrack > 6

signal

ZnnZll

NPFO

Ntrack

Z mass

Fast sim signal

signal

- 80GeV < MZ < 100GeV
- Sigma Mean = 90.7GeV, sigma = 10.6GeV

nnZ

nnH

enW

qqH

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

eeZ

signal

Recoil Mass

signal

- 120 < Mrecoil < 140GeV
- We found
- qqH is new peaking background.
- nnH is new background peaking close to signal region

nnZ

nnH

enW

qqH

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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