1. 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.

2. 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

3. Signal and Backgrounds
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

4. 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 WWqqen 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 H4n
eeH
H4n
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

5. 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 !

6. 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

7. Number of PFOs and Tracks
To eliminate low multiplicity events like Znn, Ztt
NPFO > 16
Ntrack > 6
signal
Znn Zll
NPFO
Ntrack

8. 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

9. 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

10. 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

11. 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

12. Cut Summary Number of events scaled to 250fb-1 “Left” Wen sl Znn sl
WW sl
ZZ sl
nnH
qqH
qqH H4n
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, H4n
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)

13. 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, H4n
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

14. 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