Study of Higgs Selfcouplings at ILC
J. Tian (Tsinghua U.)
Y. Takubo (Tohuko U.)
K. Fujii (KEK)
Y. Gao (Tsinghua U.)
status
investigating the full simulated backgrounds
llbbH and ννbbH which are not considered in
the analyses of ZHH(llHH) and ZHH(ννHH) at
500 GeV.
met a problem about the generators of llbbH
and ννbbH used by the ILD mass production.
previous conclusion
+
−
e + e → ZHH → (l¯l)(bb̄)(bb̄)
4.93 signal events with 0.50 background events:
ZHH excess significance: 3.6σ
+3.5
5.4
54% for cross section
68% confidence level:
−2.3
precision for Higgs self-coupling: 97%
llbbH not considered: 14.5 ab, 19.4 llbbbb
expected (comparing to current 25.6 llbbbb), if
we double the current backgrounds: 3.2σ
Mistake:
cross section of llbbH was half estimated (previous only for eebbH), should be 30.0 ab
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eebbH background
no beam polarization
!
Ecm = 500GeV, MH = 120GeV
Ldt = 2ab−1
full simulation of eebbH with the generator used by ILD mass production succeeded on KEKCC
normalized
MC
expected
pre-selection
econe1+econe2<30
econec1+econec2<10
80<mz<100
mvallqq>0.8
mvalvbbqq>0.8
mvallbbbb>0.5
llhh(llbbbb)
19700
21.18(9.5)
17.50(7.83)
15.67(7.30)
13.82(6.21)
10.47(5.78)
7.37(5.19)
4.93(3.55)
llbbbb
10924
25.64
9.31
7.79
6.18
4.12
3.67
0.50
eebbH
4986
29.0
18.6
16.2
14.0
11.2
9.10
4.71
the efficiency of last cut should similar for llbbbb and eebbH (slight higher), but
here eebbH is similar with signal !
4
strange distributions
scatter plot of M(H1) versus M(H2) for eebbH background (before cuts)
mh1
mh1:mh2
200
180
don’t they look like signal?
160
140
120
100
80
60
40
40
60
80
100
120
140
160
180
200
mh2
these events look like background from eeZH
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strange distributions
scatter plot of M(H1) versus M(H2) for eebbH background (after cuts)
mh1:mh2 {econe1+econe2<30&&econec1+econec2<10&&mz>80&&mz<100&&mvallqq>0.8&&mvalvbbqq>0.8&&mvallbbbb>0.5}
mh1
180
160
140
120
100
80
40
60
80
100
120
140
160
mh2
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strange distributions (cont’)
forward angle for eebbH background (before cuts)
angular distribution of the boson with biggest momentum
0.02
llhh(llbbbb)
0.018
eebbh
0.016
Normalized
0.014
0.012
0.01
0.008
0.006
0.004
0.002
0
−1
−0.8
−0.6
−0.4
−0.2
0
0.2
0.4
0.6
0.8
1
cos!1st
No enhancement at |cos|=1 (don’t like eebbbb case)
7
angular distribution of the boson
with highest momentum
forward angle for eebbbb background (before cuts)
angular distribution of the boson with biggest momentum
0.05
llhh(llbbbb)
llbbbb
Normalized
0.04
0.03
0.02
0.01
−1
−0.8
−0.6
−0.4
−0.2
0
0.2
0.4
0.6
0.8
1
cos!1st
enhancement at |cos|=1 is very obvious
38
similar situation for ννbbH (BG in ννHH analysis)
scatter plot of M(H1) versus M(H2) for ννbbH background (before cuts)
mh1
mh1:mh2
200
180
160
140
120
100
80
60
40
40
60
80
100
120
140
160
180
200
mh2
higher statistics, more obvious signal region is
9
check the Feynman diagrams(MadGraph)
except the QED process and fusion process, there are 32 diagrams
leading contributions come from ZZH and ZHH!
10
check the Feynman diagrams(MadGraph)
except the QED process and fusion process, there are 32 diagrams
leading contributions come from ZZH and ZHH!
10
check the Feynman diagrams(MadGraph)
except the QED process and fusion process, there are 32 diagrams
leading contributions come from ZZH and ZHH!
10
summery
this situation happens also in the ee ----> eebbbb process, where both
ZHH and ZZH contribute to the Feynman diagrams. In the full
simulation framework, Whizard is responsible for the generator and
M(H)=∞ is set for the non-Higgs process. So it is safe for the eebbbb
background.
for eebbH process, M(H)=120GeV is set. So the contribution from ZHH
involved. I think this is the reason that large parts of eebbH events
really look like signal.
from MadGraph, we know the leading contribution to eebbH should
come from ZZH except ZHH. This part is the real background. But
Whizard seems only consider the final state, we need Physsim to
generate the ZZH events for full simulation.
Suehara-san and Miyamoto-san are helping me to use Physsim for full
simulation, still some problem about the stdhep file generation
process.
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backup
discussion
once we know the real contamination for llbbH comes from ZZH, we
can estimate it from the cross section obtained by Physsim. σ(ZZH) =
0.494 fb, the expected number of llbbH events should be (2 ab-1)
0.494×2000×0.067×0.15×2 = 19.8; the expected number of llbbbb from
llbbH shoud be 19.8×0.67 = 13.3;
according to the efficiencies of llbbbb background and llHH signal, a
rough estimation for llbbH: ~ 1 event. That means 4.9 signal events
with total 1.5 background events, excess significance will change to
2.6σ (previous estimation: 4.9 S + 1.0 B = 3.2σ)
anyway, we need real full simulated ZZH data to give the answer and
I think the neural-net could be optimized if ZZH events available.
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