Introduction
Know the H → ZZ → 2l2q channel isimportant at high mH but can it alsohelp at low mH?
H → ZZ → 4l : very clean but low BR.H → ZZ → 2l2q/2l2b: less clean buthave much higher branching ratio.
Look at region where ZZ cross sectionpeaks: 140 < mH < 160 GeV
Coinsides with H → WW excess [GeV]HM
100 200 300 400 500
BR
[pb]
× σ
-410
-310
-210
-110
1
10
LHC
HIG
GS
XS W
G 2
011
SM = 7TeVs
µl = e, τν,µν,eν = ν
q = udscbbbν± l→WH
bb-l+ l→ZH
-τ+τ →VBF H
γγ
qqν± l→WW
ν-lν+ l→WW
qq-l+ l→ZZ
νν-l+ l→ZZ
-l+l
-l+ l→ZZ
This can be accessed by allowing one of the Z s to be off shellChoose the Z → ll to be off shell to reduce Z+jets background
Not a discovery channel but could be used to confirm discoveryBut can it compete with the 4l channel?
Carl Gwilliam Athens HSG2 Meeting 2/18
Datasets
Take a first look at the potential sensitivity in the low mass regionusing MC only
HSG2 dilepton skims not suitable (for electrons) due to pT cut
Monte CarloH → ZZ → llqq (PowHeg)
Applied pT reweight
Have datsets from 110 - 185 GeV, but concentrate on 140-160 GeV
Z + Drell-Yan (Pythia)
Standard pythia Z samples + 15-60 GeV Drell-Yan samples
Not using ALPGEN since don’t seem to have c/b for Drell-Yan samples
tt̄ + single top (MC@NLO)ZZ , WZ (MC@NLO)No QCD background
Carl Gwilliam Athens HSG2 Meeting 3/18
Lepton Selection
Electrons
Medium with author 1 or 3 and pT > 7 GeV and |ηclus | < 2.47Include crack regionsObject Quality cuts (including removal of bad FEBs in MC)Track isolation:
∑tracks /pT < 0.1 within ∆R = 0.2
Smearing and efficiency corrections + systematics from EPSrecommendations. No low pT corrections
Muons
STACO combined/tagged with pT > 7 GeV and |η| < 2.5Track isolation:
∑tracks /pT < 0.1 within ∆R = 0.2
Cosmic rejection: |d0| < 1 mm and |z0| < 10 mmScaling/smearing and efficiency correction + systematics from EPSrecommendations. No low pT corrections
Carl Gwilliam Athens HSG2 Meeting 4/18
Lepton Kinemtaics
Leptons forming Z candidate after mll cutelec muon
[GeV]t P
0 20 40 60 80 100 120 140 160 180 200
Eve
nts
/ bin
-110
1
10
210
310
410
510
610
710Total MC
Z
Top
Diboson
=145 GeV)H
Signal (m
ATLAS Internal
[GeV]t P
0 20 40 60 80 100 120 140 160 180 200
Eve
nts
/ bin
-110
1
10
210
310
410
510
610
710Total MC
Z
Top
Diboson
=145 GeV)H
Signal (m
ATLAS Internal
η -3 -2 -1 0 1 2 3
Eve
nts
/ bin
0
5000
10000
15000
20000
25000
30000
35000
η -3 -2 -1 0 1 2 3
Eve
nts
/ bin
0
5000
10000
15000
20000
25000
30000
35000 Total MC
Z
Other BG
ATLAS Internal
η -3 -2 -1 0 1 2 3
Eve
nts
/ bin
0
10000
20000
30000
40000
50000
η -3 -2 -1 0 1 2 3
Eve
nts
/ bin
0
10000
20000
30000
40000
50000Total MC
Z
Other BG
ATLAS Internal
Carl Gwilliam Athens HSG2 Meeting 5/18
Jet + EmissT
Selection
JetsAnti-kT 4 with pT > 20 GeV and |η| < 2.5
Following jet recommendations, take kinemtatics directly from
”AntiKt4TopoEMJets” with no OffsetEtaJES
Remove negative energy jetsRemove events with jets pointing to the bad FEB regionPile-up: reject jets with |JVF | < 0.75Recomended JES uncertainty including additional Pile-up uncertainty
b-taggingJetFitterCOMBNN with w > 0.35 (≈ 70% efficiency)
Applying correction derived by b-tagging for EPS
METMET LocHadTopo with correction for selected muons
Overlap Removal: remove elecs within ∆R = 0.2 of muons and thenjets within ∆R = 0.4 of elecs
Carl Gwilliam Athens HSG2 Meeting 6/18
Jet Kinemtaics
All jets after mll and Emiss
T cut
jett E
0 50 100 150 200 250
Eve
nts
10
210
310
410
510Total MC
Z
Top
Diboson
=145 GeV)H
Signal (m
ATLAS Internal
jetη -3 -2 -1 0 1 2 3
Eve
nts
02000400060008000
1000012000140001600018000 Total MC
Z
Top
Diboson
=145 GeV)H
Signal (m
ATLAS Internal
Carl Gwilliam Athens HSG2 Meeting 7/18
Event Selection
Triggers:
EF e20 medium || EF 2e12 mediumEF mu18 MG || EF 2mu10 || EF mu40 MSonly barrel
Primary vertex containing at least 3 tracks
Exactly 2 leptons with:
Leading pT > 20 and sub-leading pT > 7 orboth pT > 12 (14) GeV for elec (muon)20 < mll < 76 GeV (off-shell)
EmissT < 30 GeV
Tighter as larger top background
At least 2 jets with 70 < mjj < 105 GeV
Constrain mjj to mZ when constructing Higgs mass
∆φjj < 90◦
Carl Gwilliam Athens HSG2 Meeting 8/18
Event Catagories
Split events (for low and high mH selection) into 2 samples:
“tagged”: 2 b-tagged jets“untagged”: < 2 b-tagged jets
Reject events with more than 2 b-tags to reduce top background
For tagged samples always take combination with highest b weight
In this case, scale the dijet mass by 1.05 to peak closer to Z mass
For untagged sample not optimal to take 2 highest PT jets. Insteadtake all combinations of upto 3 leading jets that fulfill Mjj and highmH cuts.
Carl Gwilliam Athens HSG2 Meeting 9/18
Leptonic Z
Cut out main Z peak, keeping off-shell region
Could take upper mass cut futher off-shell
[GeV]ee m
10 20 30 40 50 60 70 80 90 100
Eve
nts
/ bin
1
10
210
310
410
510
610 Total MC
Z
Top
Diboson
=145 GeV)H
Signal (m
ATLAS Internal
electron channel
[GeV]µµ m
10 20 30 40 50 60 70 80 90 100 E
vent
s / b
in1
10
210
310
410
510
610Total MC
Z
Top
Diboson
=145 GeV)H
Signal (m
ATLAS Internal
muon channel
Carl Gwilliam Athens HSG2 Meeting 10/18
Control Plots
[GeV]missT E
0 20 40 60 80 100120140160180200220240
Eve
nts
/ bin
-210
-110
1
10
210
310
410
510
610 Total MC
Z
Top
Diboson
=145 GeV)H
Signal (m
ATLAS Internal
jet N
0 1 2 3 4 5 6 7 8 9 10
Jet
s /b
in
1
10
210
310
410
510
610
710
810Total MC
Z
Top
Diboson
=145 GeV)H
Signal (m
ATLAS Internal
Weight
-10 -5 0 5 10 15 20 25
Eve
nts
/ bin
10
210
310
410
510
610 Total MC
Z
Top
Diboson
=145 GeV)H
Signal (m
ATLAS Internal
b-Jet N
0 0.5 1 1.5 2 2.5 3 3.5 4
Eve
nts
/ bin
1
10
210
310
410
510
610
710Total MC
Z
Top
Diboson
=145 GeV)H
Signal (m
ATLAS Internal
Carl Gwilliam Athens HSG2 Meeting 11/18
∆φ
Jets slightly more back-to-back in signal than Z/DY → ∆φjj > 90◦
∆φll (offshell) very similar between signal and Z/DY
[rad.]ll
φ ∆0 0.5 1 1.5 2 2.5 3
Arb
itary
Uni
ts
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09 llbb→H
Z
ATLAS Internal
[rad.] JJ
φ ∆0 0.5 1 1.5 2 2.5 3
Arb
itary
Uni
ts0
0.02
0.04
0.06
0.08
0.1
0.12
0.14 llbb→H
Z
ATLAS Internal
Carl Gwilliam Athens HSG2 Meeting 12/18
Preliminary Results (untagged)
140 GeV 145 GeV
[GeV]lljj m
100 120 140 160 180 200 220 240
Eve
nts
/ 5 G
eV
0
500
1000
1500
2000
2500
3000
[GeV]lljj m
100 120 140 160 180 200 220 240
Eve
nts
/ 5 G
eV
0
500
1000
1500
2000
2500
3000
[GeV]lljj m
100 120 140 160 180 200 220 240
Eve
nts
/ 5 G
eV
0
500
1000
1500
2000
2500
3000-1
L dt=10.00 fb∫data
10×Signal =140 GeV)
H(m
Total BG
Z
Top
Diboson
ATLAS Internal
[GeV]lljj m
100 120 140 160 180 200 220 240
Eve
nts
/ 5 G
eV
0
500
1000
1500
2000
2500
3000
[GeV]lljj m
100 120 140 160 180 200 220 240
Eve
nts
/ 5 G
eV
0
500
1000
1500
2000
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3000
[GeV]lljj m
100 120 140 160 180 200 220 240
Eve
nts
/ 5 G
eV
0
500
1000
1500
2000
2500
3000-1
L dt=10.00 fb∫data
10×Signal =145 GeV)
H(m
Total BG
Z
Top
Diboson
ATLAS Internal
150 GeV 160 GeV
[GeV]lljj m
100 120 140 160 180 200 220 240
Eve
nts
/ 5 G
eV
0
500
1000
1500
2000
2500
3000
[GeV]lljj m
100 120 140 160 180 200 220 240
Eve
nts
/ 5 G
eV
0
500
1000
1500
2000
2500
3000
[GeV]lljj m
100 120 140 160 180 200 220 240
Eve
nts
/ 5 G
eV
0
500
1000
1500
2000
2500
3000-1
L dt=10.00 fb∫data
10×Signal =150 GeV)
H(m
Total BG
Z
Top
Diboson
ATLAS Internal
[GeV]lljj m
100 120 140 160 180 200 220 240
Eve
nts
/ 5 G
eV
0
500
1000
1500
2000
2500
3000
[GeV]lljj m
100 120 140 160 180 200 220 240
Eve
nts
/ 5 G
eV
0
500
1000
1500
2000
2500
3000
[GeV]lljj m
100 120 140 160 180 200 220 240
Eve
nts
/ 5 G
eV
0
500
1000
1500
2000
2500
3000-1
L dt=10.00 fb∫data
10×Signal =160 GeV)
H(m
Total BG
Z
Top
Diboson
ATLAS Internal
Carl Gwilliam Athens HSG2 Meeting 13/18
Z Background in Tagged Sample
As for high mH , there are insufficient statistics in the Z/Drell-Yan MCfor the final tagged distributionsInstead use untagged Z MC distribution and normalise to number ofevents in the tagged MC
[GeV]lljj m
100 120 140 160 180 200 220 240
Eve
nts
/ 5 G
eV
0
5
10
15
20
25
30Z untagged jets (scaled)
Z tagged jets
In fact, so few stats that even normalisation has ≈ 25% uncertaintyFurther study would require bigger samples
Carl Gwilliam Athens HSG2 Meeting 14/18
Preliminary Results (tagged)
140 GeV 145 GeV
[GeV]lljj m
100 120 140 160 180 200 220 240
Eve
nts
/ 5 G
eV
0
2
4
6
8
10
12
14
16
[GeV]lljj m
100 120 140 160 180 200 220 240
Eve
nts
/ 5 G
eV
0
2
4
6
8
10
12
14
16
[GeV]lljj m
100 120 140 160 180 200 220 240
Eve
nts
/ 5 G
eV
0
2
4
6
8
10
12
14
16 -1 L dt=10.00 fb∫data
=140 GeV)H
Signal (m
Total BG
Z
Top
Diboson
ATLAS Internal
[GeV]lljj m
100 120 140 160 180 200 220 240
Eve
nts
/ 5 G
eV
0
2
4
6
8
10
12
14
16
[GeV]lljj m
100 120 140 160 180 200 220 240
Eve
nts
/ 5 G
eV
0
2
4
6
8
10
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14
16
[GeV]lljj m
100 120 140 160 180 200 220 240
Eve
nts
/ 5 G
eV
0
2
4
6
8
10
12
14
16-1 L dt=10.00 fb∫data
=145 GeV)H
Signal (m
Total BG
Z
Top
Diboson
ATLAS Internal
150 GeV 160 GeV
[GeV]lljj m
100 120 140 160 180 200 220 240
Eve
nts
/ 5 G
eV
0
2
4
6
8
10
12
14
16
[GeV]lljj m
100 120 140 160 180 200 220 240
Eve
nts
/ 5 G
eV
0
2
4
6
8
10
12
14
16
[GeV]lljj m
100 120 140 160 180 200 220 240
Eve
nts
/ 5 G
eV
0
2
4
6
8
10
12
14
16-1 L dt=10.00 fb∫data
=150 GeV)H
Signal (m
Total BG
Z
Top
Diboson
ATLAS Internal
[GeV]lljj m
100 120 140 160 180 200 220 240
Eve
nts
/ 5 G
eV
0
2
4
6
8
10
12
14
16
[GeV]lljj m
100 120 140 160 180 200 220 240
Eve
nts
/ 5 G
eV
0
2
4
6
8
10
12
14
16
[GeV]lljj m
100 120 140 160 180 200 220 240
Eve
nts
/ 5 G
eV
0
2
4
6
8
10
12
14
16-1 L dt=10.00 fb∫data
=160 GeV)H
Signal (m
Total BG
Z
Top
Diboson
ATLAS Internal
Carl Gwilliam Athens HSG2 Meeting 15/18
Expected Limits
First estimate of the expected limits (MCLIMIT) witout systematics:
140 GeV 145 GeV 150 GeV 155 GeV 160 GeV
lumi (fb−1) 2 10 2 10 2 10 2 10 2 10
tagged 9.9 3.5 6.9 2.6 8.9 3.3 6.1 2.3 17.0 6.4untagged 6.1 3.7 5.7 3.5 6.5 4.3 7.3 4.7 14.7 10.1combined 5.0 2.7 4.3 2.3 5.0 3.0 4.6 2.4 11.0 6.5
Get a limit of ≈ 5× σSM with 2 fb−1
For 10 fb−1 get around 2-3 ×σSM
Untagged better at 2 fb−1 (stats) but tagged overtakes for 10 fb−1
Currently a factor of 2-3 worse than H → 4l , which has 1.5-2 ×σSMfor 2 fb−1 (but no systs)
Can probably be significantly improved using MVA though ...
Carl Gwilliam Athens HSG2 Meeting 16/18
Multi-Variant Analysis
Significant improvements should be possible using a MVA.E.g. a first look at TMVA shows good separation for DY background
MLP response-0.4 -0.2 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8
dx / (1
/N)
dN
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
2.2
2.4 SignalBackground
U/O
-flo
w (
S,B
): (
0.0,
0.0
)% /
(0.0
, 0.0
)%
TMVA response for classifier: MLP
This is definately the way to go for low mH
Didn’t have time to study further unfortunately
Carl Gwilliam Athens HSG2 Meeting 17/18
Conclusions
The low mass H → ZZ → llqq channel shows some promise but it isdifficult to conclude yet
A first look shows ≈ 5× σSM for 2 fb−1 (w/o syst) with significantimprovments possible through a MVA
Unlikely to get down to the level of the H → 4l but might not be so farbehind
Probably worth studing further but manpower needed for this
Carl Gwilliam Athens HSG2 Meeting 18/18