H ZZ llqq @ Low Mass - Nikhefivov/hsg2/GwilliamHllqqLowMass.pdf · H→ ZZ → llqq@ Low Mass Carl...

H → ZZ → llqq @ Low Mass

Carl Gwilliam

8th September 2011Athens HSG2 Meeting

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


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


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


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


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


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◦


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.


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


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


∆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


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

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



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



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



H(m

Total BG

Z

Top

Diboson

ATLAS Internal


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


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

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

=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


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 ...


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


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


H ZZ llqq @ Low Mass - Nikhefivov/hsg2/GwilliamHllqqLowMass.pdf · H→ ZZ → llqq@ Low Mass Carl...

Documents

Transcript of H ZZ llqq @ Low Mass - Nikhefivov/hsg2/GwilliamHllqqLowMass.pdf · H→ ZZ → llqq@ Low Mass Carl...