24th Nov. 2005 1

Study of VBF H->->lep+had

Junichi TanakaICEPP, Univ. of TOKYO

Discovery of Higgs and Supersymmetryto Pioneer Particle Physics in the 21th

Century

24th Nov. 2005 2

Outline

• Introduction• Study of 3rd jet by ME event generator• Study of fake BGs• Summary• Prospects : Discovery Potential of SM

Higgs

24th Nov. 2005 3

Excluded by LEP

VBF

ProductionProduction DecayDecay

114.4 < mH < 219 GeV/c2 @95% C.L.

• VBF H-> at low-mass region (mH = ~120GeV)– Large production cross section and branching ratio: xBR ~ 300fb

+-

SM Higgs Production & Decay @ LHC

LEPEWWG2005S

24th Nov. 2005 4

Vector Boson Fusion Process

• Characteristics– High Pt jets in forward region– Rapidity gap

Right fig : *3 = of 3rd jet –

average of of 2 forward jets• Signal … a dip in the center• BG … a peak in the center-> jet activities depend on

structure of color flow.

Rapidity gap

Treatment of 3rd jet is important in this mode!

PRD59(1999)14037hep-ph/9908378

(EW)

24th Nov. 2005 5

Study @ ATLAS

• VBF H-> is one of promising channels near the LEP limit. (left fig)• Reconstruct M() by using missing Et and observe a peak (right fig) o

n the right side of the Z mass peak.“VBF H->” is one of discovery channels at the low mass region.

VBF H->

ATLAS Scientific note:Eur.Phys.J.C32S2:19-54,2004

BG by Comphep

24th Nov. 2005 6

Motivation• Improvement and progress of our tools since the previous

studies (Eur.Phys.J.C32S2,19,2004, the pervious slide)

1) Events with multi jets can be generated by the recent ME event generators.

• 3rd jet was evaluated by Parton Shower (PS) model in the previous studies.

• We evaluate 3rd jet by the ME event generator.

2) Environment of reconstruction and full simulation, which will be used in 2007, is available.

• Fake rate can be estimated from the full simulation, which is based on Geant4.

• Large QCD backgrounds, which have not been considered in the previous studies, are checked by using results of the full simulation.

24th Nov. 2005 7

Type of Simulation

• Full simulation– Based on Geant4

• Initial layout– Used for detail studies

• Development of reconstruction tools, for example, tracking, reconstruction of missing Et, Particle ID tools (e, , tau, b, ).

• Fake rates, crack effect for missing Et and so on– Take much time, typically, ~15mins/event with 3GHz CPU.

• Fast simulation– Based on parameterized performance of full simulation

results– Used for the BG study, where we need high statistics.

• Most of today’s results are obtained by the fast simulation.

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Study of 3rd jet by ME event generator

24th Nov. 2005 9

ME ALPGEN + PS

• Alpgen (M.L.Mangano) is one of the ME event generators.– Studies using ME event generators becomes popular.

• ME has an advantage for producing high Pt jets.• To remove collinear and soft divergence, Pt and Rjj are required at pa

rton level.• Soft and collinear kinematical regions are covered by PS.

• We produced exclusive samples for Z+njets(n=2,3,4) with modified ALPGEN ver1 (small weight events can be produced).– “Z+njets” is a dominant BG.– MLM method is used to avoid double counting. The similar method is off

icially introduced from ver2.

24th Nov. 2005 10

To Avoid Double Counting

• We use the following method (“our MLM method”) to avoid the double counting of events: (not CKKW method)Veto events having jets (Pt>20GeV) at the outside of cone (dR=0.4), whose center is defined by the direction of a ME-parton.

ME-parton

Rec-jet

Good Bad

See T.Sasaki’s talk later

24th Nov. 2005 11

Pt and Distributions

• 3rd jets with high Pt increase.• 3rd jets become more central. It has a large effect on Central Jet Veto.• Pt and of 3rd jet depend on the number of jets. Cannot ignore them.

Pt of 3rd jet eta of 3rd jet

Alpgen sample(ver1), after lepton cut, normalized to 1fb-1.

“Z2j”“Z2j”+”Z3j”“Z2j”+”Z3j”+”Z4j”

log10Pt

with MLM methodto avoid double counting preliminary

preliminary

25GeV 100GeV

24th Nov. 2005 12

CJV Survival Probability

• Survival probability of “Z+2j” is higher than “Z+3j” and “Z+4j” due to smaller jet activity in the rapidity gap.

• ~40% of Z+njets (n=2,3,4) remains in total in case of Pt=20GeV.

(additional jets in the rapidity gap)

preliminary

• Central Jet Veto (CJV) : Veto events having additional jets with a “given” Pt in the rapidity gap.

Rapidity gap

jet

jet

No additional jetin the rapidity gap

24th Nov. 2005 13

M() Distribution

After all the selection criteria

• A amount of remaining Z+2j is large due to large cross section and high CJV survival probability.

• Z+3j and Z+4j still remains. They are not negligible.

• Since resolution becomes worse as the number of jets increases, their tails can contribute to BGs.

Z+2j

Z+3j

Z+4j

L=30fb-1

preliminary

Signal region

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Study of fake BGs

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Origin of Fake Lepton• Since cross section (x-sec) of events with QCD jets is much larger th

an that of the signal (x-sec~300fb), we suffer from these events, which have fake leptons from jets.

W j j j -> W j j : x-sec ~ >10nbj j j j -> e/ j j : x-sec ~ >10mb

• Fake Tau– 1 or 3 +- with 0, which have most of energy of a jet

• Fake Electron– Pair production of gamma (from 0)– Dalitz decay– +- and 0 with E/p~1– B/D semileptonic decay

• Fake Muon– pion punch through– pion decay in flight– B/D semileptonic decay

24th Nov. 2005 16

Tau Reconstruction and ID

• Hadronic decay: 1prong ~50% and 3prongs ~15%• We need to identify tau-jets from many QCD-jets.

• Reconstruction : two ways at ATLAS1) Cell based : start from cells, which is the similar as cone jet algo.2) Track based : start from charged track(s)

• Properties of tau-jet : for identification– 1 or 3 charged pion tracks– Narrowness and Isolation

Charged tracks : 1 or 3Photons from 0

24th Nov. 2005 17

Tau Rec/ID PerformanceCell based method

This result is parameterized and then is used in fast simulation.At least we need ~50% eff. with these fake rates for the discovery with L=10fb-1.

Light QuarkGluon

20<Pt<40GeV40<Pt<60GeV60<Pt<100GeV100<Pt<140GeV

preliminary

efficiency

fake r

ate “Fake rate of 10-2”

meanswe misidentify one QCD jetas a tau in 100 QCD jets.

Depend on Pt andtype of jet.

Gluon fake rate is betterthan quark fake rate dueto difference in narrowness.

24th Nov. 2005 18

e/ Fake Rates

• Fake rates of electron and muon from jets are obtained from QCD di-jet samples using the full simulation.– See O.Jinnouchi’s talk later.

• Typical performanceJet -> e : ~10-3

Jet -> : ~10-4

• These results are taken into account in the fast simulation to evaluate BG contributions.

24th Nov. 2005 19

W+3jets & 4jets

W j j j -> W j j j j j j -> e j j• W+3j and 4jets remain with the same order of Z+njets.• Improvement of tauID for both cases and decrease of fake electron for 4jets

are critical to decrease these BGs.

Corresponds to L ~ 10fb-1Corresponds to L ~ 70fb-1

W+3jets 4jets

(Signal region:110-135GeV)

fake(e->jet) : 10-3

preliminary

BG x-sec(fb)W+3jet

s0.26

4jets 0.30

Z+njets 0.13

preliminary

24th Nov. 2005 20

M() and Significance

• We assume that lepton(e,) efficiency is 85%.

• Significance becomes worse by a factor of 2 due to the new backgrounds.

• We need to improve tauID/fake electron and to optimize cuts.

a kind of BGsis the same asour paper(page5).

+”W+3jets” +”jjjj”

Signal (m=120)Z+nj(QCD)Z/jj(EW)tt, WWjj

L=30fb-1

BG=0.18fb BG=0.43fb BG=0.73fb

SIG=0.45fb

A B C

S/B S/B0.5

L=10fb-1

S/B0.5

L=30fb-1

A 2.5 3.3 5.8

B 1.0 2.1 3.7

C 0.61 1.6 2.9

1010

10

Signal region:110-135GeV

very preliminary

24th Nov. 2005 21

Summary

• VBF H->->lep+had is important for the SM Higgs discovery in the low mass region (mH~120GeV).

• We evaluated 3rd jet by the ME event generator.– There are differences in Pt and of 3rd jet by the number of jets.– Central jet veto survival probability and tail of M() depend on th

e number of jets.– We should use ME generators for multi-jets studies as much as p

ossible.• We studied fake BGs.

– W+3jets and 4jets become large BGs for the lep+had channel.– We need to improve our tauID and other PID performance.– We must understand QCD BGs for any LHC physics since their x-

sec is very large in the LHC. (maybe beyond our expectation…)

24th Nov. 2005 22

Discovery potential of SM Higgs

10fb-1

ATLAS

preliminary

200120GeV

Two promising channels at mTwo promising channels at mHH=~120GeV!=~120GeV!We are preparing for this discovery!We are preparing for this discovery!

NB: not include our latest results.

ATLAS-PHYS-2004-005

L=10fb-1:VBF (ll+lh) ~ 2.6 ~ 5.0@ mH=120GeV

TodayToday

24th Nov. 2005 23

Backup

24th Nov. 2005 24

ATLAS (A Toroidal LHC ApparatuS)

• Inner tracking system : 2T solenoid magnet• Liq. Ar EM calorimeter• Muon spectrometer : air-core troidal magnet

E,P resolution (P ~ 100GeV) :

~ 2%　e, ~ 1.5%Jets ~ 8%

•40MHz beam crossing•Readout channel = 160M channels•Raw data = 320Mbyte/sec (1TB/hour)

24th Nov. 2005 25

Production Decay Mass region and purpose

Gluon FusionH -> 110-140GeV Mass

H -> ZZ-> 4 l 140-1000GeV Discovery, Mass, Spin, Coupling

H -> WW 130-170GeV Discovery

Vector BosonFusion

H -> 110-140GeV Discovery, Mass, Coupling

H -> WW 130-200GeV Discovery, W coupling

H -> 110-140GeV Discovery, Mass

H -> bb 110-140GeV Yb coupling

ttH

H -> bb 110-130GeV

Yt couplingH -> 110-130GeV

H -> WW 130-180GeV

WH H -> WW 140-170GeV Discovery, W coupling

Summary of SM Higgs StudiesATLAS

24th Nov. 2005 26

Branching ratio of H-> :• BR(lep-lep) = ~12%• BR(lep-had) = ~46%“Lep-had” mode plays an important role for the Higgs discovery.

ＭＨ =120GeV

h

4M Distributions of VBF H->

Hadronic tau decay mode

factor of ~4

(e only) ATLAS

ATLAS

24th Nov. 2005 27

Fake rates

Pt(GeV) Jet -> e Jet -> BJet->e BJet-> BJet->

15-35 6.7+-0.8

2.4+-0.5

19+-8 < 3.2 25+-9

35-50 6.0+-1.3

0.6+-0.4

10+-7 < 5.1 10+-7

50-80 3.9+-0.9

2.3+-0.7

3.7+-3.7

7.5+-5.3

3.7+-3.7

80-130 3.8+-1.0

0.3+-0.3

< 4.2 < 4.2 < 4.2

130-200

3.4+-1.0

0.6+-0.4

< 5.2 5.2+-5.2

< 5.2

200- 1.4+-0.3

0.2+-0.1

< 2.4 < 2.4 < 2.4

very preliminary

in 10-4

24th Nov. 2005 28

Simulated data

• W(->ln)+3jets– Alpgen ver1– Jet condition : Pt>15GeV, |eta|<5, Rjj>0.7

Max(J1,J2)>30GeV, M(J1,J2)>200GeV, deltaEta(J1,J2)>2– x-sec = 1.4nb … very large!

• 4jets– Alpgen ver1– Jet condition : Rjj>0.7

• Forward region : Pt>30GeV, eta1xeta2<0, |eta1-eta2|>2.5Mjj>350GeV, |eta|<5.0

• Central region : Pt>20GeV, |eta|<2.7, Mjj>80GeV– x-sec = 2mb … very large!!!

24th Nov. 2005 29

tau->e

tau->h(1prong+pi0)

Missing Et

FWJet2 FWJet1

qqH -> qq-> qq lep had

We’ll look this type of event in the real data !!!We’ll look this type of event in the real data !!!

24th Nov. 2005 30

ME+PS

• Z2j samplez2j without MLM method

• Z3j samplez2j with MLM method

+ z3j without MLM method

• Z4j samplez2j with MLM method

+ z3j with MLM method+ z4j without MLM method