higgs searches in the vector boson fusion channels in atlas · • tag jets in the forward region...

Junichi KanzakiKEK

ATLAS Group

Higgs Searches in the Vector Boson Fusion Channels in ATLAS

Physics at LHCPrague, July 7th, 2003

Junichi Kanzaki, Physics at LHC, Prague, 07/07/03

Outline

• Higgs Production and Decay at LHC• Introduction to Vector Boson Fusion (VBF) Processes• Major Detector Issues for ATLAS• Analysis of VBF H -> WW* and Results• Analysis of VBF H -> tt and Results• Combined Results• Summary and Prospects


Vector boson fusionForward jet tagCentral jet veto

Higgs Production Processes at LHC

g

gg fusiong

g

gassociated ttH

t Ho

Ho

t

t

t

t

t

t

q

q

WW, ZZ fusion

q

q

q

q

Ho

Hoassociated WH, ZH

q

Ho production at hadron colliders:

W,Z

W,Z

W,ZW,Z

But : BR ( H Z¡Z¡ 4 –) = 1.4 ¥ 10—3

BR ( H Z¡Z¡ 4m–) = 3 ¥ 10—4

gg H

qq Hqq

gg,qq Hbb

gg,qq Htt

qq HZ

qq’ HW

107

106

105

104

103

102

10-1

10-2

10-3

10-4

10

1

0 200 400 600 800 1000

MH (GeV)

s (p

b)

s(pp H+X) s = 14 TeVmt = 175 GeVCTEQ4M

D_D

_118

6 .c

NLO QCDM. Spira et al.

even

ts fo

r 105

pb-

1

Gluon-gluon fusionSpecific Higgs decay mode

Heavy quark associated

Tag top decays

Vector boson associated

Tag vector boson decays


Higgs Production Cross SectionGluon gluon fusion

largest cross section

Vector boson fusionlarge cross section

Associated productionssmall cross sections

s(ppÆH+X) [pb]

÷s = 14 TeVMt = 175 GeVCTEQ4M

ggÆH

qqÆHqqqq_’ÆHW

qq_ÆHZ

gg,qq_ÆHtt

_

gg,qq_ÆHbb

_

MH [GeV]0 200 400 600 800 1000

10-4

10-3

10-2

10-1

1

10

10 2

0 100 200 300 400 500 600 700 800 900 1000


Higgs Searches before “VBF”

• gg: D. Rainwater and D. Zeppenfeld, JHEP 9712:005, 1997• tt: D. Rainwater, D. Zeppenfeld and K. Hagiwara, Phys. Rev. D59:014037, 1999• WW*: D. Rainwater and D. Zeppenfeld, Phys. Rev. D60:113004, 1999, Erratum-ibid. D61:099901, 2000.

Pioneering works in applying “VBF” to the light Higgs search by D. Rainwaterand his colleagues:

Light Higgs search before “VBF”:• gg: direct production by gluon fusion• bb: top-quark associated production ttH(->bb)

1

10

10 2

102

103

mH (GeV) S

igna

l sig

nific

ance H → γ γ

ttH (H → bb) H → ZZ(*) → 4 l

H → ZZ → llνν H → WW → lνjj

H → WW(*) → lνlν

Total significance

5 σ

∫ L dt = 30 fb-1

(no K-factors)

ATLAS

gg

ttH(->bb)

No single mode can observe light Higgs with 30fb-1

ATLAS Physics TDR (1999)


BR(H)

bb_

t+t-

cc_

gg

WW

ZZ

tt-

gg Zg

MH [GeV]50 100 200 500 1000

10-3

10-2

10-1

1

10 2 10 3

Higgs Decay Branching Ratios

• mH < 2mW bb, tt gg WW*, ZZ*

• mH ≈ 2mW WW dominates Higgs decay

• mH > 2mW WW, ZZ

100 GeV 200 GeV


Low Mass Higgs via VBF

• H -> WW* -> llvv, lvqq very effective for mH > 130 GeV

• H -> tt -> ll, lh ( + pTmiss)

sensitive in the region close to LEP direct limit• H -> gg

good around 120 GeV• H -> bb important for the Higgs coupling measurement large background from QCD processes trigger efficiency is low

Low mass region is especially important. - LEP direct limit (mH > 115 GeV)

- EW fit constraint (mH < 211 GeV, 95%C.L.)

1

10

10 2

102

103

mH (GeV) S

igna

l sig

nific

ance H → γ γ

ttH (H → bb) H → ZZ(*) → 4 l

H → ZZ → llνν H → WW → lνjj

H → WW(*) → lνlν

Total significance

5 σ

∫ L dt = 30 fb-1

(no K-factors)

ATLAS

100 GeV 200 GeV


Vector Boson Fusion Production

f

h

Forward jets

Higgs Decay

Jet

Jet

• Two high PT jets with large Dh

separation• Low QCD activities in the central region• Possibility to observe different decay modes in the same production process• Promising to observe invisible Higgs decays (relevant for beyond the SM Higgs)


Jet Pair with Large Rapidity Gap

0

0.01

0.02

0.03

0.04

0.05

-4 -2 0 2 4

η

Arb

itrar

y un

its

0

0.02

0.04

0.06

0.08

0 2 4 6 8 10

∆ η

Arb

itrar

y un

its

Comparison between VBF Higgs events vs. tt background

htag jet Dh

Higgs Signal reconstructed - dots parton level - solid hist

tt background - dashed hist


Tagging Forward Jets

0

0.2

0.4

0.6

0.8

1

0 2 4

ηE

ffici

ency

Efficiency for reconstructing a tag jet

• Efficiency is critical.

• Full simulation used for fast simulation parametrization -> parametrized for fast simulation • Double tag efficiency ~50%

≈ 0.7×0.7

Forward

Central

PT>20GeV


Central Jet Veto

0

10

20

30

0 20 40 60

PT Veto Threshold (GeV/c)

Fake

Rat

e (%

)• For Higgs signal, central jet activity is suppressed due to the lack of color exchange between the quarks. - Most background processes there is color flow in t-channel

• Pile up effects introduce fake central jets

- Small at low luminosity -> PT > 20 GeV

- Serious at high luminosity -> PT > 30 GeV or higher

Low Lumi.

High Lumi.


VBF H -> WW*

• Di-lepton mode: H -> WW* -> lnln - clean signal• Lepton + two jets mode: H -> WW* -> lnjj - larger branching ratio - large background

• Background: tt, WWjj(EW)• Lepton angular correlation is effective to suppress background for H -> WW -> ll mode.

Transverse Mass DistributionmH=160GeV (only em)


VBF H -> WW*• Lepton angular correlation shows evidence of Spin-0 resonance in H -> WW -> ll modes

MT<175GeV MT>175GeV

Signal region Outside Signal region

mH=160GeV, em modewithout lepton correlation cut


1

10

10 2

100 120 140 160 180 200 mH (GeV/c2)

Sign

al si

gnifi

canc

e

ME prediction for tt backgr. PYTHIA pred. for tt backgr.

∫ L dt = 10 fb-1

(no K-factors)ATLAS

qqH → qq WW(*)

Results of WW* channel• With 10fb-1: 145 < mH < 190 GeV (em only)

• With 30fb-1: 125 < mH < 190 GeV (all di-lepton)

• 10% uncertainty on the background is assumed - determination of the background level from data below the signal peak• Poisson statistics• Further improvement is obtained by - Neural Net approach - Likelihood ratios in significance calculation

-> 45-50% improvement -> mH >115GeV with 10fb-1 expected


VBF H -> tt• Two types of final states: - lepton+lepton mode: H -> tt -> lnnlnn - lepton+hadron mode: H -> tt -> lnnhn

• Mtt reconstruction using collinear approximation - Mass resolution ≈ 10%

• Background: - Zjj (EW and QCD) - tt and W production

Collinear approximation


mττ (GeV)

evts

/ 5

GeV

mH=120 GeV

t t

, WW EW

Z jj

012345678

80 100 120 140 160 180

VBF H -> tt

mτ τ (GeV/c2)

dσ/d

mτ

τ (fb

/10

GeV

/c2 )

MH = 130 GeV/c2

EW+QCD Zjjt tbar

0

0.5

1

1.5

100 150 200

ll mode, mH=120 GeV

for 30fb-1lh mode, mH=130 GeV

Mtt distributions


mH [GeV] 110 120 130 140 150

Combined Statistical

Significance3.7 5.7 5.7 4.8 2.4

Results of VBF H -> ttWith 30fb-1:

• tt decay modes above 5-s significance over the mass range: 115 < mH < 140 GeV

with 30 fb-1. (LEP direct limit mH > 115 GeV)

* 10% uncertainty of the background is assumed determined from Z -> tt resonance shape of real data


1

10

10 2

100 120 140 160 180 200 mH (GeV/c2)

Sig

nal s

igni

fican

ce

qqH → qq WW(*)

qqH → qq ττ VBF, combined VBF, + γγ + ttH(bb) + ZZ*

∫ L dt = 10 fb-1

(no K-factors)ATLAS

Combined Results

With 10 fb-1:

• Combining two vector boson fusion channels: 135 < mH < 190 GeV (5s)

• Adding gg, ttH and ZZ* channels: 120 GeV < mH (5s)


1

10

10 2

100 120 140 160 180 200 mH (GeV/c2)

Sig

nal s

igni

fican

ce H → γ γ ttH (H → bb) H → ZZ(*) → 4 l H → WW(*) → lνlν qqH → qq WW(*)

qqH → qq ττTotal significance

5 σ

∫ L dt = 30 fb-1

(no K-factors)ATLAS

With 30 fb-1:

• Full mass range favored by EW fit and LEP exclusion can be covered by ATLAS

• Several channels are available for a Higgs discovery.

-> gives complementarity to the discovery process

- different decay modes - different production mechanisms - different detector components

Combined Results


Summary• The vector boson fusion channels provide a large discovery potential even for a small integrated luminosities.• Tag jets in the forward region and a low jet activity in the central region of the detector allow for a significant background rejection.• The VBF H -> WW* channel provides a large discovery potential to the ATLAS experiment. ~ 135 < mH < 190 GeV/c2 (10fb-1)

• The VBF H -> tt channel also contributes in the mass region: mH < 140 GeV/c2 (30fb-1)

- Important for a measurement of the Higgs boson coupling to fermions.• Combining above channels, the full mass range up to 2mZ can be

covered with 30fb-1.


Prospects• On-going analysis for other decay modes: H -> gg, bb and invisible• Extend to intermediate mass region (H->WW and ZZ channels) WW -> lnqq mH > 300GeV -> 2mZ

WW -> lnln and ZZ->llqq (on going)

• Contribution to the measurement of Higgs properties• More understanding on the detector performance Precise estimation on the tag efficiency of forward jets• More understanding on the higher order MC generations Central jet veto is sensitive to the multi-jet production rate Tails in Z->tt background should be understood better in H->tt analysis

higgs searches in the vector boson fusion channels in atlas · • tag jets in the forward region...

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