Upload
others
View
1
Download
0
Embed Size (px)
Citation preview
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
Junichi Kanzaki, Physics at LHC, Prague, 07/07/03
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
Junichi Kanzaki, Physics at LHC, Prague, 07/07/03
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
Junichi Kanzaki, Physics at LHC, Prague, 07/07/03
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)
Junichi Kanzaki, Physics at LHC, Prague, 07/07/03
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
Junichi Kanzaki, Physics at LHC, Prague, 07/07/03
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
Junichi Kanzaki, Physics at LHC, Prague, 07/07/03
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)
Junichi Kanzaki, Physics at LHC, Prague, 07/07/03
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
Junichi Kanzaki, Physics at LHC, Prague, 07/07/03
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
Junichi Kanzaki, Physics at LHC, Prague, 07/07/03
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.
Junichi Kanzaki, Physics at LHC, Prague, 07/07/03
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)
Junichi Kanzaki, Physics at LHC, Prague, 07/07/03
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
Junichi Kanzaki, Physics at LHC, Prague, 07/07/03
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
Junichi Kanzaki, Physics at LHC, Prague, 07/07/03
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
Junichi Kanzaki, Physics at LHC, Prague, 07/07/03
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
Junichi Kanzaki, Physics at LHC, Prague, 07/07/03
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
Junichi Kanzaki, Physics at LHC, Prague, 07/07/03
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)
Junichi Kanzaki, Physics at LHC, Prague, 07/07/03
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
Junichi Kanzaki, Physics at LHC, Prague, 07/07/03
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.
Junichi Kanzaki, Physics at LHC, Prague, 07/07/03
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