Search for the Standard Model Higgs boson
decaying to a bottom-quark pairwith the ATLAS detector
Kenji Kiuchi
University of Tsukuba@TGSW 2014/Sep/29

1LHC-ATLAS experiment
ATLAS: General purpose detector@LHC

le manJura

27km ring + 8.3T dipole
8TeV: highest energy
7.7 ⇥ 1033 cm2 s 1 :highest luminosity p-p collider

available data: 8TeV, 20.3/fb~90% of delivered ! 2

Higgs->bb analysis
σ(stat.)ATLAS Prelim.

sys inc.

σ(theory )σ(theory)mH = 125.5 GeV
H → γγµ = 1.57+0.33-0.28+ 0.23- 0.22+ 0.24- 0.18+ 0.17- 0.12+ 0.35- 0.32
H → ZZ* → 4l+ 0.20- 0.13
µ = 1.44+0.40 + 0.17-0.35 - 0.10+ 0.21- 0.21
H → WW* → lνlν
+ 0.24+0.32 - 0.19
µ = 1.00+ 0.16-0.29 - 0.08

Combined
H→γ γ , ZZ*, WW*+0.21µ = 1.35-0.20

Total uncertainty± 1σ on µ>5σ>5σ
3.8σ!?

-0.6+ 0.3

H → ττ (8 TeV data only) -+ 0.3
0.4+0.5 - 0.3µ = 1.4+ 0.2-0.4 - 0.1-0.32Combinedµ = 1.30+0.18-0.17qqbb

Analyzed channels± 0.5µ = 1.09+0.36bb+ 0.14- 0.14+ 0.16
- 0.14+ 0.13
- 0.11W,Z H → bb
+0.7 ± 0.4µ = 0.2<0.1Combined
H→bb, ττno trigger

W/Z

l/v
l/v

W/Z

4.1σ+ 0.24
- 0.24+ 0.27
- 0.21+ 0.08
- 0.04vvbb, lvbb, llbb

+ 0.12- 0.12+ 0.14
- 0.11+ 0.10
- 0.08

H->bb has the largest branching ratio(~57%)
Signal strength (µ)∫
Final state have only jets -> no trigger
Next is H->bbFocus
on
W/Z
associateproduction
How to suppress & control-> trigger leptons from W/Z decayhuge background3s = 7 TeV ∫ Ldt = 4.6-4.8 fb-1
s = 8 TeV Ldt = 20.3 fb-0.5 00.5

1

1.5

2-1

https://twiki.cern.ch/twiki/bin/view/AtlasPublic/HiggsPublicResultsObject selection
ZH->llbb: 2lep.
Reco. signal

WH->lvbb: 1lep.

ZH->vvbb: 0lep.

e/mu
MET(et, phi)
leptonmissing

2 jetshigh S/Nlepton
missinghigh stat

middle stat.&S/NDeﬁne the 3 channels using loose leptons to maximize the acceptance !
Jet deﬁnition• pT > 20 GeV• |eta| < 2.5
pileup rejection
2 or 3 jets
leading jet pT > 45 GeV•

4Lepton deﬁnition• pT > 7 GeV• |eta| < 2.5(e), 2.7(mu)• isolation
0-2 lepton
leading lepton pT > 25 GeV

b-tagging
b-jet
plooseTightMedium

pb-tagging: identify b-quark origin jet(b-jet)b-quark has long life time: c⌧ ⇠ 470µm
Lxy ⇠ 5mm(pT = 50GeV ) > vertex resolution

NEW: deﬁne 3 b-tagged categories• tight(50%)…high b-jet purity• medium(70%)…conventional region• loose(80%)…additional region
5

ﬂavor fraction determination
How do we estimate W/Z+jets ﬂavor(b, c, light) fraction ?
W+2jet MC mbb(2tag) and b-tag(1tag) distribution!

mbb is most sensitive variable for higgs
but have less information about the BKG ﬂavor
b-tag weight have a strong constraint

0lep
Z+JetsW+Jetsttbar

1lep
W+Jetsttbar

62lep
Z+Jetsright side is b rich(dark color) and left side is light ﬂavor rich(light color)BDT(MVA) distribution
input variables
VariablepT v
missETpbT1
pbT2mbbR(b1 , b2 )| ET a(b1 , b2 )|(V, bb)| ET a(V, bb)|HTmin[ (`, b)]mW
Tm``
M V 1c(b1 )M V 1c(b2 )jetpT 3mbbj

0-Lepton
⇥⇥
⇥
⇥⇥
⇥
⇥
⇥⇥
⇥

1-Lepton
⇥
⇥⇥
⇥
⇥
⇥
⇥
⇥
⇥

2-Lepton
⇥
⇥
⇥
⇥
⇥
⇥
⇥
⇥
⇥

0lep

1lep⇥⇥⇥⇥⇥
Only in 3-jet events
⇥⇥⇥⇥
⇥
⇥

2jet2(Medium+)Tight btag
BDT inputs are optimized foreach channel.
Globally ﬁt using b-tag dist.
and BDT output.

2lep

7

Result

Exp. Limit: 0.8 (OLD 1.3, 60% better)Obs. Limit: 1.2
Exp. Signiﬁcance: 2.6σ
Obs. Signiﬁcance: 1.3σ
μ=0.51 0.39
Run2 data will give us an answer !!
8

http://arxiv.org/abs/1409.6212

Backup

250

-1

T0lep
200
LowMET
150

700
ATLAS Preliminary
600
500

s = 8 TeV ∫Ldt = 20.3 fb
1 lep., 2 jets, 2 Medium+Tight tagsincl. p V >120 GeV-1T1lep

400

Data 2012
VH(bb) (µ=1.0)
DibosonttSingle top
Multijet
W+hf
W+cl
W+lZ+hf
Uncertainty
Pre-fit background
VH(bb)×20300
200

50100

1.5 20 40 60 80 100 120 140 160 180 200 220

1.5 20 40 60 80 100 120 140 160 180 200 220

0.520 40 60 80 100 120 140 160 180 200 220
mbb [GeV]ATLAS Preliminary

500

400

s = 8 TeV ∫Ldt = 20.3 fb
0 lep., 2 jets, 2 Medium+Tight tagsincl. p V >120 GeV-1

T0lep

Data 2012
VH(bb) (µ=1.0)
Dibosontt
Single top
Multijet
W+hf
W+cl
Z+hf
Z+cl
Uncertainty
Pre-fit background
VH(bb)×10

Events / 25 GeV

1Data/Pred100

300

Data/Preds = 8 TeV ∫Ldt = 20.3 fb
0 lep., 2 jets, 2 Medium+Tight tags
100<p V <120 GeV

Data 2012
VH(bb) (µ=1.0)
Dibosontt
Single top
Multijet
W+hf
W+cl
W+l
Z+hf
Z+cl
Uncertainty
Pre-fit background
VH(bb)×20

Events / 25 GeV

300

ATLAS Preliminary

10.5

20 40 60 80 100 120 140 160 180 200 220
mbb [GeV]ATLAS Preliminary

100s = 8 TeV ∫Ldt = 20.3 fb
2 lep., 2 jets, 2 Medium+Tight tagsincl. p V >120 GeV-1T802lep

Data 2012
VH(bb) (µ=1.0)
Diboson
tt
Z+hf
Z+cl
Z+l
Uncertainty
Pre-fit background
VH(bb)×1060

2jet2Medium+Tight tag
LowMET: a new region
CUT base ONLY

20040!

10020Good Data/MC

1.5 20 40 60 80 100 120 140 160 180 200 220
10.5

20 40 60 80 100 120 140 160 180 200 220m [GeV]

Data/Pred

Events / 25 GeV

Data/Pred

Events / 25 GeV

Mbb distribution

2 20 40 60 80 100 120 140 160 180 200 220
101020 40 60 80 100 120 140 160 180 200 220m [GeV]

agreement

BDT training

ABCD method