How to find a Higgs boson Heather M. Gray, CERN ( ) S E R E - - PowerPoint PPT Presentation

SLIDE 1

How to find a Higgs boson

Heather M. Gray, CERN (へざあ)

W H E R E ’ S H I G G S ?

SLIDE 2

higgs discovery

[GeV]

H

m 110 115 120 125 130 135 140 145 150 Local p

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ATLAS 2011 - 2012

σ σ 1 σ 2 σ 3 σ 4 σ 5 σ 6

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γ γ

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Data S+B Fit B Fit Component σ 1 ± σ 2 ±

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(発⾒覌) Seminar on 4 July 2012 by the ATLAS and CMS collaborations

SLIDE 3

this talk

• NOT about the Higgs discovery
• NOT to discuss the latest Higgs results
• There are many and they are interesting
• Ask me about them later if you like
• But rather, to try to explain how we go about doing a Higgs

analysis using a specific example

• Example: http://link.springer.com/article/10.1007/

JHEP01(2015)069

(この話)

SLIDE 4

choose your channel I

Gluon fusion Vector Boson Fusion (VBF) (W/Z) Production ttH Production

g g t t t t H

q q W, Z W, Z H

q q W/Z H

(あなたのチャンネルを選択)

5 10 15 20

ggF VBF (W/Z)H ttH

Production Cross-section H σ [pb]

SLIDE 5

choose your channel I

Gluon fusion Vector Boson Fusion (VBF) (W/Z) Production ttH Production

g g t t t t H

q q W, Z W, Z H

q q W/Z H

(あなたのチャンネルを選択)

5 10 15 20

ggF VBF (W/Z)H ttH

Production Cross-section H σ [pb]

SLIDE 6

choose your channel II

H b b τ τ H Z Z H W W H

(あなたのチャンネルを選択)

H t t t

γ Η γ

Other 11% 0% 3% ττ 6% WW 22% bb 58%

Decay Probability

SLIDE 7

choose your channel II

H b b τ τ H Z Z H W W H

(あなたのチャンネルを選択)

H t t t

γ Η γ

Other 11% 0% 3% ττ 6% WW 22% bb 58%

Decay Probability

SLIDE 8

build a billion dollar collider

(⼗卂億ドルの加速器の建設)

SLIDE 9

and a couple million dollar detectors

CMS (Compact Muon Solenoid) ATLAS (A Toroidal ApparatuS) (そして数百万ドルの検出器)

SLIDE 10

reconstruction

• Reconstruct electrons,

muons, photons from energy deposits

• Reconstruct jets and tag b-

jets with sophisticated algorithms

• Use conversation of

(transverse) energy to calculate the missing energy (MET)

MET (再構築) jet

SLIDE 11

jet reconstruction

jet reconstruction algorithms group energy deposits together in different ways to form jets (ジェット再構築)

SLIDE 12

b-jet reconstruction

b-quarks have a longer lifetime than

• ther elementary

particles identify b-jets by reconstructing displaced vertices from tracks (ビージェット識別)

SLIDE 13

choose your cuts

W H l ν b b

• Need events containing two b-jets, 1 lepton and MET
• j1pT > 45 GeV; j2pT > 20 GeV, MV1c > 80%
• l pT > 20 GeV; isolated, MET > 20 GeV

SLIDE 14

choose discriminating variable

good discrimination poor discrimination The better the discriminating variable, the larger the separation between signal and background B S B S For the Higgs, a good variable is the mass

SLIDE 15

background

• Background events are other events

that look just like signal

• Two types of background
• Reducible
• Experimental: better isolation cut,

improved b-tagging algorithm

• Physics: different final state, e.g.

additional lepton, jets

• Irreducible = same final state as

signal

• Often different kinematics or

need to apply kinematic cuts

20 40 60 80 100 120 140 160 180 200 220 Events / 25 GeV 100 200 300 400 500 600 700

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

ATLAS

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∫

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V T

• incl. p

[GeV]

bb

m 20 40 60 80 100 120 140 160 180 200 220 Data/Pred 0.5 1 1.5

W+bb top WZ W+cl (背景)

SLIDE 16

background uncertainty

• Large uncertainties -> more difficult to extract the signal
• Uncertainties can be both statistical and systematic
• Decrease impact by either reducing background or reducing

uncertainty: e.g. estimate in a control region

(背景の不確実性)

SLIDE 17

systematic uncertainties

Z+jets Zl normalisation, 3/2-jet ratio 5% Zcl 3/2-jet ratio 26% Z+hf 3/2-jet ratio 20% Z+hf/Zbb ratio 12% ∆φ(jet1, jet2), pV

T, mbb

S W+jets Wl normalisation, 3/2-jet ratio 10% Wcl, W+hf 3/2-jet ratio 10% Wbl/Wbb ratio 35% Wbc/Wbb, Wcc/Wbb ratio 12% ∆φ(jet1, jet2), pV

T, mbb

S tt 3/2-jet ratio 20% High/low-pV

T ratio

7.5% Top-quark pT, mbb, Emiss

T

S Single top Cross section 4% (s-,t-channel), 7% (Wt) Acceptance (generator) 3%–52% mbb, pb1

T

S Diboson Cross section and acceptance (scale) 3%–29% Cross section and acceptance (PDF) 2%–4% mbb S Multijet 0-, 2-lepton channels normalisation 100% 1-lepton channel normalisation 2%–60% Template variations, reweighting S

(系統誤差)

background normalisation background shape signal scale

SLIDE 18

improving sensitivity: mass resolution

• The better the mass resolution, the

smaller the amount of background that needs to be considered

• 14% improvement in resolution

[GeV]

bb

m 20 40 60 80 100 120 140 160 180 200 Events / 4.0 GeV 0.02 0.04 0.06 0.08 0.1

Global Sequential Calib. (GSC) + Muon-in-Jet Correction + Resolution Correction 16.4 GeV -- 14.4 GeV 12% 14.1 GeV 14%

GSC

σ )/ σ

• GSC

σ Resolutions (

ATLAS Simulation

MC b b → Pythia VH, H

2 lep., 2 jets, 2 b-tags inclusive

V T

p

(質量分解能)

SLIDE 19

improving sensitivity: MVA

50 100 150 200 250 300 350 400 450 500 Events / 25 GeV 200 400 600 800 1000 Data 2012 =1.0) µ VH(bb) ( Diboson t t Single top Multijet W+hf W+cl W+l Z+hf Z+cl Z+l Uncertainty Pre-fit background 10 × VH(bb) ATLAS

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(a)

50 100 150 200 250 300 350 400 450 500 Events / 20 GeV 200 400 600 800 1000 1200 1400 1600 1800 Data 2012 =1.0) µ VH(bb) ( Diboson t t Single top Multijet W+hf W+cl W+l Z+hf Z+cl Z+l Uncertainty Pre-fit background 50 × VH(bb) ATLAS

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Ldt = 20.3 fb ∫ = 8 TeV s 0 lep., 2 jets, 2 tags >120 GeV V T p [GeV] miss T E 50 100 150 200 250 300 350 400 450 500 Data/Pred 0.5 1 1.5

(b)

0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 Events / 0.2 100 200 300 400 500 600 700 800 900 Data 2012 =1.0) µ VH(bb) ( Diboson t t Single top Multijet W+hf W+cl W+l Z+hf Uncertainty Pre-fit background 50 × VH(bb) ATLAS

• 1

Ldt = 20.3 fb ∫ = 8 TeV s 1 lep., 2 jets, 2 tags >120 GeV V T p ) 2 ,b 1 R(b ∆ 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 Data/Pred 0.5 1 1.5 2

(c)

50 100 150 200 250 300 350 400 450 500 Events / 20 GeV 500 1000 1500 2000 2500 3000 3500 4000 4500 Data 2012 =1.0) µ VH(bb) ( Diboson t t Single top Multijet W+hf W+cl W+l Z+hf Uncertainty Pre-fit background 90 × VH(bb) ATLAS

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Ldt = 20.3 fb ∫ = 8 TeV s 1 lep., 2 jets, 2 tags >120 GeV V T p [GeV] V T p 50 100 150 200 250 300 350 400 450 500 Data/Pred 0.5 1 1.5

(d)

50 100 150 200 250 300 350 400 Events / 20 GeV 50 100 150 200 250 300 Data 2012 =1.0) µ VH(bb) ( Diboson t t Single top Z+hf Z+cl Z+l Uncertainty Pre-fit background 60 × VH(bb) ATLAS

• 1

Ldt = 20.3 fb ∫ = 8 TeV s 2 lep., 2 jets, 2 tags >120 GeV V T p ) [GeV] 1 (b T p 50 100 150 200 250 300 350 400 Data/Pred 0.5 1 1.5

(e)

0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 Events / 0.2 50 100 150 200 250 Data 2012 =1.0) µ VH(bb) ( Diboson t t Single top Z+hf Z+cl Z+l Uncertainty Pre-fit background 60 × VH(bb) ATLAS

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(f)

20 40 60 80 100 120 140 160 180 200 220 Events / 25 GeV 10 20 30 40 50 60

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

ATLAS

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BDT

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0.2 0.4 0.6 0.8 1 Data/Pred 0.5 1 1.5 2

Multivariate techniques combine information from kinematic distributions into a single discriminating variable (多変量解析)

SLIDE 20

improving sensitivity: categories

• Simple idea: add cuts to divide events into categories
• Don’t throw away any events
• Separate out high S/B regions
• Information to constrain backgrounds
• For VH(bb) we categorise depending on the number of jets x

Higgs pT x b-tagging quality

• Huge improvement to sensitivity; largely from background

constraint

MJ 4% W+bb 20% VH 0% Wcl 22% Wl 16% top 34% 2% W+bb 16% VH 1% top 76% 2%

loose b-tag tight b-tag (カテゴリ)

Process Scale factor tt 0-lepton 1.36 ± 0.14 tt 1-lepton 1.12 ± 0.09 tt 2-lepton 0.99 ± 0.04 Wbb 0.83 ± 0.15 Wcl 1.14 ± 0.10 Zbb 1.09 ± 0.05 Zcl 0.88 ± 0.12

SLIDE 21

result

• Look for an excess over background prediction
• Fit rate with respect to the Standard Model

prediction

• μ= σ/σSM
• Small excess, but a little smaller than the SM

prediction

• More data needed !

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log

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Combination 8 TeV 7 TeV

0.51 0.37

− 0.40 +

(

0.22 − 0.30 − 0.25 + 0.31 +

) 0.65 0.40

− 0.43 +

(

0.24 − 0.32 − 0.28 + 0.33 +

)

• 1.61 1.46

− 1.50 +

(

0.92 − 1.13 − 0.86 + 1.22 +

) tot ( stat syst )

tot. stat.

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SLIDE 22

conclusion

• A lightening tour of the >20 years of work it took to probe the Higgs

coupling to b-quarks

• Discussed some key aspects of analysis design
• Discriminating variable selection
• Mass resolution
• Background estimate
• Systematic Uncertainties
• For bb, we’re not quite there yet, but getting very close
• Perhaps one of you, will be the one to observe it ?

(結論) W H E R E ’ S H I G G S ?